diff --git a/.gitignore b/.gitignore
index d30f756a..5d12ca97 100644
--- a/.gitignore
+++ b/.gitignore
@@ -21,3 +21,6 @@ venv/
 lib
 draft-ietf-mls-protocol.xml
 draft-ietf-mls-protocol.tls
+
+# AUTH48 temporary artifacts
+auth48/rfc9420.gen.xml
diff --git a/auth48/Makefile b/auth48/Makefile
new file mode 100644
index 00000000..8c0b6d74
--- /dev/null
+++ b/auth48/Makefile
@@ -0,0 +1,28 @@
+DRAFT=draft-ietf-mls-protocol
+RFC=9420
+RFC_ED_URL="https://www.rfc-editor.org/authors/rfc${RFC}.xml"
+
+.PHONY: prepare gen diff
+
+init:
+	curl -s ${RFC_ED_URL} | \
+		xmllint --exc-c14n - | \
+		xmllint --format - \
+		>rfc${RFC}.authors.xml 
+
+gen: rfc${RFC}.gen.xml
+
+rfc${RFC}.gen.xml: ../${DRAFT}.md
+	make -C .. ${DRAFT}.xml
+	mv ../${DRAFT}.xml .
+	python3 xml-clean.py <${DRAFT}.xml | \
+		xmllint --exc-c14n - | \
+		xmllint --format - | \
+		python3 text-clean.py >rfc${RFC}.gen.xml
+	rm ${DRAFT}.xml
+
+diff: rfc${RFC}.authors.xml rfc${RFC}.gen.xml
+	mvim -d rfc${RFC}.gen.xml rfc${RFC}.authors.xml
+
+clean:
+	rm rfc${RFC}.gen.xml
diff --git a/auth48/QUESTIONS.md b/auth48/QUESTIONS.md
new file mode 100644
index 00000000..0ef52a2c
--- /dev/null
+++ b/auth48/QUESTIONS.md
@@ -0,0 +1,885 @@
+# RFC Editor Questions
+
+## RLB Notes
+
+* This document contains the changes from PR#878, which were reviewed by the WG
+  and approved by the AD: https://github.com/mlswg/mls-protocol/pull/878
+
+* I updated Emad Omara's email address and affiliation based on his advice.
+
+* I noticed a copy/paste error in the IANA considerations, where "where this
+  credential is defined" was repeated in other registries.
+
+* Raphael noted that there was inconsistency over whether "ciphersuite" is one
+  or two words.  I have updated the document to use the two-word form, following
+  the example of RFC 8446.  Please check that all instances have been updated
+  properly. Note that this changed the title of one of the IANA registries, from
+  "MLS Ciphersuites" to "MLS Cipher Suites".
+
+* You seem to be deleting commas that do not preced independent clauses.  In
+  most cases cases, the sentence structure is sufficiently complex that these
+  these commas improve clarity, so I have re-added them.
+
+* You seem to be adding commas before "as described in..." and similar.  This is
+  almost always incorrect.  These references are not "see more details", they
+  are part of the definition of the thing they are modifying.  For example,
+  "Verify that the credential in the LeafNode is valid as described in Section
+  X" means that you need to follow the procedure in Section X.  (In one
+  instance, the with-comma version doesn't even parse!  "... an init secret
+  computed, as described in Section X")
+
+* You seem to be adding comma after one-word introductions like "Here" and
+  "Hence".  These are unnecessary and distracting.
+
+* The document is inconsistent as to whether a <t> is used inside <dd>.
+
+## Questions and Answers (Round 1)
+
+> 1) [rfced] xml2rfc returns a number of warnings and suggest that
+> viewBox be used.  Please review and let us know if you would like to make
+> any updates. 
+> 
+> Examples:
+> rfc9420.xml(434): Warning: Found SVG with width or height specified, which will make the artwork not scale.  Specify a viewBox only to let the artwork scale.
+> rfc9420.xml(568): Warning: Found SVG with width or height specified, which will make the artwork not scale.  Specify a viewBox only to let the artwork scale.
+> ...
+> rfc9420.xml(5759): Warning: Found SVG with width or height specified, which will make the artwork not scale.  Specify a viewBox only to let the artwork scale.
+> rfc9420.xml(8128): Warning: Found SVG with width or height specified, which will make the artwork not scale.  Specify a viewBox only to let the artwork scale.
+> 
+
+The document's viewBox setting is correct; xml2rfc's default is wrong.  Setting
+width and height ensures that the SVG's scale and positioning is consistent with
+the text across scaling.
+
+
+> 2) [rfced] Please insert any keywords (beyond those that appear in
+> the title) for use on https://www.rfc-editor.org/search. 
+
+The authors' XML file now has 
+
+
+> 3) [rfced] Section 2. Should the terminology be placed in alphabetical
+> order, or do you prefer the current ordering? Please review and
+> let us know your preference.
+>      
+
+We would prefer to keep the terminology in the existing order.  Since there are
+some semantic dependencies, having them in this order is clearer.
+
+
+> 4) [rfced] Please review the "type" attribute of each sourcecode
+> element in the XML file to ensure correctness.
+> 
+> Note that "tls-presentation" (not "tls") is already considered an
+> acceptable "type" per the current list of preferred values
+> (https://www.rfc-editor.org/materials/sourcecode-types.txt). Would it make
+> sense to update instances of type "tls" to "tls-presentation".  Are you
+> recommending that "tls" be added as a new type?  Note that it is also
+> acceptable to leave the "type" attribute not set.
+> 
+> In addition, review each artwork element. Specifically,
+> should any artwork element be tagged as sourcecode or another
+> element?
+>  
+
+Thanks, we were unaware of the `tls-presentation` type.  All of the `tls`
+instances have been changed to `tls-presentation`.
+
+
+> 5) [rfced] In the html and pdf outputs, the text enclosed in `<tt>` is
+> output in fixed-width font. In the txt output, there are no changes to the
+> font, and the quotation marks have been removed.
+> 
+> In the html and pdf outputs, the text enclosed in `<em>` is output in
+> italics. In the txt output, the text enclosed in `<em>` appears with an
+> underscore before and after.
+> 
+> Please review carefully and let us know if the output is acceptable or if
+> any updates are needed.
+> 
+
+Yes, this output matches our expectations.
+
+
+> 6) [rfced] We note that `<sup>` is used for superscript, but not for
+> all instances. Please review and let us know if you would like to
+> use `<sup>` for the instances that do not contain the `<sup>` element.
+> 
+
+We should use `<sup>` throughout, removing `<tt>` if necessary.  I have
+attempted to update all of the required occurrences.
+
+
+> 7) [rfced] Would it be correct to say that a member "sends" a Welcome
+> message to a new client instead of "broadcasts" it since the
+> Welcome is only being distributed to one client?
+> 
+> Original:
+>    Any member of the group can download a KeyPackage for a new client
+>    and broadcast Add and Commit messages that the current group will
+>    use to update their state, and a Welcome message that the new client
+>    can use to initialize its state and join the group.
+> 
+> Perhaps:
+>    Any member of the group can download a KeyPackage for a new client
+>    and broadcast Add and Commit messages that the current group will
+>    use to update their state, and send a Welcome message that
+>    the new client can use to initialize its state and join the group.
+> 
+
+I have edited this to be serial: "download ... broadcast ... send".
+
+
+> 8) [rfced] Please review whether any of the notes in this document
+> should be in the `<aside>` element. It is defined as "a container for
+> content that is semantically less important or tangential to the
+> content that surrounds it" (https://authors.ietf.org/en/rfcxml-vocabulary#aside).
+> 
+
+I don't think this is necessary.
+
+
+> 9) [rfced] May we shorten the title for Figure 4 by moving the second
+> sentence to a note below the figure? The note may be indented or
+> in an `<aside>` element. Please let us know your preference.
+> 
+> Original:
+>    Figure 4: Client B proposes to update its key, and client A commits the
+>    proposal. As a result, the keys for both B and A updated, so
+>    the group has post-compromise security with respect to both
+>    of them.
+> 
+> Perhaps:
+>    Figure 4: Client B proposes to update its key, and client A commits the
+>    proposal
+> 
+>    Note: As a result of client A commiting client B's proposal,
+>    the keys for both A and B updated, so the group has
+>    post-compromise security with respect to both of them.
+> 
+
+I added an appropriate note to the main text, at the point where this figure is
+referenced.
+
+
+> 10) [rfced] Is the intended meaning that the right subtree is the same
+> as the left subtree? If so, may we update the text in parentheses
+> for clarity as follows?
+> 
+> Original:
+>    For a given parent node, its left subtree is the subtree with
+>    its left child as head (respectively right subtree).
+> 
+> Perhaps:
+>    For a given parent node, its left subtree is the subtree with its
+>    left child as the head (and respectively, its right subtree has its
+>    right child as the head).
+> 
+
+No, the intent is that you swap left for right, so you have a right subtree with
+the right child as its head.  I expanded the sentence to be less terse.
+
+
+> 11) [rfced] Tables 2, 3, 5, 7, and 15 do not have titles. If you would
+> like to add titles, please provide the desired text.
+> 
+
+These tables do not need labels; they're just example data. In fact, I would be
+happy to suppress the "Table N" notation, but that doesn't seem to be possible
+within the bounds of RFC 7991.
+
+
+> 12) [rfced] We capitalized the following instance of "r" and "s" to
+> match use in RFC 8032; please let us know of any objections.
+> 
+> Original:
+>    In particular, ECDSA signatures are DER-encoded and EdDSA
+>    signatures are defined as the concatenation of r and s as
+>    specified in [RFC8032].
+> 
+> Current:
+>    In particular, ECDSA signatures are DER encoded, and EdDSA
+>    signatures are defined as the concatenation of R and S,
+>    as specified in [RFC8032].
+> 
+
+This is correct, thanks.
+
+
+> 13) [rfced] We do not see "SealBase" or "OpenBase" in RFC 9180, but we
+> do see "Seal()" and "Open()". We assume that these are one in the
+> same; however, if any further updates are needed for consistency,
+> please let us know.
+> 
+> Current:
+>    Here, the functions SealBase and OpenBase are defined [RFC9180],
+>    using the HPKE algorithms specified by the group's ciphersuite.
+> 
+
+SealBase/OpenBase are defined in the "Single-Shot APIs" section of RFC 9180,
+with `<MODE> = Base`.  I clarified.
+
+https://datatracker.ietf.org/doc/html/rfc9180#name-single-shot-apis
+
+
+> 14) [rfced] The following lines are over the 72-character limit (and
+> over the 69-character limit for ASCII artwork). Please let us
+> know how you would like to shorten/wrap the lines.
+> 
+> Section 5.2:
+>   MakeKeyPackageRef(value) = RefHash("MLS 1.0 KeyPackage Reference", value) (4 over)
+>   MakeProposalRef(value)   = RefHash("MLS 1.0 Proposal Reference", value)   (2 over)
+> 
+> Figure 12:
+>   Proposal        Commit     Application Data   (1 over)
+>   V           |                  +- Asymmetric  (9 over)
+>   FramedContentAuthData |     |   Sign / Verify (12 over)
+>   +- Symmetric        (8 over)
+>   Protect / Unprotect (18 over)
+> 
+> Section 7.7
+>   +- - new member (2 over)
+> 
+> Section 12.4.3.1:
+>   encrypted_group_info, group_secrets) (10 over)
+> 
+> Section 12.4.3.1:
+>   encrypted_group_info, kem_output, ciphertext) (9 over)
+> 
+> Appendix D:
+>   raise Exception("Cannot truncate a tree with 0 or 1 nodes") (2 over)
+> 
+
+I have reformatted these to fit.  The one in Appendix D was actually a bug in
+the example code!
+
+
+> 15) [rfced] We notice that the text "See IANA registry for registered
+> values" is included within four of the sourcecode elements (see
+> Sections 5.3, 6, 7.2, and 12.1).  Would it be better to refer to the registry name and add a pointer to the relevant section in the document? 
+> 
+> One example:
+> 
+> Original:
+>    Using the terminology from [RFC6125], a Credential provides
+>    "presented identifiers", and it is up to the application to
+>    supply a "reference identifier" for the authenticated client,
+>    if any.
+> 
+>    // See IANA registry for registered values
+>    uint16 CredentialType;
+> 
+>    struct {
+>        opaque cert_data<V>;
+>    } Certificate;
+> 
+>    struct {
+>        CredentialType credential_type;
+>        select (Credential.credential_type) {
+>            case basic:
+>                opaque identity<V>;
+> 
+>            case x509:
+>                Certificate certificates<V>;
+>        };
+>    } Credential;
+> 
+> Perhaps:
+>    Using the terminology from [RFC6125], a Credential provides
+>    "presented identifiers", and it is up to the application to
+>    supply a "reference identifier" for the authenticated client,
+>    if any.
+> 
+>    Note: See the "MLS Credential Types" registry (Section 17.5).
+> 
+>    uint16 CredentialType;
+> 
+>    struct {
+>        opaque cert_data<V>;
+>    } Certificate;
+> 
+>    struct {
+>        CredentialType credential_type;
+>        select (Credential.credential_type) {
+>            case basic:
+>                opaque identity<V>;
+> 
+>            case x509:
+>                Certificate certificates<V>;
+>        };
+>    } Credential;
+> 
+
+I updated the `// See IANA` comments to explicitly reference the relevant
+registries by name.  Referencing by name is better than by section, since we
+want folks to look at the actual registry, not the initial values defined in
+this document.
+
+
+> 16) [rfced] FYI: We added "and" before "represented". If that is not
+> correct, please let us know.
+> 
+> Original:
+>    Whether these fields can be computed by the client represented by the
+>    LeafNode depends on when the LeafNode was created.
+> 
+> Current:
+>    Whether these fields can be computed by the client and represented by the
+>    LeafNode depends on when the LeafNode was created.
+> 
+
+I rewrote this sentence to clarify.  FWIW, the original is correct, in that the
+"client represented by the LeafNode" is the one doing the computing.
+
+
+> 17) [rfced] The first part of the following sentence does not parse;
+> is there text missing after "ratchet"? Please let us know how we
+> may update this for clarity.
+> 
+> Original:
+>    After generating fresh key material and applying it to ratchet
+>    forward their local tree state as described in the Section 7.4, the
+>    generator broadcasts this update to other members of the group in a
+>    Commit message, who apply it to keep their local views of the tree in
+>    sync with the sender's.
+> 
+
+"Ratchet" is being used as a verb here, with "ratchet forward" meaning "move
+forward irreversibly".  With that in mind, I think it does parse, but I changed
+"ratchet forward" to "update" to simplify.
+
+
+> 18) [rfced] Section 7.9. Should this example have a figure number?
+> 
+> Original:
+> 
+>          Y
+>        __|__
+>       /     \
+>      _       _
+>     / \     / \
+>    E   _   G   _
+> 
+
+No, this is just an intermediate value in a computation, not noteworthy enough
+to merit a figure number / label.
+
+
+> 19) [rfced] Is this text intended to be a serial list? Also, should
+> "credential" be singular or plural (i.e., "a credential" or "credentials")?
+> 
+> Original:
+>     -  Ratchet tree: A tree with a single node, a leaf containing an
+>        HPKE public key and credential for the creator
+> 
+> Perhaps:
+>     -  Ratchet tree: A tree with a single node, a leaf containing an
+>        HPKE public key, and a credential for the creator
+> 
+
+No, the single node is a leaf node with the specified contents.  I changed "a
+leaf" to "a leaf node".
+
+
+> 20) [rfced] To avoid redundancy, would you like to streamline this
+>  list by updating the introductory sentence, removing "It
+>  contains" from each bullet point, and rephrasing the last point
+>  as follows?
+> 
+> Original:
+>   For a regular, i.e. not external, commit the list is invalid if any of the following occurs:
+> 
+>   * It contains an individual proposal that is invalid as specified in Section 12.1.
+>   * It contains an Update proposal generated by the committer.
+>   * It contains a Remove proposal that removes the committer.
+>   * It contains multiple Update and/or Remove proposals that apply to the same leaf.
+>     If the committer has received multiple such proposals they SHOULD prefer any Remove
+>     received, or the most recent Update if there are no Removes.
+>   * It contains multiple Add proposals that contain KeyPackages that represent the same
+>     client according to the application (for example, identical signature keys).
+>   * It contains an Add proposal with a KeyPackage that represents a client already in
+>     the group according to the application, unless there is a Remove proposal in the
+>     list removing the matching client from the group.
+>   * It contains multiple PreSharedKey proposals that reference the same PreSharedKeyID.
+>   * It contains multiple GroupContextExtensions proposals.
+>   * It contains a ReInit proposal together with any other proposal. If the committer has
+>     received other proposals during the epoch, they SHOULD prefer them over the ReInit
+>     proposal, allowing the ReInit to be resent and applied in a subsequent epoch.
+>   * It contains an ExternalInit proposal.
+>   * It contains a proposal with a non-default proposal type that is not supported by
+>     some members of the group that will process the Commit (i.e., members being added or
+>     removed by the Commit do not need to support the proposal type).
+>   * After processing the commit the ratchet tree is invalid, in particular, if it contains
+>     any leaf node that is invalid according to Section 7.3.
+> 
+> Perhaps:
+>   For a regular, i.e., not external, commit, the list is invalid if it contains any of the
+>   following:
+> 
+>   * An individual proposal that is invalid as specified in Section 12.1
+>   * An Update proposal generated by the committer
+>   * A Remove proposal that removes the committer
+>   * Multiple Update and/or Remove proposals that apply to the same leaf.
+>     If the committer has received multiple such proposals they SHOULD prefer any Remove
+>     received, or the most recent Update if there are no Removes.
+>   * Multiple Add proposals that contain KeyPackages that represent the same
+>     client according to the application (for example, identical signature keys)
+>   * An Add proposal with a KeyPackage that represents a client already in
+>     the group according to the application, unless there is a Remove proposal in the
+>     list removing the matching client from the group
+>   * Multiple PreSharedKey proposals that reference the same PreSharedKeyID
+>   * Multiple GroupContextExtensions proposals
+>   * A ReInit proposal together with any other proposal. If the committer has
+>     received other proposals during the epoch, they SHOULD prefer them over the ReInit
+>     proposal, allowing the ReInit to be resent and applied in a subsequent epoch.
+>   * An ExternalInit proposal
+>   * A proposal with a non-default proposal type that is not supported by
+>     some members of the group that will process the Commit (i.e., members being added or
+>     removed by the Commit do not need to support the proposal type)
+>   * An invalid ratchet tree after processing the commit, in particular, if it contains any
+>     node that is invalid according to Section 7.3
+> 
+ 
+Even though it's more verbose, I think this is clearer with the repetition.  In
+particular, the "multiple" lines read better with the explicit "It contains",
+and you avoid the problem of the parallelism of the last bullet.  So I would let
+this stand.
+
+ 
+> 21) [rfced] Is the intended meaning that the proposals field is
+> populated from "Proposals received during the current epoch" and
+> "an empty path field" (option A) or that an initial Commit object
+> and an empty path field should be constructed (option B)? Please
+> clarify.
+> 
+> Original:
+>    *  Construct an initial Commit object with the proposals field
+>       populated from Proposals received during the current epoch,
+>       and an empty path field.
+> 
+> Perhaps:
+> A) *  Construct an initial Commit object with the proposals field
+>       populated from Proposals received during the current epoch
+>       and from an empty path field.
+> 
+> or
+> 
+> B) *  Construct an initial Commit object with the proposals field
+>       populated from Proposals received during the current epoch,
+>       and construct an empty path field.
+> 
+
+The intent is that the initial Commit has (a) proposals and (b) an empty path.
+I changed "and an empty path field" to "and with the path field empty" to be
+parallel to "with the proposals field"
+
+
+> 22) [rfced] In order for the following list to be parallel, we would
+> like to update the first 3 points with verbs as shown below.
+> Please let us know if this is agreeable or if you prefer otherwise.
+> 
+> Original:
+>    *  Construct a GroupInfo reflecting the new state:
+> 
+>       -  Group ID, epoch, tree, confirmed transcript hash, interim
+>          transcript hash, and group context extensions from the new
+>          state
+> 
+>       -  The confirmation_tag from the FramedContentAuthData object
+> 
+>       -  Other extensions as defined by the application
+> 
+>       -  Optionally derive an external keypair as described in Section 8
+>          (required for External Commits, see Section 12.4.3.2)
+> 
+>       -  Sign the GroupInfo using the member's private signing key
+> 
+>       -  Encrypt the GroupInfo using the key and nonce derived from the
+>          joiner_secret for the new epoch (see Section 12.4.3.1)
+> Perhaps:
+>    *  Construct a GroupInfo reflecting the new state as follows:
+> 
+>       -  Derive a Group ID, an epoch, a tree, a confirmed transcript hash,
+>          an interim transcript hash, and group context extensions from the
+>          new state.
+> 
+>       -  Create the confirmation_tag from the FramedContentAuthData object.
+> 
+>       -  Use other extensions as defined by the application.
+> 
+>       -  Optionally derive an external key pair, as described in Section 8
+>          (required for External Commits; see Section 12.4.3.2).
+> 
+>       -  Sign the GroupInfo using the member's private signing key.
+> 
+>       -  Encrypt the GroupInfo using the key and nonce derived from the
+>          joiner_secret for the new epoch (see Section 12.4.3.1).
+> 
+
+I added verbs to the first three bullets.
+
+
+> 23) [rfced] Should the first 2 items in this list perhaps be a part of
+> the introductory sentence since they seem to be informational
+> rather than actions?
+> 
+> Original:
+>    *  Construct a new group state using the information in the GroupInfo
+>       object.
+> 
+>       -  The GroupContext is the group_context field from the GroupInfo
+>          object.
+> 
+>       -  The new member's position in the tree is at the leaf my_leaf,
+>          as defined above.
+> 
+>       -  Update the leaf my_leaf with the private key corresponding to
+>          the public key in the node.
+> 
+>       -  If the path_secret value is set in the GroupSecrets object:
+>          Identify the lowest common ancestor of the leaf node my_leaf
+>          [...]
+> 
+> Perhaps:
+>    *  Construct a new group state using the information in the GroupInfo
+>       object. Note that the GroupContext is the group_context field from
+>       the GroupInfo object, and the new member's position in the tree is
+>       at the leaf my_leaf, as defined above.
+> 
+>       -  Update the leaf my_leaf with the private key corresponding to
+>          the public key in the node.
+> 
+>       -  If the path_secret value is set in the GroupSecrets object:
+>          Identify the lowest common ancestor of the leaf node my_leaf
+>          [...]
+> 
+
+I rewrote the first three bullets to be parallel.
+
+
+> 24) [rfced] Please clarify how "or in the path field of a Commit"
+> relates to this sentence. Is the uniqueness of keys in leaf
+> nodes or in the path field of a Commit?
+> 
+> Original:
+>    Uniqueness of keys in leaf nodes is assured by explicit checks on
+>    leaf nodes being added to the tree by Add or Update proposals, or in
+>    the path field of a Commit.
+> 
+> Perhaps:
+>    Uniqueness of keys in leaf nodes, or in the path field of a Commit,
+>    is assured by explicit checks on leaf nodes being added to the tree
+>    by Add or Update proposals.
+> 
+
+The three ways a leaf can be added to the tree are: in an Add proposal, in an
+Update proposal, or in the `path` field of a Commit.  I updated to use that
+phrasing.
+
+
+> 25) [rfced] Note that we have updated the "MLS Extension Types" registry and added the IETF as the change controller for the media type registration per this note from IANA.  Please let us know if any updates are needed.
+> 
+> NOTE: We've listed the IETF as the change controller for the media type registration, and per the authors, have changed "KP, GI" to "KP, GI, LN" in the MLS Extension Types. We understand that these changes will be included in the AUTH48 edits.
+> 
+
+Thanks, these changes are correct.
+
+
+> 26) [rfced] Section 17.1. Regarding the author note below, please note
+> that at the time of writing, "draft-ietf-tls-rfc8447bis" has not
+> entered EDIT state yet. Currently, the sections are similar but
+> not identical. Please review and let us know if any further
+> changes are desired. Note that we added one instance of "it" as
+> outlined below.
+> 
+> Author note:
+>    "This section should be the same as the corresponding
+>    text in draft-ietf-tls-rfc8447bis.  Please align the two documents if
+>    they have diverged in the approval process".
+> 
+> Original:
+>    The IETF could recommend mechanisms that have limited applicability,
+>    but will provide applicability statements that describe any
+>    limitations of the mechanism or necessary constraints on its use.
+> 
+> Current:
+>    The IETF could recommend mechanisms that have limited applicability,
+>    but it will provide applicability statements that describe any
+>    limitations of the mechanism or necessary constraints on its use.
+>      
+
+I have added the "it" here.  I'm OK with any divergence due to copy-editing
+changes that might get introduced in EDIT state.
+
+
+> 27) [rfced] Should the citations for RFCs 8446 and 9180 follow "The
+>      mapping of ciphersuites" or "TLS signature schemes" instead of
+>      "is as follows"? We see that "ciphersuites" are mentioned in both
+>      of these references. "HPKE", "HMAC", and "TLS" are mentioned in
+>      RFC 9180 and "HMAC" and "TLS" are mentioned in RFC 8446.
+> 
+> Original:
+>    The mapping of ciphersuites to HPKE primitives, HMAC hash functions,
+>    and TLS signature schemes is as follows [RFC9180] [RFC8446]:
+> 
+> Perhaps:
+>    The mapping of ciphersuites [RFC8446][RFC9180] to HPKE primitives,
+>    HMAC hash functions, and TLS signature schemes is as follows:
+> 
+
+The "ciphersuites" at the beginning of the sentence are MLS ciphersuites, so the
+placement in your "perhaps" is not appropriate.  I have moved the citations so
+that they immediately follow the relevant parameters, but would also be OK with
+them at the end.
+
+
+> 28) [rfced] FYI: We have removed instances of "MIME" from Section
+> 17.10 per guidance from IANA. Note that it states the following
+> under the "Media Types" registry
+> (https://www.iana.org/assignments/media-types/media-types.xhtml):
+> 
+>    [RFC2046] specifies that Media Types (formerly known as MIME types) and Media
+>    Subtypes will be assigned and listed by the IANA.
+> 
+
+Acknowledged.
+
+
+> 29) [rfced] Should the text about "Provisional registration" be included in this document?  It does not appear in the IANA registration - see https://www.iana.org/assignments/media-types/message/mls. 
+> 
+>    Provisional registration? (standards tree only):  No
+> 
+
+I have removed this entry.
+
+ 
+> 30) [rfced] Appendices A and B: Would it be correct to add "and" to
+> the list of letters in these tree examples for consistency as shown below?
+> 
+> Appendix A:
+> a)
+> Original:
+>    To construct the tree in Figure 11:
+>    * A creates a group with B, ..., G
+>    * F sends an empty Commit, setting X, Y, W
+>    * G removes C and D, blanking V, U, and setting Y, W
+> 
+> Perhaps:
+>    To construct the tree in Figure 11:
+>    * A creates a group with B, ..., G
+>    * F sends an empty Commit, setting X, Y, and W
+>    * G removes C and D, blanking V and U, and setting Y and W
+> 
+> b)
+> Original:
+>    To construct the tree in Figure 13:
+>    *  A creates a group with B, C, D
+> 
+> Perhaps:
+>    To construct the tree in Figure 13:
+>    *  A creates a group with B, C, and D
+> 
+> Appendix B:
+> c)
+> Original:
+>    2.  B adds C, D: set B', X', Y
+> 
+>    3.  C sends empty Commit: set C', Z', Y'
+> 
+> Perhaps:
+>    2.  B adds C and D: set B', X', and Y
+> 
+>    3.  C sends empty Commit: set C', Z', and Y'
+> 
+
+I have added these "and"s.
+
+
+> 31) [rfced] Terminology
+> 
+> a) Throughout the text, the following terminology appears to be used
+> inconsistently. Please review these occurrences and let us know if/how they
+> may be made consistent. 
+> 
+>  - Client vs. client
+>      (Note: all instances are lowercase except "...to a Client" and
+>       "...receiving Client"; should these be made lowercase?)
+> 
+> 
+>  - Commit vs. commit
+>      (some examples: in a Commit, for each Commit, processing a Commit, sends a Commit,
+>       malformed Commit, malformed commit, creating a commit, an external commit,
+>       in the commit, the first commit)
+> 
+>  - Credential vs. credential
+>      (some examples: encoded in Credentials, the Credentials, a Credential, a credential,
+>       this credential, credential type)
+> 
+>  - External Commit vs. external Commit vs. external commit
+> 
+>  - Fetch vs. fetch (1 instance each)
+> 
+>  - Parent Hash vs. parent hash
+> 
+>  - Proposal vs. proposal
+>      (Please clarify if all capitalized instances are referring to a
+>      "Proposal message" or "Proposal object" or if updates are needed.
+>      Note that there are instances of "Add Proposal" vs. "Add proposal"
+>      and "a proposal or a commit" vs. "the Proposal and Commit".)
+> 
+>  - Secret Tree vs. secret tree
+> 
+>  - Update vs. update
+>      (Please clarify if all capitalized instances are referring to an
+>       "Update message" or "Update proposal" or if updates are needed.)
+> 
+
+I have normalized on:
+
+* Lower-case "client" everywhere
+* Upper-case "Commit" whenever it refers to a Commit object
+* Upper-case "Credential" only when referring to a Credential object
+* "external Commit"
+* Lower-case "fetch"
+* Lower-case "parent hash"
+* Upper-case "Proposal" only when referring to a Proposal object, not an
+  abstract proposal or a specific type of proposal (e.g., an "Add proposal")
+* Lower-case "secret tree"
+* Upper-case "Update" when it refers to an Update proposal, lower-case otherwise
+
+Basically, when use upper case only when we refer to a struct with a TLS-syntax definition.
+
+Also, I noticed that some struct names were in `<tt>` (e.g., `PreSharedKeyID`).  That
+should not be done, so I removed the `<tt>` around them.
+
+
+> b) The Web Portion of the RFC Style Guide
+> (https://www.rfc-editor.org/styleguide/part2/) recommends that once an
+> abbreviation has been introduced, the abbreviated form should be used
+> thereafter. After the first expansion of the following terms, would you
+> like to use the abbreviated forms thereafter?
+> 
+>  - Authentication Service
+>  - Delivery Service
+>  - forward secrecy
+>  - post-compromise security
+>  - pre-shared key
+
+These acronyms are quite short, two letters in most cases.  So I think it's
+helpful to use the full definition in some places.  I would leave the references
+as-is.
+
+
+> c) Some numbers are spelled out and some are represented as digits for
+> bits and bytes, for instance, "4 bytes" vs. "four bytes". Would you
+> like to make this consistent by using digits when referring to bits
+> and bytes? Please let us know your preference.
+
+I would keep the current balance.  Words are used except for two
+instances:
+
+* "from 0 bytes to 2<sup>30</sup> bytes"
+* "integers are encoded in 1, 2, or 4 bytes and can encode 6-, 14-, or 30-bit values"
+
+The first of these makes sense for the "0" to be parallel to the exponentiated
+value.  For the second, the digits make the correspondence between byte length
+and bit length clearer.
+
+
+> d) Note that we updated the following terms to the latter forms as
+> indicated below; please let us know of any objections.
+> 
+>  - cipher suite -> ciphersuite (for consistency)
+>  - hybrid public-key encryption (HPKE) -> hybrid public key encryption (HPKE) (per RFC 9180)
+>  - Input Key Material (IKM) -> Input Keying Material (IKM) (per use in other RFCs)
+>  - keypair -> key pair (for consistency)
+>  - public-key encryption -> public key encrption (per IANA registry and use in other RFCs)
+>  - Signature algorithm -> signature algorithm (for consistency and per 8032)
+ 
+I concur with these changes.
+
+ 
+> 32) [rfced] Please review the "Inclusive Language" portion of the online
+> Style Guide <https://www.rfc-editor.org/styleguide/part2/#inclusive_language> and let
+> us know if any changes are needed.
+> 
+> In addition, please consider whether "tradition" should be updated for clarity. 
+> While the NIST website
+> <https://www.nist.gov/nist-research-library/nist-technical-series-publications-author-instructions#table1>
+> indicates that this term is potentially biased, it is also ambiguous. 
+> "Tradition" is a subjective term, as it is not the same for everyone.
+>  
+
+This was discussed during IESG review:
+
+https://mailarchive.ietf.org/arch/msg/mls/jSMQHXxcY3bX8S-xefyjbX4KFzE/
+
+## Questions and Answers (Round 2)
+
+> 1) We read the comments in the GitHub PR that were included above the
+> Questions/Answers. Overall, we are okay with the changes made. Note that we
+> did not look further into whether or not `<t>` is used inside `<dd>`.  Please let
+> us know if there are specific instances that need review.
+> 
+> > * The document is inconsistent as to whether a `<t>` is used inside `<dd>`
+
+I reviewed, and did not find any instances of `<t>` inside of `<dd>`.  I may have
+been mistaken in this comment.
+
+
+> 2) Thank you for providing the keywords.  Note that they are formatted as follows in XML file:
+>  
+>    <keyword>security</keyword>
+>    <keyword>authenticated key exchange</keyword>
+>    <keyword>end-to-end encryption</keyword>
+
+Thanks, that looks correct to me.  I have adjusted the Markdown so that the
+generated XML matches this format.
+
+
+> 3) In Section 3.2, we removed the period from the title of Figure 4 (sentence form is okay but with no period).
+> 
+> 4) In Section 12.1.8, we reverted the hyphen in “newly-hired” as we do not hyphenate adverbs that end in “ly”.
+> 
+> 5) In Sections 12.1.8 and 12.4, we added quote marks to the IANA registry name - 2 instances.
+> 
+> Old:
+>    MLS Proposal Types registry
+> 
+> Current:
+>    "MLS Proposal Types” registry
+
+These changes are good with me.
+
+
+> 6) In Section 12.4.2, the text does not parse with the removal of “is”. Please let us know how you would like to update (perhaps remove “that”).
+> 
+> Current:
+>   *  Verify that the signature on the FramedContent message as
+>       described in Section 6.1.
+
+Sorry, I should have removed the "that", so that it reads "Verify the signature..."
+
+
+> 7) In Section 15.1, we reverted the change to the citation spacing. Our practice is to separate citations with one space.
+> 
+> Old:
+>   Application messages MAY be padded to provide some resistance against
+>   traffic analysis techniques over encrypted traffic [CLINIC][HCJ16].
+> 
+> New:
+>   Application messages MAY be padded to provide some resistance against
+>   traffic analysis techniques over encrypted traffic
+>   [CLINIC] [HCJ16].
+
+This is fine.  I think the spacing is an artifact of the XML generation.
+
+
+> 8) As requested, we checked that all instances of “ciphersuite” were updated to “cipher suite” - no additional changes.
+
+Thanks!
+
+
+> 9) FYI - The AD will need to approve some changes that are beyond editorial, and IANA will need to make some changes to their registries. We will request these actions once all author changes are complete.
+
+Duly noted.
diff --git a/auth48/README.md b/auth48/README.md
new file mode 100644
index 00000000..d1f639d3
--- /dev/null
+++ b/auth48/README.md
@@ -0,0 +1,64 @@
+Approving AUTH48 with Markdown Alignment
+====================================================
+
+The Makefile included in this directory provides some quick commands to
+facilitate AUTH48 negotiations.  You should update the `RFC` variable to reflect
+your assigned RFC number and the `DRAFT` variable to reflect your draft name.
+
+The overall process flow here is:
+
+1. RFC Editor provides XML file in email.  This becomes the basis of the
+   authors' XML file, which will include the authors' proposed changes to the
+   RFC Editor's XML.
+2. Authors generate an XML file from the WG's Markdown file.
+3. Authors review the diff between generated XML file and the authors' XML file.
+   * Changes that the authors approve of should be reflected in the Markdown
+     (and thus the generated XML)
+   * Changes that the authors reject should be reverted in the authors' XML
+     file.
+4. Steps 2 and 3 are repeated until there are effectively no differences between
+   the generated and authors XML files (some XML technicalities may remain). At
+   this point, the authors' XML file reflects only the approved changes, and the
+   approved changes are reflected in the Markedown.
+5. Authors reply to the RFC editor with the authors' XML file.  If there are
+   further rounds of negotiation, the process repeats.
+
+## Initializing the repo
+
+```
+make init
+```
+
+This command downloads the RFC Editor's proposed XML file and canonicalizes it
+to facilitate comparison.  The canonicalized XML is the basis for the authors'
+XML file, which we will use to communicate changes back to the RFC Editor.
+
+This should only be done once.  If you re-run `make init` again after making
+changes to the authors' XML file, it will reset the authors' XML file to a
+pristine state.
+
+## Generating the XML
+
+```
+make gen
+```
+
+This command generates the XML from the WG's Markdown file, applies some
+normalization to deal with known incompatibilities between `kramdown-rfc` and
+the RFC Editor's XML, and canonicalizes it.  While `make diff` will
+auto-generate the generated XML if it is not present, this command can be useful
+for regenerating once the diff is open, after making changes to the Markdown.
+
+
+## Reviewing the diff
+
+```
+make diff
+```
+
+This command generates the XML from the WG's Markdown file and opens a MacVim
+window with a diff between the generated XML and the authors' XML.  (For
+non-macOS platforms, you can change `mvim -d` to `vimdiff`.)  In this view, you
+can make edits to the authors' XML file to reflect gaps from the generated XML.
+To make edits to the generated XML file, put the edits in the Markdown and run
+`make gen`.
diff --git a/auth48/rfc9420.authors.xml b/auth48/rfc9420.authors.xml
new file mode 100644
index 00000000..f6cd0707
--- /dev/null
+++ b/auth48/rfc9420.authors.xml
@@ -0,0 +1,8167 @@
+<?xml version="1.0"?>
+<rfc category="std" consensus="true" docName="draft-ietf-mls-protocol-20" ipr="trust200902" number="9420" obsoletes="" sortRefs="true" submissionType="IETF" symRefs="true" tocInclude="true" updates="" version="3" xml:lang="en">
+  <front>
+    <title abbrev="MLS">The Messaging Layer Security (MLS) Protocol</title>
+    <seriesInfo name="RFC" value="9420"/>
+    <author fullname="Richard Barnes" initials="R." surname="Barnes">
+      <organization>Cisco</organization>
+      <address>
+        <email>rlb@ipv.sx</email>
+      </address>
+    </author>
+    <author fullname="Benjamin Beurdouche" initials="B." surname="Beurdouche">
+      <organization>Inria &amp; Mozilla</organization>
+      <address>
+        <email>ietf@beurdouche.com</email>
+      </address>
+    </author>
+    <author fullname="Raphael Robert" initials="R." surname="Robert">
+      <organization>Phoenix R&amp;D</organization>
+      <address>
+        <email>ietf@raphaelrobert.com</email>
+      </address>
+    </author>
+    <author fullname="Jon Millican" initials="J." surname="Millican">
+      <organization>Meta Platforms</organization>
+      <address>
+        <email>jmillican@meta.com</email>
+      </address>
+    </author>
+    <author fullname="Emad Omara" initials="E." surname="Omara">
+      <organization/>
+      <address>
+        <email>emad.omara@gmail.com</email>
+      </address>
+    </author>
+    <author fullname="Katriel Cohn-Gordon" initials="K." surname="Cohn-Gordon">
+      <organization>University of Oxford</organization>
+      <address>
+        <email>me@katriel.co.uk</email>
+      </address>
+    </author>
+    <date month="May" year="2023"/>
+    <area>sec</area>
+    <workgroup>mls</workgroup>
+    <keyword>security</keyword>
+    <keyword>authenticated key exchange</keyword>
+    <keyword>end-to-end encryption</keyword>
+    <abstract>
+      <t>Messaging applications are increasingly making use of end-to-end
+security mechanisms to ensure that messages are only accessible to
+the communicating endpoints, and not to any servers involved in delivering
+messages.  Establishing keys to provide such protections is
+challenging for group chat settings, in which more than two
+clients need to agree on a key but may not be online at the same
+time.  In this document, we specify a key establishment
+protocol that provides efficient asynchronous group key establishment
+with forward secrecy (FS) and post-compromise security (PCS) for groups
+in size ranging from two to thousands.</t>
+    </abstract>
+  </front>
+  <middle>
+    <section anchor="introduction">
+      <name>Introduction</name>
+      <t>A group of users who want to send each other encrypted messages needs
+a way to derive shared symmetric encryption keys. For two parties,
+this problem has been studied thoroughly, with the Double Ratchet
+emerging as a common solution <xref target="DoubleRatchet"/> <xref target="Signal"/>.
+Channels implementing the Double Ratchet enjoy fine-grained forward secrecy
+as well as post-compromise security, but are nonetheless efficient
+enough for heavy use over low-bandwidth networks.</t>
+      <t>For a group of size greater than two, a common strategy is to
+distribute symmetric "sender keys" over existing 1:1
+secure channels, and then for each member to send messages to the
+group encrypted with their own sender key. On the one hand, using sender keys
+improves efficiency relative to pairwise transmission of individual messages, and
+it provides forward secrecy (with the addition of a hash ratchet).
+On the other hand, it is difficult to achieve post-compromise security with
+sender keys, requiring a number of key update messages that scales as the square
+of the group size.
+An adversary who learns a sender key can often indefinitely and
+passively eavesdrop on that member's messages.  Generating and
+distributing a new sender key provides a form of post-compromise
+security with regard to that sender.  However, it requires
+computation and communications resources that scale linearly with
+the size of the group.</t>
+      <t>In this document, we describe a protocol based on tree structures
+that enables asynchronous group keying with forward secrecy and
+post-compromise security.  Based on earlier work on "asynchronous
+ratcheting trees" <xref target="ART"/>, the protocol presented here uses an
+asynchronous key-encapsulation mechanism for tree structures.
+This mechanism allows the members of the group to derive and update
+shared keys with costs that scale as the log of the group size.</t>
+    </section>
+    <section anchor="terminology">
+      <name>Terminology</name>
+      <t>The key words "<bcp14>MUST</bcp14>", "<bcp14>MUST NOT</bcp14>", "<bcp14>REQUIRED</bcp14>", "<bcp14>SHALL</bcp14>", "<bcp14>SHALL NOT</bcp14>",
+"<bcp14>SHOULD</bcp14>", "<bcp14>SHOULD NOT</bcp14>", "<bcp14>RECOMMENDED</bcp14>", "<bcp14>NOT RECOMMENDED</bcp14>", "<bcp14>MAY</bcp14>", and
+"<bcp14>OPTIONAL</bcp14>" in this document are to be interpreted as described in
+BCP 14 <xref target="RFC2119"/> <xref target="RFC8174"/> when, and only when, they appear in all
+      capitals, as shown here.</t>
+      <dl>
+        <dt>Client:</dt>
+        <dd>An agent that uses this protocol to establish shared cryptographic
+state with other clients.  A client is defined by the
+cryptographic keys it holds.</dd>
+        <dt>Group:</dt>
+        <dd>A group represents a logical collection of clients that share a common
+secret value at any given time.  Its state is represented as a linear
+sequence of epochs in which each epoch depends on its predecessor.</dd>
+        <dt>Epoch:</dt>
+        <dd>A state of a group in which a specific set of authenticated clients hold
+shared cryptographic state.</dd>
+        <dt>Member:</dt>
+        <dd>A client that is included in the shared state of a group and hence
+has access to the group's secrets.</dd>
+        <dt>Key Package:</dt>
+        <dd>A signed object describing a client's identity and capabilities, including
+a hybrid public key encryption (HPKE) <xref target="RFC9180"/> public key that
+can be used to encrypt to that client. Other clients can use a client's
+KeyPackage to introduce the client to a new group.</dd>
+        <dt>Group Context:</dt>
+        <dd>An object that summarizes the shared, public state of the group. The group
+context is typically distributed in a signed GroupInfo message, which is provided
+to new members to help them join a group.</dd>
+        <dt>Signature Key:</dt>
+        <dd>A signing key pair used to authenticate the sender of a message.</dd>
+        <dt>Proposal:</dt>
+        <dd>A message that proposes a change to the group, e.g., adding or removing a
+member.</dd>
+        <dt>Commit:</dt>
+        <dd>A message that implements the changes to the group proposed in a set of
+Proposals.</dd>
+        <dt>PublicMessage:</dt>
+        <dd>An MLS protocol message that is signed by its sender and authenticated as
+coming from a member of the group in a particular epoch, but not encrypted.</dd>
+        <dt>PrivateMessage:</dt>
+        <dd>An MLS protocol message that is signed by its sender, authenticated as
+coming from a member of the group in a particular epoch, and encrypted so
+that it is confidential to the members of the group in that epoch.</dd>
+        <dt>Handshake Message:</dt>
+        <dd>A PublicMessage or PrivateMessage carrying an MLS Proposal or Commit
+object, as opposed to application data.</dd>
+        <dt>Application Message:</dt>
+        <dd>A PrivateMessage carrying application data.</dd>
+      </dl>
+      <t>Terminology specific to tree computations is described in
+<xref target="ratchet-tree-terminology"/>.</t>
+      <t>In general, symmetric values are referred to as "keys" or "secrets"
+interchangeably.  Either term denotes a value that <bcp14>MUST</bcp14> be kept confidential to
+a client.  When labeling individual values, we typically use "secret" to refer
+to a value that is used to derive further secret values and "key" to refer to a
+value that is used with an algorithm such as Hashed Message Authentication Code
+(HMAC) or an Authenticated Encryption with Associated Data (AEAD) algorithm.</t>
+      <t>The PublicMessage and PrivateMessage formats are defined in <xref target="message-framing"/>.
+Security notions such as forward secrecy and post-compromise
+security are defined in <xref target="security-considerations"/>.</t>
+      <t>As detailed in <xref target="grease"/>, MLS uses the "Generate Random Extensions And Sustain
+Extensibility" (GREASE) approach to maintaining extensibility, where senders insert random
+values into fields in which receivers are required to ignore unknown values.
+Specific "GREASE values" for this purpose are registered in the appropriate IANA
+registries.</t>
+      <section anchor="presentation-language">
+        <name>Presentation Language</name>
+        <t>We use the TLS presentation language <xref target="RFC8446"/> to describe the structure of
+protocol messages.  In addition to the base syntax, we add two additional
+features: the ability for fields to be optional and the ability for vectors to
+have variable-size length headers.</t>
+        <section anchor="optional-value">
+          <name>Optional Value</name>
+          <t>An optional value is encoded with a presence-signaling octet, followed by the
+value itself if present.  When decoding, a presence octet with a value other
+than 0 or 1 <bcp14>MUST</bcp14> be rejected as malformed.</t>
+          <sourcecode type="tls-presentation">
+struct {
+    uint8 present;
+    select (present) {
+        case 0: struct{};
+        case 1: T value;
+    };
+} optional&lt;T&gt;;
+</sourcecode>
+        </section>
+        <section anchor="variable-size-vector-length-headers">
+          <name>Variable-Size Vector Length Headers</name>
+          <t>In the TLS presentation language, vectors are encoded as a sequence of encoded
+elements prefixed with a length.  The length field has a fixed size set by
+specifying the minimum and maximum lengths of the encoded sequence of elements.</t>
+          <t>In MLS, there are several vectors whose sizes vary over significant ranges.  So
+instead of using a fixed-size length field, we use a variable-size length using
+a variable-length integer encoding based on the one described in
+<xref section="16" sectionFormat="of" target="RFC9000"/>. They differ only in that the one here requires a minimum-size
+encoding. Instead of presenting min and max values, the vector description
+simply includes a <tt>V</tt>. For example:</t>
+          <sourcecode type="tls-presentation">
+struct {
+    uint32 fixed&lt;0..255&gt;;
+    opaque variable&lt;V&gt;;
+} StructWithVectors;
+</sourcecode>
+          <t>Such a vector can represent values with length from 0 bytes to 2<sup>30</sup> bytes.
+The variable-length integer encoding reserves the two most significant bits
+of the first byte to encode the base 2 logarithm of the integer encoding length
+in bytes.  The integer value is encoded on the remaining bits, so that the
+overall value is in network byte order.
+The encoded value <bcp14>MUST</bcp14> use the smallest number of bits required to
+represent the value.  When decoding, values using more bits than necessary <bcp14>MUST</bcp14>
+be treated as malformed.</t>
+          <t>This means that integers are encoded in 1, 2, or 4 bytes and can encode 6-,
+14-, or 30-bit values, respectively.</t>
+          <table anchor="integer-summary">
+            <name>Summary of Integer Encodings</name>
+            <thead>
+              <tr>
+                <th align="left">Prefix</th>
+                <th align="left">Length</th>
+                <th align="left">Usable Bits</th>
+                <th align="left">Min</th>
+                <th align="left">Max</th>
+              </tr>
+            </thead>
+            <tbody>
+              <tr>
+                <td align="left">00</td>
+                <td align="left">1</td>
+                <td align="left">6</td>
+                <td align="left">0</td>
+                <td align="left">63</td>
+              </tr>
+              <tr>
+                <td align="left">01</td>
+                <td align="left">2</td>
+                <td align="left">14</td>
+                <td align="left">64</td>
+                <td align="left">16383</td>
+              </tr>
+              <tr>
+                <td align="left">10</td>
+                <td align="left">4</td>
+                <td align="left">30</td>
+                <td align="left">16384</td>
+                <td align="left">1073741823</td>
+              </tr>
+              <tr>
+                <td align="left">11</td>
+                <td align="left">invalid</td>
+                <td align="left">-</td>
+                <td align="left">-</td>
+                <td align="left">-</td>
+              </tr>
+            </tbody>
+          </table>
+          <t>Vectors that start with the prefix "11" are invalid and <bcp14>MUST</bcp14> be rejected.</t>
+          <t>For example:</t>
+          <ul spacing="normal">
+            <li>The four-byte length value 0x9d7f3e7d decodes to 494878333.</li>
+            <li>The two-byte length value 0x7bbd decodes to 15293.</li>
+            <li>The single-byte length value 0x25 decodes to 37.</li>
+          </ul>
+          <t>The following figure adapts the pseudocode provided in <xref target="RFC9000"/> to add a
+check for minimum-length encoding:</t>
+          <sourcecode type="pseudocode">
+ReadVarint(data):
+  // The length of variable-length integers is encoded in the
+  // first two bits of the first byte.
+  v = data.next_byte()
+  prefix = v &gt;&gt; 6
+  if prefix == 3:
+    raise Exception('invalid variable length integer prefix')
+
+  length = 1 &lt;&lt; prefix
+
+  // Once the length is known, remove these bits and read any
+  // remaining bytes.
+  v = v &amp; 0x3f
+  repeat length-1 times:
+    v = (v &lt;&lt; 8) + data.next_byte()
+
+  // Check if the value would fit in half the provided length.
+  if prefix &gt;= 1 &amp;&amp; v &lt; (1 &lt;&lt; (8*(length/2) - 2)):
+    raise Exception('minimum encoding was not used')
+
+  return v
+</sourcecode>
+          <t>The use of variable-size integers for vector lengths allows vectors to grow
+very large, up to 2<sup>30</sup> bytes.  Implementations should take care not to allow
+vectors to overflow available storage.  To facilitate debugging of potential
+interoperability problems, implementations <bcp14>SHOULD</bcp14> provide a clear error when
+such an overflow condition occurs.</t>
+        </section>
+      </section>
+    </section>
+    <section anchor="protocol-overview">
+      <name>Protocol Overview</name>
+      <t>MLS is designed to operate in the context described in
+<xref target="I-D.ietf-mls-architecture"/>. In particular, we assume that the following
+services are provided:</t>
+      <ul spacing="normal">
+        <li>An Authentication Service (AS) that enables group members to authenticate the
+credentials presented by other group members.</li>
+        <li>A Delivery Service (DS) that routes MLS messages among the participants in the
+protocol.</li>
+      </ul>
+      <t>MLS assumes a trusted AS but a largely untrusted DS. <xref target="authentication-service-compromise"/>
+describes the impact of compromise or
+misbehavior of an AS. MLS is designed to protect the confidentiality and integrity of
+the group data even in the face of a compromised DS;
+in general, the DS is only expected to reliably deliver messages.
+<xref target="delivery-service-compromise"/> describes the impact of compromise or
+misbehavior of a DS.</t>
+      <t>The core functionality of MLS is continuous group authenticated key exchange
+(AKE).  As with other authenticated key exchange protocols (such as TLS), the
+participants in the protocol agree on a common secret value, and each
+participant can verify the identity of the other participants. That secret
+can then be used to protect messages sent from one participant in the
+group to the other participants using the MLS framing layer
+or can be exported for use with other protocols. MLS provides
+group AKE in the sense that there can be more than two participants in the
+protocol, and continuous group AKE in the sense that the set of participants in
+the protocol can change over time.</t>
+      <t>The core organizing principles of MLS are <em>groups</em> and <em>epochs</em>.  A group
+represents a logical collection of clients that share a common secret value at
+any given time.  The history of a group is divided into a linear sequence of
+epochs.  In each epoch, a set of authenticated <em>members</em> agree on an <em>epoch
+secret</em> that is known only to the members of the group in that epoch.  The set
+of members involved in the group can change from one epoch to the next, and MLS
+ensures that only the members in the current epoch have access to the epoch
+secret.  From the epoch secret, members derive further shared secrets for
+message encryption, group membership authentication, and so on.</t>
+      <t>The creator of an MLS group creates the group's first epoch unilaterally, with
+no protocol interactions.  Thereafter, the members of the group advance their
+shared cryptographic state from one epoch to another by exchanging MLS messages.</t>
+      <ul spacing="normal">
+        <li>A <em>KeyPackage</em> object describes a client's capabilities and provides keys that
+can be used to add the client to a group.</li>
+        <li>A <em>Proposal</em> message proposes a change to be made in the next epoch, such as
+adding or removing a member.</li>
+        <li>A <em>Commit</em> message initiates a new epoch by instructing members of the group
+to implement a collection of proposals.</li>
+        <li>A <em>Welcome</em> message provides a new member to the group with the information to
+initialize their state for the epoch in which they were added or in which they
+want to add themselves to the group.</li>
+      </ul>
+      <t>KeyPackage and Welcome messages are used to initiate a group or introduce new
+members, so they are exchanged between group members and clients not yet in the
+group. A client publishes a KeyPackage via the DS, thus enabling other
+clients to add it to groups. When a group member wants to add a new member to a
+group, it uses the new member's KeyPackage to add them and constructs a Welcome
+message with which the new member can initialize their local state.</t>
+      <t>Proposal and Commit messages are sent from one member of a group to the others.
+MLS provides a common framing layer for sending messages within a group:
+A <em>PublicMessage</em> provides sender authentication for unencrypted Proposal and Commit
+messages.  A <em>PrivateMessage</em> provides encryption and authentication for
+both Proposal/Commit messages as well as any application data.</t>
+      <section anchor="cryptographic-state-and-evolution">
+        <name>Cryptographic State and Evolution</name>
+        <t>The cryptographic state at the core of MLS is divided into three areas of responsibility:</t>
+        <figure>
+          <name>Overview of MLS Group Evolution</name>
+          <artset>
+            <artwork type="svg">
+              <svg xmlns="http://www.w3.org/2000/svg" class="diagram" font-family="monospace" font-size="13px" height="400" text-anchor="middle" version="1.1" viewBox="0 0 584 400" width="584">
+                <path d="M 8,128 L 8,288" fill="none" stroke="black"/>
+                <path d="M 208,48 L 208,200" fill="none" stroke="black"/>
+                <path d="M 208,216 L 208,368" fill="none" stroke="black"/>
+                <path d="M 272,64 L 272,96" fill="none" stroke="black"/>
+                <path d="M 272,120 L 272,192" fill="none" stroke="black"/>
+                <path d="M 272,216 L 272,288" fill="none" stroke="black"/>
+                <path d="M 272,312 L 272,352" fill="none" stroke="black"/>
+                <path d="M 336,48 L 336,200" fill="none" stroke="black"/>
+                <path d="M 336,216 L 336,368" fill="none" stroke="black"/>
+                <path d="M 576,128 L 576,288" fill="none" stroke="black"/>
+                <path d="M 48,208 L 72,208" fill="none" stroke="black"/>
+                <path d="M 200,208 L 216,208" fill="none" stroke="black"/>
+                <path d="M 336,208 L 352,208" fill="none" stroke="black"/>
+                <path d="M 512,208 L 528,208" fill="none" stroke="black"/>
+                <path d="M 8,128 L 48,208" fill="none" stroke="black"/>
+                <path d="M 536,208 L 576,288" fill="none" stroke="black"/>
+                <path d="M 8,288 L 48,208" fill="none" stroke="black"/>
+                <path d="M 536,208 L 576,128" fill="none" stroke="black"/>
+                <path d="M 224,32 C 215.16936,32 208,39.16936 208,48" fill="none" stroke="black"/>
+                <path d="M 320,32 C 328.83064,32 336,39.16936 336,48" fill="none" stroke="black"/>
+                <path d="M 224,384 C 215.16936,384 208,376.83064 208,368" fill="none" stroke="black"/>
+                <path d="M 320,384 C 328.83064,384 336,376.83064 336,368" fill="none" stroke="black"/>
+                <polygon class="arrowhead" fill="black" points="536,208 524,202.4 524,213.6" transform="rotate(0,528,208)"/>
+                <polygon class="arrowhead" fill="black" points="360,208 348,202.4 348,213.6" transform="rotate(0,352,208)"/>
+                <polygon class="arrowhead" fill="black" points="280,352 268,346.4 268,357.6" transform="rotate(90,272,352)"/>
+                <polygon class="arrowhead" fill="black" points="280,288 268,282.4 268,293.6" transform="rotate(90,272,288)"/>
+                <polygon class="arrowhead" fill="black" points="280,192 268,186.4 268,197.6" transform="rotate(90,272,192)"/>
+                <polygon class="arrowhead" fill="black" points="280,96 268,90.4 268,101.6" transform="rotate(90,272,96)"/>
+                <polygon class="arrowhead" fill="black" points="224,208 212,202.4 212,213.6" transform="rotate(0,216,208)"/>
+                <polygon class="arrowhead" fill="black" points="80,208 68,202.4 68,213.6" transform="rotate(0,72,208)"/>
+                <g class="text">
+                  <text x="272" y="36">...</text>
+                  <text x="360" y="84">Key</text>
+                  <text x="412" y="84">Schedule</text>
+                  <text x="276" y="116">epoch_secret</text>
+                  <text x="56" y="148">Ratchet</text>
+                  <text x="532" y="148">Secret</text>
+                  <text x="52" y="164">Tree</text>
+                  <text x="532" y="164">Tree</text>
+                  <text x="136" y="212">commit_secret</text>
+                  <text x="276" y="212">epoch_secret</text>
+                  <text x="432" y="212">encryption_secret</text>
+                  <text x="276" y="308">epoch_secret</text>
+                  <text x="272" y="388">...</text>
+                </g>
+              </svg>
+            </artwork>
+            <artwork type="ascii-art">
+                          .-    ...    -.
+                         |               |
+                         |       |       |
+                         |       |       | Key Schedule
+                         |       V       |
+                         |  epoch_secret |
+.                        |       |       |                             .
+|\ Ratchet               |       |       |                     Secret /|
+| \ Tree                 |       |       |                      Tree / |
+|  \                     |       |       |                          /  |
+|   \                    |       V       |                         /   |
+|    +--&gt; commit_secret --&gt; epoch_secret --&gt; encryption_secret --&gt;+    |
+|   /                    |       |       |                         \   |
+|  /                     |       |       |                          \  |
+| /                      |       |       |                           \ |
+|/                       |       |       |                            \|
+'                        |       V       |                             '
+                         |  epoch_secret |
+                         |       |       |
+                         |       |       |
+                         |       V       |
+                         |               |
+                          '-    ...    -'
+</artwork>
+          </artset>
+        </figure>
+        <ul spacing="normal">
+          <li>A <em>ratchet tree</em> that represents the membership of the group, providing group
+members a way to authenticate each other and efficiently encrypt messages to
+subsets of the group.  Each epoch has a distinct ratchet tree. It seeds the
+<em>key schedule</em>.</li>
+          <li>
+            <t>A <em>key schedule</em> that describes the chain of key derivations used to progress from
+epoch to epoch (mainly using the <em>init_secret</em> and <em>epoch_secret</em>), as well as the derivation of
+a variety of other secrets (see <xref target="epoch-derived-secrets"/>). For example:
+            </t>
+            <ul spacing="normal">
+              <li>An <em>encryption secret</em> that is used to initialize the secret tree for the
+epoch.</li>
+              <li>An <em>exporter secret</em> that allows other protocols to leverage MLS as a
+generic authenticated group key exchange.</li>
+              <li>A <em>resumption secret</em> that members can use to prove their membership in the
+group, e.g., when creating a subgroup or a successor group.</li>
+            </ul>
+          </li>
+          <li>A <em>secret tree</em> derived from the key schedule that represents shared secrets
+used by the members of the group for encrypting and authenticating messages.
+Each epoch has a distinct secret tree.</li>
+        </ul>
+        <t>Each member of the group maintains a partial view of these components of the group's
+state.  MLS messages are used to initialize these views and keep them in sync as
+the group transitions between epochs.</t>
+        <t>Each new epoch is initiated with a Commit message.  The Commit instructs
+existing members of the group to update their views of the ratchet tree by applying
+a set of Proposals, and uses the updated ratchet tree to distribute fresh
+entropy to the group.  This fresh entropy is provided only to members in the new
+epoch and not to members who have been removed. Commits thus maintain the property that
+the epoch secret is confidential to the members in the current epoch.</t>
+        <t>For each Commit that adds one or more members to the group, there are one or more corresponding
+Welcome messages.  Each Welcome message provides new members with the information
+they need to initialize their views of the key schedule and ratchet tree, so
+that these views align with the views held by other members of the group
+in this epoch.</t>
+      </section>
+      <section anchor="example-protocol-execution">
+        <name>Example Protocol Execution</name>
+        <t>There are three major operations in the life of a group:</t>
+        <ul spacing="normal">
+          <li>Adding a member, initiated by a current member;</li>
+          <li>Updating the keys that represent a member in the tree; and</li>
+          <li>Removing a member.</li>
+        </ul>
+        <t>Each of these operations is "proposed" by sending a message of the corresponding
+type (Add / Update / Remove).  The state of the group is not changed, however,
+until a Commit message is sent to provide the group with fresh entropy.  In this
+section, we show each proposal being committed immediately, but in more advanced
+deployment cases, an application might gather several proposals before
+committing them all at once.  In the illustrations below, we show the Proposal
+and Commit messages directly, while in reality they would be sent encapsulated in
+PublicMessage or PrivateMessage objects.</t>
+        <t>Before the initialization of a group, clients publish KeyPackages to a directory
+provided by the DS (see <xref target="prepublish-flow"/>).</t>
+        <figure anchor="prepublish-flow">
+          <name>Clients A, B, and C publish KeyPackages to the directory</name>
+          <artset>
+            <artwork type="svg">
+              <svg xmlns="http://www.w3.org/2000/svg" class="diagram" font-family="monospace" font-size="13px" height="288" text-anchor="middle" version="1.1" viewBox="0 0 568 288" width="568">
+                <path d="M 8,128 L 8,272" fill="none" stroke="black"/>
+                <path d="M 144,128 L 144,152" fill="none" stroke="black"/>
+                <path d="M 144,168 L 144,272" fill="none" stroke="black"/>
+                <path d="M 280,128 L 280,152" fill="none" stroke="black"/>
+                <path d="M 280,168 L 280,200" fill="none" stroke="black"/>
+                <path d="M 280,216 L 280,272" fill="none" stroke="black"/>
+                <path d="M 416,128 L 416,272" fill="none" stroke="black"/>
+                <path d="M 536,128 L 536,272" fill="none" stroke="black"/>
+                <path d="M 400,64 L 456,64" fill="none" stroke="black"/>
+                <path d="M 488,64 L 544,64" fill="none" stroke="black"/>
+                <path d="M 8,160 L 408,160" fill="none" stroke="black"/>
+                <path d="M 144,208 L 408,208" fill="none" stroke="black"/>
+                <path d="M 280,256 L 408,256" fill="none" stroke="black"/>
+                <path d="M 400,64 C 391.16936,64 384,71.16936 384,80" fill="none" stroke="black"/>
+                <path d="M 456,64 C 464.83064,64 472,56.83064 472,48" fill="none" stroke="black"/>
+                <path d="M 488,64 C 479.16936,64 472,56.83064 472,48" fill="none" stroke="black"/>
+                <path d="M 544,64 C 552.83064,64 560,71.16936 560,80" fill="none" stroke="black"/>
+                <polygon class="arrowhead" fill="black" points="416,256 404,250.4 404,261.6" transform="rotate(0,408,256)"/>
+                <polygon class="arrowhead" fill="black" points="416,208 404,202.4 404,213.6" transform="rotate(0,408,208)"/>
+                <polygon class="arrowhead" fill="black" points="416,160 404,154.4 404,165.6" transform="rotate(0,408,160)"/>
+                <g class="text">
+                  <text x="436" y="36">Delivery</text>
+                  <text x="504" y="36">Service</text>
+                  <text x="528" y="100">Group</text>
+                  <text x="8" y="116">A</text>
+                  <text x="144" y="116">B</text>
+                  <text x="280" y="116">C</text>
+                  <text x="416" y="116">Directory</text>
+                  <text x="536" y="116">Channel</text>
+                  <text x="64" y="148">KeyPackageA</text>
+                  <text x="200" y="196">KeyPackageB</text>
+                  <text x="336" y="244">KeyPackageC</text>
+                </g>
+              </svg>
+            </artwork>
+            <artwork type="ascii-art">
+                                                  Delivery Service
+                                                          |
+                                                .--------' '--------.
+                                               |                     |
+                                                               Group
+A                B                C            Directory       Channel
+|                |                |                |              |
+| KeyPackageA    |                |                |              |
++-------------------------------------------------&gt;|              |
+|                |                |                |              |
+|                | KeyPackageB    |                |              |
+|                +--------------------------------&gt;|              |
+|                |                |                |              |
+|                |                | KeyPackageC    |              |
+|                |                +---------------&gt;|              |
+|                |                |                |              |
+</artwork>
+          </artset>
+        </figure>
+        <t><xref target="create-flow"/> shows how these pre-published KeyPackages are used to create a group.
+When client A wants to establish a group with clients B and C, it first initializes a
+group state containing only itself and downloads KeyPackages for B and C. For
+each member, A generates an Add proposal and a Commit message to add that member and then
+broadcasts the two messages to the group. Client A also generates a Welcome message and sends it
+directly to the new member (there's no need to send it to the group). Only after
+A has received its Commit message back from the Delivery Service does it update its
+state to reflect the new member's addition.</t>
+        <t>Once A has updated its state, the new member has processed the Welcome, and any
+other group members have processed the Commit, they will all have consistent
+representations of the group state, including a group secret that is known only
+to the members the group. The new member will be able to read and send new
+messages to the group, but messages sent before they were added to the group
+will not be accessible.</t>
+        <figure anchor="create-flow">
+          <name>Client A creates a group with clients B and C</name>
+          <artset>
+            <artwork type="svg">
+              <svg xmlns="http://www.w3.org/2000/svg" class="diagram" font-family="monospace" font-size="13px" height="512" text-anchor="middle" version="1.1" viewBox="0 0 560 512" width="560">
+                <path d="M 8,64 L 8,496" fill="none" stroke="black"/>
+                <path d="M 128,104 L 128,152" fill="none" stroke="black"/>
+                <path d="M 128,168 L 128,264" fill="none" stroke="black"/>
+                <path d="M 128,280 L 128,344" fill="none" stroke="black"/>
+                <path d="M 128,360 L 128,392" fill="none" stroke="black"/>
+                <path d="M 128,408 L 128,456" fill="none" stroke="black"/>
+                <path d="M 128,472 L 128,496" fill="none" stroke="black"/>
+                <path d="M 248,104 L 248,152" fill="none" stroke="black"/>
+                <path d="M 248,168 L 248,264" fill="none" stroke="black"/>
+                <path d="M 248,280 L 248,344" fill="none" stroke="black"/>
+                <path d="M 248,360 L 248,456" fill="none" stroke="black"/>
+                <path d="M 368,64 L 368,152" fill="none" stroke="black"/>
+                <path d="M 368,168 L 368,264" fill="none" stroke="black"/>
+                <path d="M 368,280 L 368,344" fill="none" stroke="black"/>
+                <path d="M 368,360 L 368,456" fill="none" stroke="black"/>
+                <path d="M 528,64 L 528,496" fill="none" stroke="black"/>
+                <path d="M 16,96 L 368,96" fill="none" stroke="black"/>
+                <path d="M 8,160 L 520,160" fill="none" stroke="black"/>
+                <path d="M 8,208 L 120,208" fill="none" stroke="black"/>
+                <path d="M 16,272 L 528,272" fill="none" stroke="black"/>
+                <path d="M 8,352 L 520,352" fill="none" stroke="black"/>
+                <path d="M 8,400 L 240,400" fill="none" stroke="black"/>
+                <path d="M 16,464 L 528,464" fill="none" stroke="black"/>
+                <path d="M 136,480 L 528,480" fill="none" stroke="black"/>
+                <polygon class="arrowhead" fill="black" points="528,352 516,346.4 516,357.6" transform="rotate(0,520,352)"/>
+                <polygon class="arrowhead" fill="black" points="528,160 516,154.4 516,165.6" transform="rotate(0,520,160)"/>
+                <polygon class="arrowhead" fill="black" points="248,400 236,394.4 236,405.6" transform="rotate(0,240,400)"/>
+                <polygon class="arrowhead" fill="black" points="144,480 132,474.4 132,485.6" transform="rotate(180,136,480)"/>
+                <polygon class="arrowhead" fill="black" points="128,208 116,202.4 116,213.6" transform="rotate(0,120,208)"/>
+                <polygon class="arrowhead" fill="black" points="24,464 12,458.4 12,469.6" transform="rotate(180,16,464)"/>
+                <polygon class="arrowhead" fill="black" points="24,272 12,266.4 12,277.6" transform="rotate(180,16,272)"/>
+                <polygon class="arrowhead" fill="black" points="24,96 12,90.4 12,101.6" transform="rotate(180,16,96)"/>
+                <g class="text">
+                  <text x="528" y="36">Group</text>
+                  <text x="8" y="52">A</text>
+                  <text x="128" y="52">B</text>
+                  <text x="248" y="52">C</text>
+                  <text x="368" y="52">Directory</text>
+                  <text x="528" y="52">Channel</text>
+                  <text x="128" y="68">|</text>
+                  <text x="248" y="68">|</text>
+                  <text x="132" y="84">KeyPackageB,</text>
+                  <text x="232" y="84">KeyPackageC</text>
+                  <text x="420" y="132">Add(A-&gt;AB)</text>
+                  <text x="424" y="148">Commit(Add)</text>
+                  <text x="68" y="196">Welcome(B)</text>
+                  <text x="420" y="244">Add(A-&gt;AB)</text>
+                  <text x="424" y="260">Commit(Add)</text>
+                  <text x="428" y="324">Add(AB-&gt;ABC)</text>
+                  <text x="424" y="340">Commit(Add)</text>
+                  <text x="188" y="388">Welcome(C)</text>
+                  <text x="428" y="436">Add(AB-&gt;ABC)</text>
+                  <text x="424" y="452">Commit(Add)</text>
+                  <text x="248" y="500">|</text>
+                  <text x="368" y="500">|</text>
+                </g>
+              </svg>
+            </artwork>
+            <artwork type="ascii-art">
+                                                               Group
+A              B              C          Directory            Channel
+|              |              |              |                   |
+|         KeyPackageB, KeyPackageC           |                   |
+|&lt;-------------------------------------------+                   |
+|              |              |              |                   |
+|              |              |              | Add(A-&gt;AB)        |
+|              |              |              | Commit(Add)       |
++---------------------------------------------------------------&gt;|
+|              |              |              |                   |
+|  Welcome(B)  |              |              |                   |
++-------------&gt;|              |              |                   |
+|              |              |              |                   |
+|              |              |              | Add(A-&gt;AB)        |
+|              |              |              | Commit(Add)       |
+|&lt;---------------------------------------------------------------+
+|              |              |              |                   |
+|              |              |              |                   |
+|              |              |              | Add(AB-&gt;ABC)      |
+|              |              |              | Commit(Add)       |
++---------------------------------------------------------------&gt;|
+|              |              |              |                   |
+|              |  Welcome(C)  |              |                   |
++----------------------------&gt;|              |                   |
+|              |              |              |                   |
+|              |              |              | Add(AB-&gt;ABC)      |
+|              |              |              | Commit(Add)       |
+|&lt;---------------------------------------------------------------+
+|              |&lt;------------------------------------------------+
+|              |              |              |                   |
+</artwork>
+          </artset>
+        </figure>
+        <t>Subsequent additions of group members proceed in the same way.  Any
+member of the group can download a KeyPackage for a new client,
+broadcast Add and Commit messages that the current group will use to update
+their state, and send a Welcome message that the new client can use to
+initialize its state and join the group.</t>
+        <t>To enforce the forward secrecy and post-compromise security of messages, each
+member periodically updates the keys that represent them to the group.  A member
+does this by sending a Commit (possibly with no proposals) or by sending an
+Update message that is committed by another member (see <xref target="update-flow"/>).
+Once the other members of
+the group have processed these messages, the group's secrets will be unknown to
+an attacker that had compromised the secrets corresponding to the sender's leaf in the tree.
+At the end of the scenario shown in <xref target="update-flow"/>, the group has
+post-compromise security with respect to both A and B.</t>
+        <t>Update messages <bcp14>SHOULD</bcp14> be sent at regular intervals of time as long as the group
+is active, and members that don't update <bcp14>SHOULD</bcp14> eventually be removed from the
+group. It's left to the application to determine an appropriate amount of time
+between Updates. Since the purpose of sending an Update is to proactively
+constrain a compromise window, the right frequency is usually on the order of
+hours or days, not milliseconds. For example, an application might send an
+Update each time a member sends an application message after receiving any
+message from another member, or daily if no application messages are sent.</t>
+        <t>The MLS architecture recommends that MLS be operated over a secure transport
+(see <xref section="7.1" sectionFormat="of" target="I-D.ietf-mls-architecture"/>).  Such transport protocols
+will typically provide functions such as congestion control that manage the
+impact of an MLS-using application on other applications sharing the same
+network.  Applications should take care that they do not send MLS messages at a
+rate that will cause problems such as network congestion, especially if they are
+not following the above recommendation (e.g., sending MLS directly over UDP instead).</t>
+        <figure anchor="update-flow">
+          <name>Client B proposes to update its key, and client A commits the proposal</name>
+          <artset>
+            <artwork type="svg">
+              <svg xmlns="http://www.w3.org/2000/svg" class="diagram" font-family="monospace" font-size="13px" height="304" text-anchor="middle" version="1.1" viewBox="0 0 528 304" width="528">
+                <path d="M 8,64 L 8,288" fill="none" stroke="black"/>
+                <path d="M 128,64 L 128,120" fill="none" stroke="black"/>
+                <path d="M 128,136 L 128,200" fill="none" stroke="black"/>
+                <path d="M 128,248 L 128,288" fill="none" stroke="black"/>
+                <path d="M 248,64 L 248,88" fill="none" stroke="black"/>
+                <path d="M 248,152 L 248,200" fill="none" stroke="black"/>
+                <path d="M 248,264 L 248,288" fill="none" stroke="black"/>
+                <path d="M 368,64 L 368,88" fill="none" stroke="black"/>
+                <path d="M 368,168 L 368,200" fill="none" stroke="black"/>
+                <path d="M 488,64 L 488,288" fill="none" stroke="black"/>
+                <path d="M 128,96 L 480,96" fill="none" stroke="black"/>
+                <path d="M 16,128 L 488,128" fill="none" stroke="black"/>
+                <path d="M 136,144 L 488,144" fill="none" stroke="black"/>
+                <path d="M 256,160 L 488,160" fill="none" stroke="black"/>
+                <path d="M 8,208 L 480,208" fill="none" stroke="black"/>
+                <path d="M 16,240 L 488,240" fill="none" stroke="black"/>
+                <path d="M 136,256 L 488,256" fill="none" stroke="black"/>
+                <path d="M 256,272 L 488,272" fill="none" stroke="black"/>
+                <polygon class="arrowhead" fill="black" points="488,208 476,202.4 476,213.6" transform="rotate(0,480,208)"/>
+                <polygon class="arrowhead" fill="black" points="488,96 476,90.4 476,101.6" transform="rotate(0,480,96)"/>
+                <polygon class="arrowhead" fill="black" points="264,272 252,266.4 252,277.6" transform="rotate(180,256,272)"/>
+                <polygon class="arrowhead" fill="black" points="264,160 252,154.4 252,165.6" transform="rotate(180,256,160)"/>
+                <polygon class="arrowhead" fill="black" points="144,256 132,250.4 132,261.6" transform="rotate(180,136,256)"/>
+                <polygon class="arrowhead" fill="black" points="144,144 132,138.4 132,149.6" transform="rotate(180,136,144)"/>
+                <polygon class="arrowhead" fill="black" points="24,240 12,234.4 12,245.6" transform="rotate(180,16,240)"/>
+                <polygon class="arrowhead" fill="black" points="24,128 12,122.4 12,133.6" transform="rotate(180,16,128)"/>
+                <g class="text">
+                  <text x="488" y="36">Group</text>
+                  <text x="8" y="52">A</text>
+                  <text x="128" y="52">B</text>
+                  <text x="184" y="52">...</text>
+                  <text x="248" y="52">Z</text>
+                  <text x="368" y="52">Directory</text>
+                  <text x="496" y="52">Channel</text>
+                  <text x="176" y="84">Update(B)</text>
+                  <text x="248" y="116">|</text>
+                  <text x="368" y="116">|</text>
+                  <text x="416" y="116">Update(B)</text>
+                  <text x="64" y="196">Commit(Upd)</text>
+                  <text x="128" y="228">|</text>
+                  <text x="248" y="228">|</text>
+                  <text x="368" y="228">|</text>
+                  <text x="424" y="228">Commit(Upd)</text>
+                  <text x="368" y="292">|</text>
+                </g>
+              </svg>
+            </artwork>
+            <artwork type="ascii-art">
+                                                          Group
+A              B     ...      Z          Directory        Channel
+|              |              |              |              |
+|              | Update(B)    |              |              |
+|              +-------------------------------------------&gt;|
+|              |              |              | Update(B)    |
+|&lt;----------------------------------------------------------+
+|              |&lt;-------------------------------------------+
+|              |              |&lt;----------------------------+
+|              |              |              |              |
+| Commit(Upd)  |              |              |              |
++----------------------------------------------------------&gt;|
+|              |              |              | Commit(Upd)  |
+|&lt;----------------------------------------------------------+
+|              |&lt;-------------------------------------------+
+|              |              |&lt;----------------------------+
+|              |              |              |              |
+</artwork>
+          </artset>
+        </figure>
+        <t>Members are removed from the group in a similar way, as shown in <xref target="remove-flow"/>.
+Any member of the group can send a Remove proposal followed by a
+Commit message.  The Commit message provides new entropy to all members of the
+group except the removed member.  This new entropy is added to the epoch secret
+for the new epoch so that it is not known to the removed member.
+Note that this does not necessarily imply that any member
+is actually allowed to evict other members; groups can
+enforce access control policies on top of these
+basic mechanisms.</t>
+        <figure anchor="remove-flow">
+          <name>Client Z removes client B from the group</name>
+          <artset>
+            <artwork type="svg">
+              <svg xmlns="http://www.w3.org/2000/svg" class="diagram" font-family="monospace" font-size="13px" height="240" text-anchor="middle" version="1.1" viewBox="0 0 520 240" width="520">
+                <path d="M 8,64 L 8,224" fill="none" stroke="black"/>
+                <path d="M 128,64 L 128,168" fill="none" stroke="black"/>
+                <path d="M 128,184 L 128,224" fill="none" stroke="black"/>
+                <path d="M 248,64 L 248,168" fill="none" stroke="black"/>
+                <path d="M 248,200 L 248,224" fill="none" stroke="black"/>
+                <path d="M 368,64 L 368,104" fill="none" stroke="black"/>
+                <path d="M 368,120 L 368,168" fill="none" stroke="black"/>
+                <path d="M 488,64 L 488,224" fill="none" stroke="black"/>
+                <path d="M 248,112 L 480,112" fill="none" stroke="black"/>
+                <path d="M 16,176 L 488,176" fill="none" stroke="black"/>
+                <path d="M 136,192 L 488,192" fill="none" stroke="black"/>
+                <path d="M 256,208 L 488,208" fill="none" stroke="black"/>
+                <polygon class="arrowhead" fill="black" points="488,112 476,106.4 476,117.6" transform="rotate(0,480,112)"/>
+                <polygon class="arrowhead" fill="black" points="264,208 252,202.4 252,213.6" transform="rotate(180,256,208)"/>
+                <polygon class="arrowhead" fill="black" points="144,192 132,186.4 132,197.6" transform="rotate(180,136,192)"/>
+                <polygon class="arrowhead" fill="black" points="24,176 12,170.4 12,181.6" transform="rotate(180,16,176)"/>
+                <g class="text">
+                  <text x="488" y="36">Group</text>
+                  <text x="8" y="52">A</text>
+                  <text x="128" y="52">B</text>
+                  <text x="184" y="52">...</text>
+                  <text x="248" y="52">Z</text>
+                  <text x="368" y="52">Directory</text>
+                  <text x="488" y="52">Channel</text>
+                  <text x="296" y="84">Remove(B)</text>
+                  <text x="304" y="100">Commit(Rem)</text>
+                  <text x="416" y="148">Remove(B)</text>
+                  <text x="424" y="164">Commit(Rem)</text>
+                  <text x="368" y="228">|</text>
+                </g>
+              </svg>
+            </artwork>
+            <artwork type="ascii-art">
+                                                          Group
+A              B     ...      Z          Directory       Channel
+|              |              |              |              |
+|              |              | Remove(B)    |              |
+|              |              | Commit(Rem)  |              |
+|              |              +----------------------------&gt;|
+|              |              |              |              |
+|              |              |              | Remove(B)    |
+|              |              |              | Commit(Rem)  |
+|&lt;----------------------------------------------------------+
+|              |&lt;-------------------------------------------+
+|              |              |&lt;----------------------------+
+|              |              |              |              |
+</artwork>
+          </artset>
+        </figure>
+        <t>Note that the flows in this section are examples; applications can arrange
+message flows in other ways.  For example:</t>
+        <ul spacing="normal">
+          <li>Welcome messages don't necessarily need to be sent directly to new joiners.
+Since they are encrypted to new joiners, they could be distributed more
+broadly, say if the application only had access to a broadcast channel for the
+group.</li>
+          <li>Proposal messages don't need to be immediately sent to all group members.  They need to
+be available to the committer before generating a Commit, and to other members before
+processing the Commit.</li>
+          <li>The sender of a Commit doesn't necessarily have to wait to receive its own
+Commit back before advancing its state. It only needs to know that its Commit
+will be the next one applied by the group, say based on a promise from an
+orchestration server.</li>
+        </ul>
+      </section>
+      <section anchor="external-joins">
+        <name>External Joins</name>
+        <t>In addition to the Welcome-based flow for adding a new member to the group, it
+is also possible for a new member to join by means of an "external Commit".
+This mechanism can be used when the existing members don't have a KeyPackage for
+the new member, for example, in the case of an "open" group that can be joined
+by new members without asking permission from existing members.</t>
+        <t><xref target="groupinfo-flow"/> shows a typical  message flow for an external join. To enable
+a new member to join the group in this way, a member of the group (A, B)
+publishes a GroupInfo object that includes the GroupContext for the group as
+well as a public key that can be used to encrypt a secret to the existing
+members of the group.  When the new member Z wishes to join, they download the
+GroupInfo object and use it to form a Commit of a special form that adds Z to
+the group (as detailed in <xref target="joining-via-external-commits"/>).  The existing
+members of the group process this external Commit in a similar way to a normal
+Commit, advancing to a new epoch in which Z is now a member of the group.</t>
+        <figure anchor="groupinfo-flow">
+          <name>Client A publishes a GroupInfo object, and Client Z uses it to join the group</name>
+          <artwork>
+                                                          Group
+A              B              Z          Directory        Channel
+|              |              |              |              |
+| GroupInfo    |              |              |              |
++-------------------------------------------&gt;|              |
+|              |              | GroupInfo    |              |
+|              |              |&lt;-------------+              |
+|              |              |              |              |
+|              |              | Commit(ExtZ) |              |
+|              |              +----------------------------&gt;|
+|              |              |              | Commit(ExtZ) |
+|&lt;----------------------------------------------------------+
+|              |&lt;-------------------------------------------+
+|              |              |&lt;----------------------------+
+|              |              |              |              |
+</artwork>
+        </figure>
+      </section>
+      <section anchor="relationships-between-epochs">
+        <name>Relationships between Epochs</name>
+        <t>A group has a single linear sequence of epochs. Groups and epochs are generally
+independent of one another. However, it can sometimes be useful to link epochs
+cryptographically, either within a group or across groups. MLS derives a
+resumption pre-shared key (PSK) from each epoch to allow entropy extracted from
+one epoch to be injected into a future epoch.  A group member that wishes to
+inject a PSK issues a PreSharedKey proposal (<xref target="presharedkey"/>) describing the
+PSK to be injected.  When this proposal is committed, the corresponding PSK will
+be incorporated into the key schedule as described in <xref target="pre-shared-keys"/>.</t>
+        <t>Linking epochs in this way
+guarantees that members entering the new epoch agree on a key if and only if
+they were members of the group during the epoch from which the resumption key
+was extracted.</t>
+        <t>MLS supports two ways to tie a new group to an existing group, which are illustrated in
+Figures <xref format="counter" target="psk-reinit"/> and <xref format="counter" target="psk-branch"/>. Reinitialization
+closes one group and creates a new group comprising the same members with
+different parameters. Branching starts a new group with a subset of the original
+group's participants (with no effect on the original group).  In both cases,
+the new group is linked to the old group via a resumption PSK.</t>
+        <figure anchor="psk-reinit">
+          <name>Reinitializing a Group</name>
+          <artset>
+            <artwork type="svg">
+              <svg xmlns="http://www.w3.org/2000/svg" class="diagram" font-family="monospace" font-size="13px" height="240" text-anchor="middle" version="1.1" viewBox="0 0 272 240" width="272">
+                <path d="M 48,40 L 48,112" fill="none" stroke="black"/>
+                <path d="M 224,136 L 224,208" fill="none" stroke="black"/>
+                <path d="M 56,80 L 72,80" fill="none" stroke="black"/>
+                <polygon class="arrowhead" fill="black" points="232,208 220,202.4 220,213.6" transform="rotate(90,224,208)"/>
+                <polygon class="arrowhead" fill="black" points="64,80 52,74.4 52,85.6" transform="rotate(180,56,80)"/>
+                <polygon class="arrowhead" fill="black" points="56,112 44,106.4 44,117.6" transform="rotate(90,48,112)"/>
+                <g class="text">
+                  <text x="56" y="36">epoch_A_[n-1]</text>
+                  <text x="108" y="84">ReInit</text>
+                  <text x="48" y="132">epoch_A_[n]</text>
+                  <text x="224" y="132">epoch_B_[0]</text>
+                  <text x="48" y="148">.</text>
+                  <text x="48" y="164">.</text>
+                  <text x="136" y="164">PSK(usage=reinit)</text>
+                  <text x="132" y="180">.....................&gt;</text>
+                  <text x="224" y="228">epoch_B_[1]</text>
+                </g>
+              </svg>
+            </artwork>
+            <artwork type="ascii-art">
+epoch_A_[n-1]
+     |
+     |
+     |&lt;-- ReInit
+     |
+     V
+epoch_A_[n]           epoch_B_[0]
+     .                     |
+     .  PSK(usage=reinit)  |
+     .....................&gt;|
+                           |
+                           V
+                      epoch_B_[1]
+</artwork>
+          </artset>
+        </figure>
+        <figure anchor="psk-branch">
+          <name>Branching a Group</name>
+          <artset>
+            <artwork type="svg">
+              <svg xmlns="http://www.w3.org/2000/svg" class="diagram" font-family="monospace" font-size="13px" height="144" text-anchor="middle" version="1.1" viewBox="0 0 272 144" width="272">
+                <path d="M 48,40 L 48,112" fill="none" stroke="black"/>
+                <path d="M 224,40 L 224,112" fill="none" stroke="black"/>
+                <polygon class="arrowhead" fill="black" points="232,112 220,106.4 220,117.6" transform="rotate(90,224,112)"/>
+                <polygon class="arrowhead" fill="black" points="56,112 44,106.4 44,117.6" transform="rotate(90,48,112)"/>
+                <g class="text">
+                  <text x="48" y="36">epoch_A_[n]</text>
+                  <text x="224" y="36">epoch_B_[0]</text>
+                  <text x="136" y="68">PSK(usage=branch)</text>
+                  <text x="136" y="84">....................&gt;</text>
+                  <text x="56" y="132">epoch_A_[n+1]</text>
+                  <text x="224" y="132">epoch_B_[1]</text>
+                </g>
+              </svg>
+            </artwork>
+            <artwork type="ascii-art">
+epoch_A_[n]           epoch_B_[0]
+     |                     |
+     |  PSK(usage=branch)  |
+     |....................&gt;|
+     |                     |
+     V                     V
+epoch_A_[n+1]         epoch_B_[1]
+</artwork>
+          </artset>
+        </figure>
+        <t>Applications may also choose to use resumption PSKs to link epochs in other
+ways.  For example, <xref target="psk-reinject"/> shows a case where a resumption PSK
+from epoch <tt>n</tt> is injected into epoch <tt>n+k</tt>.  This demonstrates that the members
+of the group at epoch <tt>n+k</tt> were also members at epoch <tt>n</tt>, irrespective of any
+changes to these members' keys due to Updates or Commits.</t>
+        <figure anchor="psk-reinject">
+          <name>Reinjecting Entropy from an Earlier Epoch</name>
+          <artset>
+            <artwork type="svg">
+              <svg xmlns="http://www.w3.org/2000/svg" class="diagram" font-family="monospace" font-size="13px" height="304" text-anchor="middle" version="1.1" viewBox="0 0 248 304" width="248">
+                <path d="M 48,40 L 48,176" fill="none" stroke="black"/>
+                <path d="M 48,200 L 48,272" fill="none" stroke="black"/>
+                <polygon class="arrowhead" fill="black" points="56,272 44,266.4 44,277.6" transform="rotate(90,48,272)"/>
+                <polygon class="arrowhead" fill="black" points="56,176 44,170.4 44,181.6" transform="rotate(90,48,176)"/>
+                <g class="text">
+                  <text x="48" y="36">epoch_A_[n]</text>
+                  <text x="156" y="68">PSK(usage=application)</text>
+                  <text x="136" y="84">.....................</text>
+                  <text x="216" y="100">.</text>
+                  <text x="216" y="116">.</text>
+                  <text x="40" y="132">.</text>
+                  <text x="56" y="132">.</text>
+                  <text x="216" y="132">...</text>
+                  <text x="216" y="148">.</text>
+                  <text x="216" y="164">.</text>
+                  <text x="216" y="180">.</text>
+                  <text x="64" y="196">epoch_A_[n+k-1]</text>
+                  <text x="216" y="196">.</text>
+                  <text x="216" y="212">.</text>
+                  <text x="216" y="228">.</text>
+                  <text x="136" y="244">&lt;....................</text>
+                  <text x="56" y="292">epoch_A_[n+k]</text>
+                </g>
+              </svg>
+            </artwork>
+            <artwork type="ascii-art">
+epoch_A_[n]
+     |
+     |  PSK(usage=application)
+     |.....................
+     |                    .
+     |                    .
+    ...                  ...
+     |                    .
+     |                    .
+     V                    .
+epoch_A_[n+k-1]           .
+     |                    .
+     |                    .
+     |&lt;....................
+     |
+     V
+epoch_A_[n+k]
+</artwork>
+          </artset>
+        </figure>
+      </section>
+    </section>
+    <section anchor="ratchet-tree-concepts">
+      <name>Ratchet Tree Concepts</name>
+      <t>The protocol uses "ratchet trees" for deriving shared secrets among a group of
+clients.  A ratchet tree is an arrangement of secrets and key pairs among the
+members of a group in a way that allows for secrets to be efficiently updated to
+reflect changes in the group.</t>
+      <t>Ratchet trees allow a group to efficiently remove any member by encrypting new
+entropy to a subset of the group.  A ratchet tree assigns shared keys to
+subgroups of the overall group, so that, for example, encrypting to all but one
+member of the group requires only <tt>log(N)</tt> encryptions to subtrees, instead of the <tt>N-1</tt>
+encryptions that would be needed to encrypt to each participant individually
+(where N is the number of members in the group).</t>
+      <t>This remove operation allows MLS to efficiently achieve
+post-compromise security.  In an Update proposal or a full Commit message, an old (possibly
+compromised) representation of a member is efficiently removed from the group and
+replaced with a freshly generated instance.</t>
+      <section anchor="ratchet-tree-terminology">
+        <name>Ratchet Tree Terminology</name>
+        <t>Trees consist of <em>nodes</em>. A node is a
+<em>leaf</em> if it has no children; otherwise, it is a <em>parent</em>.
+All parents in our trees have precisely
+two children, a <em>left</em> child and a <em>right</em> child. A node is the <em>root</em>
+of a tree if it has no parent, and <em>intermediate</em> if it has both
+children and a parent. The <em>descendants</em> of a node are that node's
+children, and the descendants of its children.  We say a tree
+<em>contains</em> a node if that node is a descendant of the root of the tree,
+or if the node itself is the root of the tree. Nodes are <em>siblings</em> if they share the same parent.</t>
+        <t>A <em>subtree</em> of a tree is the tree given by any node (the <em>head</em> of the
+subtree) and its descendants. The <em>size</em> of a tree or subtree is the
+number of leaf nodes it contains.  For a given parent node, its <em>left
+subtree</em> is the subtree with its left child as head and its
+<em>right subtree</em> is the subtree with its right child as head.</t>
+        <t>Every tree used in this protocol is a perfect binary tree, that is, a complete
+balanced binary tree with 2<sup>d</sup> leaves all at the same depth <tt>d</tt>.  This
+structure is unique for a given depth <tt>d</tt>.</t>
+        <t>There are multiple ways that an implementation might represent a ratchet tree in
+memory.  A convenient property of left-balanced binary trees (including the
+complete trees used here) is that they can be represented as an array of nodes,
+with node relationships computed based on the nodes' indices in the array.  A
+more traditional representation based on linked node objects may also be used.
+Appendices <xref format="counter" target="array-based-trees"/> and <xref format="counter" target="link-based-trees"/> provide some details on how to
+implement the tree operations required for MLS in these representations.  MLS
+places no requirements on implementations' internal representations of ratchet
+trees.  An implementation may use any tree representation and associated
+algorithms, as long as they produce correct protocol messages.</t>
+        <section anchor="ratchet-tree-nodes">
+          <name>Ratchet Tree Nodes</name>
+          <t>Each leaf node in a ratchet tree is given an <em>index</em> (or <em>leaf index</em>), starting
+at 0 from the left to 2<sup>d</sup> - 1 at the right (for a tree with 2<sup>d</sup> leaves). A tree
+with 2<sup>d</sup> leaves has 2<sup>d+1</sup> - 1 nodes, including parent nodes.</t>
+          <t>Each node in a ratchet tree is either <em>blank</em> (containing no value) or it holds
+an HPKE public key with some associated data:</t>
+          <ul spacing="normal">
+            <li>A public key (for the HPKE scheme in use; see <xref target="cipher-suites"/>)</li>
+            <li>A credential (only for leaf nodes; see <xref target="credentials"/>)</li>
+            <li>An ordered list of "unmerged" leaves (see <xref target="views"/>)</li>
+            <li>A hash of certain information about the node's parent, as of the last time the
+node was changed (see <xref target="parent-hashes"/>).</li>
+          </ul>
+          <t>As described in <xref target="views"/>, different members know different subsets of the set
+of private keys corresponding to the public keys in nodes in the tree.  The
+private key corresponding to a parent node is known only to members at leaf
+nodes that are descendants of that node.  The private key corresponding to a leaf
+node is known only to the member at that leaf node.  A leaf node is <em>unmerged</em>
+relative to one of its ancestor nodes if the member at the leaf node does not
+know the private key corresponding to the ancestor node.</t>
+          <t>Every node, regardless of whether the node is blank or populated, has
+a corresponding <em>hash</em> that summarizes the contents of the subtree
+below that node.  The rules for computing these hashes are described
+in <xref target="tree-hashes"/>.</t>
+          <t>The <em>resolution</em> of a node is an ordered list of non-blank nodes
+that collectively cover all non-blank descendants of the node.
+The resolution of the root contains the set of keys that are collectively necessary to
+encrypt to every node in the group. The resolution
+of a node is effectively a depth-first, left-first enumeration of the nearest
+non-blank nodes below the node:</t>
+          <ul spacing="normal">
+            <li>The resolution of a non-blank node comprises the node itself,
+followed by its list of unmerged leaves, if any.</li>
+            <li>The resolution of a blank leaf node is the empty list.</li>
+            <li>The resolution of a blank intermediate node is the result of
+concatenating the resolution of its left child with the resolution
+of its right child, in that order.</li>
+          </ul>
+          <t>For example, consider the following subtree, where the <tt>_</tt> character
+represents a blank node and unmerged leaves are indicated in square
+brackets:</t>
+          <figure anchor="resolution-tree">
+            <name>A Tree with Blanks and Unmerged Leaves</name>
+            <artwork type="ascii-art">
+               ...
+               /
+              _
+        ______|______
+       /             \
+      X[B]            _
+    __|__           __|__
+   /     \         /     \
+  _       _       Y       _
+ / \     / \     / \     / \
+A   B   _   D   E   F   _   H
+
+0   1   2   3   4   5   6   7
+</artwork>
+          </figure>
+          <t>In this tree, we can see all of the above rules in play:</t>
+          <ul spacing="normal">
+            <li>The resolution of node X is the list [X, B].</li>
+            <li>The resolution of leaf 2 or leaf 6 is the empty list [].</li>
+            <li>The resolution of top node is the list [X, B, Y, H].</li>
+          </ul>
+        </section>
+        <section anchor="paths-through-a-ratchet-tree">
+          <name>Paths through a Ratchet Tree</name>
+          <t>The <em>direct path</em> of a root is the empty list. The direct path of any other node
+is the concatenation of that node's parent along with the parent's direct path.</t>
+          <t>The <em>copath</em> of a node is the node's sibling concatenated with the list of
+siblings of all the nodes in its direct path, excluding the root.</t>
+          <t>The <em>filtered direct path</em> of a leaf node L is the node's direct path, with any
+node removed whose child on the copath of L has an empty resolution (keeping in
+mind that any unmerged leaves of the copath child count toward its resolution).
+The removed nodes do not need their own key pairs because encrypting to the
+node's key pair would be equivalent to encrypting to its non-copath child.</t>
+          <t>For example, consider the following tree (where blank nodes are indicated with
+<tt>_</tt>, but also assigned a label for reference):</t>
+          <figure anchor="full-tree">
+            <name>A Complete Tree with Five Members, with Labels for Blank Parent Nodes</name>
+            <artset>
+              <artwork type="svg">
+                <svg xmlns="http://www.w3.org/2000/svg" class="diagram" font-family="monospace" font-size="13px" height="240" text-anchor="middle" version="1.1" viewBox="0 0 256 240" width="256">
+                  <path d="M 56,104 L 56,128" fill="none" stroke="black"/>
+                  <path d="M 120,48 L 120,64" fill="none" stroke="black"/>
+                  <path d="M 184,112 L 184,128" fill="none" stroke="black"/>
+                  <path d="M 72,64 L 168,64" fill="none" stroke="black"/>
+                  <path d="M 40,128 L 72,128" fill="none" stroke="black"/>
+                  <path d="M 168,128 L 200,128" fill="none" stroke="black"/>
+                  <path d="M 72,128 L 80,144" fill="none" stroke="black"/>
+                  <path d="M 92,168 L 96,176" fill="none" stroke="black"/>
+                  <path d="M 168,64 L 176,80" fill="none" stroke="black"/>
+                  <path d="M 200,128 L 208,144" fill="none" stroke="black"/>
+                  <path d="M 220,168 L 224,176" fill="none" stroke="black"/>
+                  <path d="M 32,144 L 40,128" fill="none" stroke="black"/>
+                  <path d="M 64,80 L 72,64" fill="none" stroke="black"/>
+                  <path d="M 80,176 L 84,168" fill="none" stroke="black"/>
+                  <path d="M 160,144 L 168,128" fill="none" stroke="black"/>
+                  <path d="M 208,176 L 212,168" fill="none" stroke="black"/>
+                  <g class="text">
+                    <text x="120" y="36">W</text>
+                    <text x="136" y="36">=</text>
+                    <text x="164" y="36">root</text>
+                    <text x="64" y="100">_=U</text>
+                    <text x="184" y="100">Y</text>
+                    <text x="24" y="164">T</text>
+                    <text x="96" y="164">_=V</text>
+                    <text x="152" y="164">X</text>
+                    <text x="224" y="164">_=Z</text>
+                    <text x="16" y="180">/</text>
+                    <text x="32" y="180">\</text>
+                    <text x="144" y="180">/</text>
+                    <text x="160" y="180">\</text>
+                    <text x="8" y="196">A</text>
+                    <text x="40" y="196">B</text>
+                    <text x="72" y="196">_</text>
+                    <text x="104" y="196">_</text>
+                    <text x="136" y="196">E</text>
+                    <text x="168" y="196">F</text>
+                    <text x="200" y="196">G</text>
+                    <text x="240" y="196">_=H</text>
+                    <text x="8" y="228">0</text>
+                    <text x="40" y="228">1</text>
+                    <text x="72" y="228">2</text>
+                    <text x="104" y="228">3</text>
+                    <text x="136" y="228">4</text>
+                    <text x="168" y="228">5</text>
+                    <text x="200" y="228">6</text>
+                    <text x="232" y="228">7</text>
+                  </g>
+                </svg>
+              </artwork>
+              <artwork type="ascii-art">
+              W = root
+              |
+        .-----+-----.
+       /             \
+      _=U             Y
+      |               |
+    .-+-.           .-+-.
+   /     \         /     \
+  T       _=V     X       _=Z
+ / \     / \     / \     / \
+A   B   _   _   E   F   G   _=H
+
+0   1   2   3   4   5   6   7
+</artwork>
+            </artset>
+          </figure>
+          <t>In this tree, the direct paths, copaths, and filtered direct paths for the leaf
+nodes are as follows:</t>
+          <table>
+            <thead>
+              <tr>
+                <th align="left">Node</th>
+                <th align="left">Direct path</th>
+                <th align="left">Copath</th>
+                <th align="left">Filtered Direct Path</th>
+              </tr>
+            </thead>
+            <tbody>
+              <tr>
+                <td align="left">A</td>
+                <td align="left">T, U, W</td>
+                <td align="left">B, V, Y</td>
+                <td align="left">T, W</td>
+              </tr>
+              <tr>
+                <td align="left">B</td>
+                <td align="left">T, U, W</td>
+                <td align="left">A, V, Y</td>
+                <td align="left">T, W</td>
+              </tr>
+              <tr>
+                <td align="left">E</td>
+                <td align="left">X, Y, W</td>
+                <td align="left">F, Z, U</td>
+                <td align="left">X, Y, W</td>
+              </tr>
+              <tr>
+                <td align="left">F</td>
+                <td align="left">X, Y, W</td>
+                <td align="left">E, Z, U</td>
+                <td align="left">X, Y, W</td>
+              </tr>
+              <tr>
+                <td align="left">G</td>
+                <td align="left">Z, Y, W</td>
+                <td align="left">H, X, U</td>
+                <td align="left">Y, W</td>
+              </tr>
+            </tbody>
+          </table>
+        </section>
+      </section>
+      <section anchor="views">
+        <name>Views of a Ratchet Tree</name>
+        <t>We generally assume that each participant maintains a complete and
+up-to-date view of the public state of the group's ratchet tree,
+including the public keys for all nodes and the credentials
+associated with the leaf nodes.</t>
+        <t>No participant in an MLS group knows the private key associated with
+every node in the tree. Instead, each member is assigned to a leaf of the tree,
+which determines the subset of private keys it knows. The
+credential stored at that leaf is one provided by the member.</t>
+        <t>In particular, MLS maintains the members' views of the tree in such
+a way as to maintain the <em>tree invariant</em>:</t>
+        <blockquote>
+          The private key for a node in the tree is known to a member of
+          the group only if the node's subtree contains that member's leaf.
+        </blockquote>
+        <t>In other words, if a node is not blank, then it holds a public key.
+The corresponding private key is known only to members occupying
+leaves below that node.</t>
+        <t>The reverse implication is not true: A member may not know the private key of
+an intermediate node above them.  Such a member has an <em>unmerged</em> leaf at the
+intermediate node.  Encrypting to an intermediate node requires encrypting to
+the node's public key, as well as the public keys of all the unmerged leaves
+below it.  A leaf is unmerged with regard to all of its ancestors when it is
+first added, because the process of adding the leaf does not give it access to
+the private keys for all of the nodes above it in the tree.  Leaves are "merged"
+as they receive the private keys for nodes, as described in
+<xref target="ratchet-tree-evolution"/>.</t>
+        <t>For example, consider a four-member group (A, B, C, D) where the node above the
+right two members is blank.  (This is what it would look like if A created a
+group with B, C, and D.)  Then the public state of the tree and the views of the
+private keys of the tree held by each participant would be as follows, where <tt>_</tt>
+represents a blank node, <tt>?</tt> represents an unknown private key, and <tt>pk(X)</tt>
+represents the public key corresponding to the private key <tt>X</tt>:</t>
+        <artwork type="ascii-art">
+         Public Tree
+============================
+            pk(ABCD)
+          /          \
+    pk(AB)            _
+     / \             / \
+pk(A)   pk(B)   pk(C)   pk(D)
+
+
+ Private @ A       Private @ B       Private @ C       Private @ D
+=============     =============     =============     =============
+     ABCD              ABCD              ABCD              ABCD
+    /   \             /   \             /   \             /   \
+  AB      _         AB      _         ?       _         ?       _
+ / \     / \       / \     / \       / \     / \       / \     / \
+A   ?   ?   ?     ?   B   ?   ?     ?   ?   C   ?     ?   ?   ?   D
+</artwork>
+        <t>Note how the tree invariant applies: Each member knows only their own leaf,
+the private key AB is known only to A and B, and the private key ABCD
+is known to all four members. This also illustrates another important
+point: it is possible for there to be "holes" on the path from a member's leaf
+to the root in which the member knows the key both above and below
+a given node, but not for that node, as in the case with D.</t>
+      </section>
+    </section>
+    <section anchor="cryptographic-objects">
+      <name>Cryptographic Objects</name>
+      <section anchor="cipher-suites">
+        <name>Cipher Suites</name>
+        <t>Each MLS session uses a single cipher suite that specifies the
+following primitives to be used in group key computations:</t>
+        <ul spacing="normal">
+          <li>
+            <t>HPKE parameters:
+            </t>
+            <ul spacing="normal">
+              <li>A Key Encapsulation Mechanism (KEM)</li>
+              <li>A Key Derivation Function (KDF)</li>
+              <li>An Authenticated Encryption with Associated Data (AEAD) encryption algorithm</li>
+            </ul>
+          </li>
+          <li>A hash algorithm</li>
+          <li>A Message Authentication Code (MAC) algorithm</li>
+          <li>A signature algorithm</li>
+        </ul>
+        <t>MLS uses HPKE for public key encryption <xref target="RFC9180"/>.  The
+<tt>DeriveKeyPair</tt> function associated to the KEM for the cipher suite maps octet
+strings to HPKE key pairs.  As in HPKE, MLS assumes that an AEAD algorithm
+produces a single ciphertext output from AEAD encryption (aligning with
+<xref target="RFC5116"/>), as opposed to a separate ciphertext and tag.</t>
+        <t>Cipher suites are represented with the CipherSuite type. The cipher suites are
+defined in <xref target="mls-cipher-suites"/>.</t>
+        <section anchor="public-keys">
+          <name>Public Keys</name>
+          <t>HPKE public keys are opaque values in a format defined by the underlying
+protocol (see <xref section="4" sectionFormat="of" target="RFC9180"/> for more information).</t>
+          <sourcecode type="tls-presentation">
+opaque HPKEPublicKey&lt;V&gt;;
+</sourcecode>
+          <t>Signature public keys are likewise represented as opaque values in a format
+defined by the cipher suite's signature scheme.</t>
+          <sourcecode type="tls-presentation">
+opaque SignaturePublicKey&lt;V&gt;;
+</sourcecode>
+          <t>For cipher suites using the Edwards-curve Digital Signature Algorithm (EdDSA)
+signature schemes (Ed25519 or Ed448), the public key is in the format specified
+in <xref target="RFC8032"/>.</t>
+          <t>For cipher suites using the Elliptic Curve Digital Signature Algorithm (ECDSA)
+with the NIST curves (P-256, P-384, or P-521), the public key is represented as
+an encoded UncompressedPointRepresentation struct, as defined in <xref target="RFC8446"/>.</t>
+        </section>
+        <section anchor="signing">
+          <name>Signing</name>
+          <t>The signature algorithm specified in a group's cipher suite is the mandatory algorithm
+to be used for signing messages within the group.  It
+<bcp14>MUST</bcp14> be the same as the signature algorithm specified in the credentials in the
+leaves of the tree (including the leaf node information in KeyPackages used to
+add new members).</t>
+          <t>The signatures used in this document are encoded as specified in <xref target="RFC8446"/>.
+In particular, ECDSA signatures are DER encoded, and EdDSA signatures are defined
+as the concatenation of <tt>R</tt> and <tt>S</tt>, as specified in <xref target="RFC8032"/>.</t>
+          <t>To disambiguate different signatures used in MLS, each signed value is prefixed
+by a label as shown below:</t>
+          <sourcecode type="pseudocode">
+SignWithLabel(SignatureKey, Label, Content) =
+    Signature.Sign(SignatureKey, SignContent)
+
+VerifyWithLabel(VerificationKey, Label, Content, SignatureValue) =
+    Signature.Verify(VerificationKey, SignContent, SignatureValue)
+</sourcecode>
+          <t>Where SignContent is specified as:</t>
+          <sourcecode type="tls-presentation">
+struct {
+    opaque label&lt;V&gt;;
+    opaque content&lt;V&gt;;
+} SignContent;
+</sourcecode>
+          <t>And its fields are set to:</t>
+          <sourcecode type="pseudocode">
+label = "MLS 1.0 " + Label;
+content = Content;
+</sourcecode>
+          <t>The functions <tt>Signature.Sign</tt> and <tt>Signature.Verify</tt> are defined by the
+signature algorithm.  If MLS extensions require signatures by group members,
+they should reuse the SignWithLabel construction, using a distinct label.  To
+avoid collisions in these labels, an IANA registry is defined in
+<xref target="mls-signature-labels"/>.</t>
+        </section>
+        <section anchor="public-key-encryption">
+          <name>Public Key Encryption</name>
+          <t>As with signing, MLS includes a label and context in encryption operations to
+avoid confusion between ciphertexts produced for different purposes.  Encryption
+and decryption including this label and context are done as follows:</t>
+          <sourcecode type="pseudocode">
+EncryptWithLabel(PublicKey, Label, Context, Plaintext) =
+  SealBase(PublicKey, EncryptContext, "", Plaintext)
+
+DecryptWithLabel(PrivateKey, Label, Context, KEMOutput, Ciphertext) =
+  OpenBase(KEMOutput, PrivateKey, EncryptContext, "", Ciphertext)
+</sourcecode>
+          <t>Where EncryptContext is specified as:</t>
+          <sourcecode type="tls-presentation">
+struct {
+  opaque label&lt;V&gt;;
+  opaque context&lt;V&gt;;
+} EncryptContext;
+</sourcecode>
+          <t>And its fields are set to:</t>
+          <artwork>
+label = "MLS 1.0 " + Label;
+context = Context;
+</artwork>
+          <t>The functions <tt>SealBase</tt> and <tt>OpenBase</tt> are defined in <xref section="6.1" sectionFormat="of" target="RFC9180"/> (with "Base" as the MODE), using the HPKE algorithms specified by the
+group's cipher suite.  If MLS extensions require HPKE encryption operations, they
+should reuse the EncryptWithLabel construction, using a distinct label.  To
+avoid collisions in these labels, an IANA registry is defined in
+<xref target="mls-public-key-encryption-labels"/>.</t>
+        </section>
+      </section>
+      <section anchor="hash-based-identifiers">
+        <name>Hash-Based Identifiers</name>
+        <t>Some MLS messages refer to other MLS objects by hash.  For example, Welcome
+messages refer to KeyPackages for the members being welcomed, and Commits refer
+to Proposals they cover.  These identifiers are computed as follows:</t>
+        <sourcecode type="tls-presentation">
+opaque HashReference&lt;V&gt;;
+
+HashReference KeyPackageRef;
+HashReference ProposalRef;
+</sourcecode>
+        <sourcecode type="pseudocode">
+MakeKeyPackageRef(value)
+  = RefHash("MLS 1.0 KeyPackage Reference", value)
+
+MakeProposalRef(value)
+  = RefHash("MLS 1.0 Proposal Reference", value)
+
+RefHash(label, value) = Hash(RefHashInput)
+</sourcecode>
+        <t>Where RefHashInput is defined as:</t>
+        <sourcecode type="tls-presentation">
+struct {
+  opaque label&lt;V&gt;;
+  opaque value&lt;V&gt;;
+} RefHashInput;
+</sourcecode>
+        <t>And its fields are set to:</t>
+        <sourcecode type="pseudocode">
+label = label;
+value = value;
+</sourcecode>
+        <t>For a KeyPackageRef, the <tt>value</tt> input is the encoded KeyPackage, and the
+cipher suite specified in the KeyPackage determines the KDF used.  For a
+ProposalRef, the <tt>value</tt> input is the AuthenticatedContent carrying the
+Proposal.  In the latter two cases, the KDF is determined by the group's
+cipher suite.</t>
+      </section>
+      <section anchor="credentials">
+        <name>Credentials</name>
+        <t>Each member of a group presents a credential that provides one or more
+identities for the member and associates them with the member's signing key.
+The identities and signing key are verified by the Authentication Service in use
+for a group.</t>
+        <t>It is up to the application to decide which identifiers to use at
+the application level.  For example,
+a certificate in an X509Credential may attest to several domain names or email
+addresses in its subjectAltName extension.  An application may decide to
+present all of these to a user, or if it knows a "desired" domain name or email
+address, it can check that the desired identifier is among those attested.
+Using the terminology from <xref target="RFC6125"/>, a credential provides "presented
+identifiers", and it is up to the application to supply a "reference identifier"
+for the authenticated client, if any.</t>
+        <sourcecode type="tls-presentation">
+// See the "MLS Credential Types" IANA registry for values
+uint16 CredentialType;
+
+struct {
+    opaque cert_data&lt;V&gt;;
+} Certificate;
+
+struct {
+    CredentialType credential_type;
+    select (Credential.credential_type) {
+        case basic:
+            opaque identity&lt;V&gt;;
+
+        case x509:
+            Certificate certificates&lt;V&gt;;
+    };
+} Credential;
+</sourcecode>
+        <t>A "basic" credential is a bare assertion of an identity, without any additional
+information.  The format of the encoded identity is defined by the application.</t>
+        <t>For an X.509 credential, each entry in the <tt>certificates</tt> field represents a single DER-encoded
+X.509 certificate. The chain is ordered such that the first entry (certificates[0]) is
+the end-entity certificate. The public key encoded in the
+<tt>subjectPublicKeyInfo</tt> of the end-entity certificate <bcp14>MUST</bcp14> be identical to the
+<tt>signature_key</tt> in the LeafNode containing this credential. A chain <bcp14>MAY</bcp14> omit any
+non-leaf certificates that supported peers are known to already possess.</t>
+        <section anchor="credential-validation">
+          <name>Credential Validation</name>
+          <t>The application using MLS is responsible for specifying which identifiers it
+finds acceptable for each member in a group.  In other words, following the
+model that <xref target="RFC6125"/> describes for TLS, the application maintains a list of
+"reference identifiers" for the members of a group, and the credentials provide
+"presented identifiers".  A member of a group is authenticated by first
+validating that the member's credential legitimately represents some presented
+identifiers, and then ensuring that the reference identifiers for the member are
+authenticated by those presented identifiers.</t>
+          <t>The parts of the system that perform these functions are collectively referred
+to as the Authentication Service (AS) <xref target="I-D.ietf-mls-architecture"/>.  A
+member's credential is said to be <em>validated with the AS</em> when the AS verifies
+that the credential's presented identifiers are correctly associated with the
+<tt>signature_key</tt> field in the member's LeafNode, and that those
+identifiers match the reference identifiers for the member.</t>
+          <t>Whenever a new credential is introduced in the group, it <bcp14>MUST</bcp14> be validated with
+the AS.  In particular, at the following events in the protocol:</t>
+          <ul spacing="normal">
+            <li>When a member receives a KeyPackage that it will use in an Add proposal to add
+a new member to the group</li>
+            <li>When a member receives a GroupInfo object that it will use to join a group,
+either via a Welcome or via an external Commit</li>
+            <li>When a member receives an Add proposal adding a member to the group</li>
+            <li>When a member receives an Update proposal whose LeafNode has a new credential
+for the member</li>
+            <li>When a member receives a Commit with an UpdatePath whose LeafNode has a new
+credential for the committer</li>
+            <li>When an <tt>external_senders</tt> extension is added to the group</li>
+            <li>When an existing <tt>external_senders</tt> extension is updated</li>
+          </ul>
+          <t>In cases where a member's credential is being replaced, such as the Update and
+Commit cases above, the AS <bcp14>MUST</bcp14> also verify that the set of presented
+identifiers in the new credential is valid as a successor to the set of
+presented identifiers in the old credential, according to the application's
+policy.</t>
+        </section>
+        <section anchor="credential-expiry-and-revocation">
+          <name>Credential Expiry and Revocation</name>
+          <t>In some credential schemes, a valid credential can "expire" or become invalid
+after a certain point in time. For example, each X.509 certificate has a
+<tt>notAfter</tt> field, expressing a time after which the certificate is not valid.</t>
+          <t>Expired credentials can cause operational problems in light of the validation
+requirements of <xref target="credential-validation"/>.  Applications can apply some
+operational practices and adaptations to Authentication Service policies to
+moderate these impacts.</t>
+          <t>In general, to avoid operational problems such as new joiners rejecting expired
+credentials in a group, applications that use such credentials should ensure to
+the extent practical that all of the credentials in use in a group are valid at
+all times.</t>
+          <t>If a member finds that its credential has expired (or will soon), it should
+issue an Update or Commit that replaces it with a valid credential.  For this
+reason, members <bcp14>SHOULD</bcp14> accept Update proposals and Commits issued by members
+with expired credentials, if the credential in the Update or Commit is valid.</t>
+          <t>Similarly, when a client is processing messages sent some time in the past
+(e.g., syncing up with a group after being offline), the client <bcp14>SHOULD</bcp14> accept
+signatures from members with expired credentials, since the credential may
+have been valid at the time the message was sent.</t>
+          <t>If a member finds that another member's credential has expired, they may issue a
+Remove that removes that member.  For example, an application could require a
+member preparing to issue a Commit to check the tree for expired credentials and
+include Remove proposals for those members in its Commit.  In situations where
+the group tree is known to the DS, the DS could also monitor the tree for
+expired credentials and issue external Remove proposals.</t>
+          <t>Some credential schemes also allow credentials to be revoked.  Revocation is
+similar to expiry in that a previously valid credential becomes invalid.
+As such, most of the considerations above also apply to revoked credentials.
+However, applications may want to treat revoked credentials differently, e.g.,
+by removing members with revoked credentials while allowing members with expired
+credentials time to update.</t>
+        </section>
+        <section anchor="uniquely-identifying-clients">
+          <name>Uniquely Identifying Clients</name>
+          <t>MLS implementations will presumably provide applications with a way to request
+protocol operations with regard to other clients (e.g., removing clients).  Such
+functions will need to refer to the other clients using some identifier.  MLS
+clients have a few types of identifiers, with different operational properties.</t>
+          <t>Internally to the protocol, group members are uniquely identified by their leaf
+index. However, a leaf index is only valid for referring to members in a given
+epoch. The same leaf index may represent a different member, or no member at
+all, in a subsequent epoch.</t>
+          <t>The Credentials presented by the clients in a group authenticate
+application-level identifiers for the clients.  However, these identifiers may not
+uniquely identify clients.  For example, if a user has multiple devices that are
+all present in an MLS group, then those devices' clients could all present the
+user's application-layer identifiers.</t>
+          <t>If needed, applications may add application-specific identifiers to the
+<tt>extensions</tt> field of a LeafNode object with the <tt>application_id</tt> extension.</t>
+          <sourcecode type="tls-presentation">
+opaque application_id&lt;V&gt;;
+</sourcecode>
+          <t>However, applications <bcp14>MUST NOT</bcp14> rely on the data in an <tt>application_id</tt> extension
+as if it were authenticated by the Authentication Service, and <bcp14>SHOULD</bcp14> gracefully
+handle cases where the identifier presented is not unique.</t>
+        </section>
+      </section>
+    </section>
+    <section anchor="message-framing">
+      <name>Message Framing</name>
+      <t>Handshake and application messages use a common framing structure.
+This framing provides encryption to ensure confidentiality within the
+group, as well as signing to authenticate the sender.</t>
+      <t>In most of the protocol, messages are handled in the form of
+AuthenticatedContent objects.  These structures contain the content of the
+message itself as well as information to authenticate the sender (see
+<xref target="content-authentication"/>).  The additional protections required to transmit
+these messages over an untrusted channel (group membership authentication or
+AEAD encryption) are added by encoding the AuthenticatedContent as a
+PublicMessage or PrivateMessage message, which can then be sent as an MLSMessage.
+Likewise, these protections are enforced (via membership verification or AEAD
+decryption) when decoding a PublicMessage or PrivateMessage into an
+AuthenticatedContent object.</t>
+      <t>PrivateMessage represents a signed and encrypted message, with
+protections for both the content of the message and related
+metadata.  PublicMessage represents a message that is only signed,
+and not encrypted.  Applications <bcp14>MUST</bcp14> use PrivateMessage to encrypt
+application messages and <bcp14>SHOULD</bcp14> use PrivateMessage to encode
+handshake messages, but they <bcp14>MAY</bcp14> transmit handshake messages encoded
+as PublicMessage objects in cases where it is necessary for the
+Delivery Service to examine such messages.</t>
+      <sourcecode type="tls-presentation">
+enum {
+    reserved(0),
+    mls10(1),
+    (65535)
+} ProtocolVersion;
+
+enum {
+    reserved(0),
+    application(1),
+    proposal(2),
+    commit(3),
+    (255)
+} ContentType;
+
+enum {
+    reserved(0),
+    member(1),
+    external(2),
+    new_member_proposal(3),
+    new_member_commit(4),
+    (255)
+} SenderType;
+
+struct {
+    SenderType sender_type;
+    select (Sender.sender_type) {
+        case member:
+            uint32 leaf_index;
+        case external:
+            uint32 sender_index;
+        case new_member_commit:
+        case new_member_proposal:
+            struct{};
+    };
+} Sender;
+
+// See the "MLS Wire Formats" IANA registry for values
+uint16 WireFormat;
+
+struct {
+    opaque group_id&lt;V&gt;;
+    uint64 epoch;
+    Sender sender;
+    opaque authenticated_data&lt;V&gt;;
+
+    ContentType content_type;
+    select (FramedContent.content_type) {
+        case application:
+          opaque application_data&lt;V&gt;;
+        case proposal:
+          Proposal proposal;
+        case commit:
+          Commit commit;
+    };
+} FramedContent;
+
+struct {
+    ProtocolVersion version = mls10;
+    WireFormat wire_format;
+    select (MLSMessage.wire_format) {
+        case mls_public_message:
+            PublicMessage public_message;
+        case mls_private_message:
+            PrivateMessage private_message;
+        case mls_welcome:
+            Welcome welcome;
+        case mls_group_info:
+            GroupInfo group_info;
+        case mls_key_package:
+            KeyPackage key_package;
+    };
+} MLSMessage;
+</sourcecode>
+      <t>Messages from senders that aren't in the group are sent as PublicMessage. See
+Sections <xref format="counter" target="external-proposals"/> and <xref format="counter" target="joining-via-external-commits"/> for more details.</t>
+      <t>The following structure is used to fully describe the data transmitted in
+plaintexts or ciphertexts.</t>
+      <sourcecode type="tls-presentation">
+struct {
+    WireFormat wire_format;
+    FramedContent content;
+    FramedContentAuthData auth;
+} AuthenticatedContent;
+</sourcecode>
+      <t>The following figure illustrates how the various structures described in this
+section relate to each other, and the high-level operations used to produce and
+consume them:</t>
+      <figure>
+        <name>Relationships among MLS Objects</name>
+        <artset>
+          <artwork type="svg">
+            <svg xmlns="http://www.w3.org/2000/svg" class="diagram" font-family="monospace" font-size="13px" height="480" text-anchor="middle" version="1.1" viewBox="0 0 520 480" width="520">
+              <path d="M 64,48 L 64,64" fill="none" stroke="black"/>
+              <path d="M 88,144 L 88,176" fill="none" stroke="black"/>
+              <path d="M 88,208 L 88,224" fill="none" stroke="black"/>
+              <path d="M 112,304 L 112,336" fill="none" stroke="black"/>
+              <path d="M 112,368 L 112,416" fill="none" stroke="black"/>
+              <path d="M 160,128 L 160,144" fill="none" stroke="black"/>
+              <path d="M 160,224 L 160,256" fill="none" stroke="black"/>
+              <path d="M 184,48 L 184,96" fill="none" stroke="black"/>
+              <path d="M 184,128 L 184,256" fill="none" stroke="black"/>
+              <path d="M 184,288 L 184,304" fill="none" stroke="black"/>
+              <path d="M 256,304 L 256,336" fill="none" stroke="black"/>
+              <path d="M 256,368 L 256,448" fill="none" stroke="black"/>
+              <path d="M 304,48 L 304,64" fill="none" stroke="black"/>
+              <path d="M 304,400 L 304,416" fill="none" stroke="black"/>
+              <path d="M 336,144 L 336,240" fill="none" stroke="black"/>
+              <path d="M 336,304 L 336,336" fill="none" stroke="black"/>
+              <path d="M 392,400 L 392,416" fill="none" stroke="black"/>
+              <path d="M 480,400 L 480,416" fill="none" stroke="black"/>
+              <path d="M 64,64 L 304,64" fill="none" stroke="black"/>
+              <path d="M 88,144 L 160,144" fill="none" stroke="black"/>
+              <path d="M 336,176 L 352,176" fill="none" stroke="black"/>
+              <path d="M 88,224 L 160,224" fill="none" stroke="black"/>
+              <path d="M 112,304 L 256,304" fill="none" stroke="black"/>
+              <path d="M 336,320 L 352,320" fill="none" stroke="black"/>
+              <path d="M 112,416 L 480,416" fill="none" stroke="black"/>
+              <path d="M 320,128 C 328.83064,128 336,135.16936 336,144" fill="none" stroke="black"/>
+              <path d="M 320,256 C 328.83064,256 336,248.83064 336,240" fill="none" stroke="black"/>
+              <path d="M 320,288 C 328.83064,288 336,295.16936 336,304" fill="none" stroke="black"/>
+              <path d="M 320,352 C 328.83064,352 336,344.83064 336,336" fill="none" stroke="black"/>
+              <polygon class="arrowhead" fill="black" points="264,448 252,442.4 252,453.6" transform="rotate(90,256,448)"/>
+              <polygon class="arrowhead" fill="black" points="264,336 252,330.4 252,341.6" transform="rotate(90,256,336)"/>
+              <polygon class="arrowhead" fill="black" points="192,256 180,250.4 180,261.6" transform="rotate(90,184,256)"/>
+              <polygon class="arrowhead" fill="black" points="192,96 180,90.4 180,101.6" transform="rotate(90,184,96)"/>
+              <polygon class="arrowhead" fill="black" points="168,256 156,250.4 156,261.6" transform="rotate(90,160,256)"/>
+              <polygon class="arrowhead" fill="black" points="120,336 108,330.4 108,341.6" transform="rotate(90,112,336)"/>
+              <polygon class="arrowhead" fill="black" points="96,176 84,170.4 84,181.6" transform="rotate(90,88,176)"/>
+              <g class="text">
+                <text x="68" y="36">Proposal</text>
+                <text x="188" y="36">Commit</text>
+                <text x="296" y="36">Application</text>
+                <text x="364" y="36">Data</text>
+                <text x="176" y="116">FramedContent</text>
+                <text x="404" y="180">Asymmetric</text>
+                <text x="88" y="196">FramedContentAuthData</text>
+                <text x="380" y="196">Sign</text>
+                <text x="408" y="196">/</text>
+                <text x="444" y="196">Verify</text>
+                <text x="188" y="276">AuthenticatedContent</text>
+                <text x="400" y="324">Symmetric</text>
+                <text x="392" y="340">Protect</text>
+                <text x="432" y="340">/</text>
+                <text x="480" y="340">Unprotect</text>
+                <text x="112" y="356">PublicMessage</text>
+                <text x="252" y="356">PrivateMessage</text>
+                <text x="304" y="388">Welcome</text>
+                <text x="388" y="388">KeyPackage</text>
+                <text x="480" y="388">GroupInfo</text>
+                <text x="260" y="468">MLSMessage</text>
+              </g>
+            </svg>
+          </artwork>
+          <artwork type="ascii-art">
+    Proposal        Commit     Application Data
+       |              |              |
+       +--------------+--------------+
+                      |
+                      V
+               FramedContent
+                   |  |                -.
+          +--------+  |                  |
+          |           |                  |
+          V           |                  +-- Asymmetric
+FramedContentAuthData |                  |   Sign / Verify
+          |           |                  |
+          +--------+  |                  |
+                   |  |                  |
+                   V  V                -'
+             AuthenticatedContent
+                      |                -.
+             +--------+--------+         |
+             |                 |         +-- Symmetric
+             V                 V         |   Protect / Unprotect
+       PublicMessage    PrivateMessage -'
+             |                 |
+             |                 |  Welcome  KeyPackage  GroupInfo
+             |                 |     |          |          |
+             +-----------------+-----+----------+----------+
+                               |
+                               V
+                           MLSMessage
+</artwork>
+        </artset>
+      </figure>
+      <section anchor="content-authentication">
+        <name>Content Authentication</name>
+        <t>FramedContent is authenticated using the FramedContentAuthData structure.</t>
+        <sourcecode type="tls-presentation">
+struct {
+    ProtocolVersion version = mls10;
+    WireFormat wire_format;
+    FramedContent content;
+    select (FramedContentTBS.content.sender.sender_type) {
+        case member:
+        case new_member_commit:
+            GroupContext context;
+        case external:
+        case new_member_proposal:
+            struct{};
+    };
+} FramedContentTBS;
+
+opaque MAC&lt;V&gt;;
+
+struct {
+    /* SignWithLabel(., "FramedContentTBS", FramedContentTBS) */
+    opaque signature&lt;V&gt;;
+    select (FramedContent.content_type) {
+        case commit:
+            /*
+              MAC(confirmation_key,
+                  GroupContext.confirmed_transcript_hash)
+            */
+            MAC confirmation_tag;
+        case application:
+        case proposal:
+            struct{};
+    };
+} FramedContentAuthData;
+</sourcecode>
+        <t>The signature is computed using <tt>SignWithLabel</tt> with label
+<tt>"FramedContentTBS"</tt> and with a content that covers the message content and
+the wire format that will be used for this message. If the sender's
+<tt>sender_type</tt> is <tt>member</tt>, the content also covers the GroupContext for the
+current epoch so that signatures are specific to a given group and epoch.</t>
+        <t>The sender <bcp14>MUST</bcp14> use the private key corresponding to the following signature key
+depending on the sender's <tt>sender_type</tt>:</t>
+        <ul spacing="normal">
+          <li><tt>member</tt>: The signature key contained in the LeafNode at the index
+indicated by <tt>leaf_index</tt> in the ratchet tree.</li>
+          <li><tt>external</tt>: The signature key at the index
+indicated by <tt>sender_index</tt> in the <tt>external_senders</tt> group context
+extension (see <xref target="external-senders-extension"/>). The
+<tt>content_type</tt> of the message <bcp14>MUST</bcp14> be <tt>proposal</tt> and the <tt>proposal_type</tt> <bcp14>MUST</bcp14> be a value that is allowed for external senders.</li>
+          <li><tt>new_member_commit</tt>: The signature key in the LeafNode in
+  the Commit's path (see <xref target="joining-via-external-commits"/>). The
+  <tt>content_type</tt> of the message <bcp14>MUST</bcp14> be <tt>commit</tt>.</li>
+          <li><tt>new_member_proposal</tt>: The signature key in the LeafNode in
+  the KeyPackage embedded in an external Add proposal. The
+  <tt>content_type</tt> of the message <bcp14>MUST</bcp14> be <tt>proposal</tt> and the
+  <tt>proposal_type</tt> of the Proposal <bcp14>MUST</bcp14> be <tt>add</tt>.</li>
+        </ul>
+        <t>Recipients of an MLSMessage <bcp14>MUST</bcp14> verify the signature with the key depending on
+the <tt>sender_type</tt> of the sender as described above.</t>
+        <t>The confirmation tag value confirms that the members of the group have arrived
+at the same state of the group. A FramedContentAuthData is said to be valid when both
+the <tt>signature</tt> and <tt>confirmation_tag</tt> fields are valid.</t>
+      </section>
+      <section anchor="encoding-and-decoding-a-public-message">
+        <name>Encoding and Decoding a Public Message</name>
+        <t>Messages that are authenticated but not encrypted are encoded using the PublicMessage structure.</t>
+        <sourcecode type="tls-presentation">
+struct {
+    FramedContent content;
+    FramedContentAuthData auth;
+    select (PublicMessage.content.sender.sender_type) {
+        case member:
+            MAC membership_tag;
+        case external:
+        case new_member_commit:
+        case new_member_proposal:
+            struct{};
+    };
+} PublicMessage;
+</sourcecode>
+        <t>The <tt>membership_tag</tt> field in the PublicMessage object authenticates the sender's
+membership in the group. For messages sent by members, it <bcp14>MUST</bcp14> be set to the
+following value:</t>
+        <sourcecode type="tls-presentation">
+struct {
+  FramedContentTBS content_tbs;
+  FramedContentAuthData auth;
+} AuthenticatedContentTBM;
+</sourcecode>
+        <sourcecode type="pseudocode">
+membership_tag = MAC(membership_key, AuthenticatedContentTBM)
+</sourcecode>
+        <t>When decoding a PublicMessage into an AuthenticatedContent,
+the application <bcp14>MUST</bcp14> check <tt>membership_tag</tt> and <bcp14>MUST</bcp14> check that the
+FramedContentAuthData is valid.</t>
+      </section>
+      <section anchor="encoding-and-decoding-a-private-message">
+        <name>Encoding and Decoding a Private Message</name>
+        <t>Authenticated and encrypted messages are encoded using the PrivateMessage structure.</t>
+        <sourcecode type="tls-presentation">
+struct {
+    opaque group_id&lt;V&gt;;
+    uint64 epoch;
+    ContentType content_type;
+    opaque authenticated_data&lt;V&gt;;
+    opaque encrypted_sender_data&lt;V&gt;;
+    opaque ciphertext&lt;V&gt;;
+} PrivateMessage;
+</sourcecode>
+        <t><tt>encrypted_sender_data</tt> and <tt>ciphertext</tt> are encrypted using the AEAD function
+specified by the cipher suite in use, using the SenderData
+and PrivateMessageContent structures as input.</t>
+        <section anchor="content-encryption">
+          <name>Content Encryption</name>
+          <t>Content to be encrypted is encoded in a PrivateMessageContent structure.</t>
+          <sourcecode type="tls-presentation">
+struct {
+    select (PrivateMessage.content_type) {
+        case application:
+          opaque application_data&lt;V&gt;;
+
+        case proposal:
+          Proposal proposal;
+
+        case commit:
+          Commit commit;
+    };
+
+    FramedContentAuthData auth;
+    opaque padding[length_of_padding];
+} PrivateMessageContent;
+</sourcecode>
+          <t>The <tt>padding</tt> field is set by the sender, by first encoding the content (via the
+<tt>select</tt>) and the <tt>auth</tt> field, and then appending the chosen number of zero bytes.
+A receiver identifies the padding field in a plaintext decoded from
+<tt>PrivateMessage.ciphertext</tt> by first decoding the content and the <tt>auth</tt> field;
+then the <tt>padding</tt> field comprises any remaining octets of plaintext.  The
+<tt>padding</tt> field <bcp14>MUST</bcp14> be filled with all zero bytes.  A receiver <bcp14>MUST</bcp14> verify that
+there are no non-zero bytes in the <tt>padding</tt> field, and if this check fails, the
+enclosing PrivateMessage <bcp14>MUST</bcp14> be rejected as malformed.  This check ensures that
+the padding process is deterministic, so that, for example, padding cannot be
+used as a covert channel.</t>
+          <t>In the MLS key schedule, the sender creates two distinct key ratchets for
+handshake and application messages for each member of the group. When encrypting
+a message, the sender looks at the ratchets it derived for its own member and
+chooses an unused generation from either the handshake ratchet or the application ratchet,
+depending on the content type of the message. This generation of the ratchet is
+used to derive a provisional nonce and key.</t>
+          <t>Before use in the encryption operation, the nonce is XORed with a fresh random
+value to guard against reuse.  Because the key schedule generates nonces
+deterministically, a client <bcp14>MUST</bcp14> keep persistent state as to where in the key
+schedule it is; if this persistent state is lost or corrupted, a client might
+reuse a generation that has already been used, causing reuse of a key/nonce pair.</t>
+          <t>To avoid this situation, the sender of a message <bcp14>MUST</bcp14> generate a fresh random
+four-byte "reuse guard" value and XOR it with the first four bytes of the nonce
+from the key schedule before using the nonce for encryption.  The sender <bcp14>MUST</bcp14>
+include the reuse guard in the <tt>reuse_guard</tt> field of the sender data object, so
+that the recipient of the message can use it to compute the nonce to be used for
+decryption.</t>
+          <artwork type="ascii-art">
++-+-+-+-+---------...---+
+|   Key Schedule Nonce  |
++-+-+-+-+---------...---+
+           XOR
++-+-+-+-+---------...---+
+| Guard |       0       |
++-+-+-+-+---------...---+
+           ===
++-+-+-+-+---------...---+
+| Encrypt/Decrypt Nonce |
++-+-+-+-+---------...---+
+</artwork>
+          <t>The Additional Authenticated Data (AAD) input to the encryption
+contains an object of the following form, with the values used to
+identify the key and nonce:</t>
+          <sourcecode type="tls-presentation">
+struct {
+    opaque group_id&lt;V&gt;;
+    uint64 epoch;
+    ContentType content_type;
+    opaque authenticated_data&lt;V&gt;;
+} PrivateContentAAD;
+</sourcecode>
+          <t>When decoding a PrivateMessageContent, the application <bcp14>MUST</bcp14> check that the
+FramedContentAuthData is valid.</t>
+          <t>It is up to the application to decide what <tt>authenticated_data</tt> to provide and
+how much padding to add to a given message (if any).  The overall size of the
+AAD and ciphertext <bcp14>MUST</bcp14> fit within the limits established for the group's AEAD
+algorithm in <xref target="I-D.irtf-cfrg-aead-limits"/>.</t>
+        </section>
+        <section anchor="sender-data-encryption">
+          <name>Sender Data Encryption</name>
+          <t>The "sender data" used to look up the key for content encryption is
+encrypted with the cipher suite's AEAD with a key and nonce derived from both the
+<tt>sender_data_secret</tt> and a sample of the encrypted content. Before being
+encrypted, the sender data is encoded as an object of the following form:</t>
+          <sourcecode type="tls-presentation">
+struct {
+    uint32 leaf_index;
+    uint32 generation;
+    opaque reuse_guard[4];
+} SenderData;
+</sourcecode>
+          <t>When constructing a SenderData object from a Sender object, the sender <bcp14>MUST</bcp14> verify
+Sender.sender_type is <tt>member</tt> and use Sender.leaf_index for
+SenderData.leaf_index.</t>
+          <t>The <tt>reuse_guard</tt> field contains a fresh random value used to avoid nonce reuse
+in the case of state loss or corruption, as described in <xref target="content-encryption"/>.</t>
+          <t>The key and nonce provided to the AEAD are computed as the KDF of the first
+<tt>KDF.Nh</tt> bytes of the ciphertext generated in the previous section. If the
+length of the ciphertext is less than <tt>KDF.Nh</tt>, the whole ciphertext is used.
+In pseudocode, the key and nonce are derived as:</t>
+          <sourcecode type="pseudocode">
+ciphertext_sample = ciphertext[0..KDF.Nh-1]
+
+sender_data_key = ExpandWithLabel(sender_data_secret, "key",
+                      ciphertext_sample, AEAD.Nk)
+sender_data_nonce = ExpandWithLabel(sender_data_secret, "nonce",
+                      ciphertext_sample, AEAD.Nn)
+</sourcecode>
+          <t>The AAD for the SenderData ciphertext is the
+first three fields of PrivateMessage:</t>
+          <sourcecode type="tls-presentation">
+struct {
+    opaque group_id&lt;V&gt;;
+    uint64 epoch;
+    ContentType content_type;
+} SenderDataAAD;
+</sourcecode>
+          <t>When parsing a SenderData struct as part of message decryption, the recipient
+<bcp14>MUST</bcp14> verify that the leaf index indicated in the <tt>leaf_index</tt> field identifies a
+non-blank node.</t>
+        </section>
+      </section>
+    </section>
+    <section anchor="ratchet-tree-operations">
+      <name>Ratchet Tree Operations</name>
+      <t>The ratchet tree for an epoch describes the membership of a group in that epoch,
+providing public key encryption (HPKE) keys that can be used to encrypt to subsets of
+the group as well as information to authenticate the members.  In order to
+reflect changes to the membership of the group from one epoch to the next,
+corresponding changes are made to the ratchet tree.  In this section, we
+describe the content of the tree and the required operations.</t>
+      <section anchor="parent-node-contents">
+        <name>Parent Node Contents</name>
+        <t>As discussed in <xref target="ratchet-tree-nodes"/>, the nodes of a ratchet tree contain
+several types of data describing individual members (for leaf nodes) or
+subgroups of the group (for parent nodes).  Parent nodes are simpler:</t>
+        <sourcecode type="tls-presentation">
+struct {
+    HPKEPublicKey encryption_key;
+    opaque parent_hash&lt;V&gt;;
+    uint32 unmerged_leaves&lt;V&gt;;
+} ParentNode;
+</sourcecode>
+        <t>The <tt>encryption_key</tt> field contains an HPKE public key whose private key is held only
+by the members at the leaves among its descendants.  The <tt>parent_hash</tt> field
+contains a hash of this node's parent node, as described in <xref target="parent-hashes"/>.
+The <tt>unmerged_leaves</tt> field lists the leaves under this parent node that are
+unmerged, according to their indices among all the leaves in the tree.  The
+entries in the <tt>unmerged_leaves</tt> vector <bcp14>MUST</bcp14> be sorted in increasing order.</t>
+      </section>
+      <section anchor="leaf-node-contents">
+        <name>Leaf Node Contents</name>
+        <t>A leaf node in the tree describes all the details of an individual client's
+appearance in the group, signed by that client. It is also used in client
+KeyPackage objects to store the information that will be needed to add a
+client to a group.</t>
+        <sourcecode type="tls-presentation">
+enum {
+    reserved(0),
+    key_package(1),
+    update(2),
+    commit(3),
+    (255)
+} LeafNodeSource;
+
+struct {
+    ProtocolVersion versions&lt;V&gt;;
+    CipherSuite cipher_suites&lt;V&gt;;
+    ExtensionType extensions&lt;V&gt;;
+    ProposalType proposals&lt;V&gt;;
+    CredentialType credentials&lt;V&gt;;
+} Capabilities;
+
+struct {
+    uint64 not_before;
+    uint64 not_after;
+} Lifetime;
+
+// See the "MLS Extension Types" IANA registry for values
+uint16 ExtensionType;
+
+struct {
+    ExtensionType extension_type;
+    opaque extension_data&lt;V&gt;;
+} Extension;
+
+struct {
+    HPKEPublicKey encryption_key;
+    SignaturePublicKey signature_key;
+    Credential credential;
+    Capabilities capabilities;
+
+    LeafNodeSource leaf_node_source;
+    select (LeafNode.leaf_node_source) {
+        case key_package:
+            Lifetime lifetime;
+
+        case update:
+            struct{};
+
+        case commit:
+            opaque parent_hash&lt;V&gt;;
+    };
+
+    Extension extensions&lt;V&gt;;
+    /* SignWithLabel(., "LeafNodeTBS", LeafNodeTBS) */
+    opaque signature&lt;V&gt;;
+} LeafNode;
+
+struct {
+    HPKEPublicKey encryption_key;
+    SignaturePublicKey signature_key;
+    Credential credential;
+    Capabilities capabilities;
+
+    LeafNodeSource leaf_node_source;
+    select (LeafNodeTBS.leaf_node_source) {
+        case key_package:
+            Lifetime lifetime;
+
+        case update:
+            struct{};
+
+        case commit:
+            opaque parent_hash&lt;V&gt;;
+    };
+
+    Extension extensions&lt;V&gt;;
+
+    select (LeafNodeTBS.leaf_node_source) {
+        case key_package:
+            struct{};
+
+        case update:
+            opaque group_id&lt;V&gt;;
+            uint32 leaf_index;
+
+        case commit:
+            opaque group_id&lt;V&gt;;
+            uint32 leaf_index;
+    };
+} LeafNodeTBS;
+</sourcecode>
+        <t>The <tt>encryption_key</tt> field contains an HPKE public key whose private key is held only
+by the member occupying this leaf (or in the case of a LeafNode in a KeyPackage
+object, the issuer of the KeyPackage). The <tt>signature_key</tt> field contains the
+member's public signing key. The <tt>credential</tt> field contains information
+authenticating both the member's identity and the provided signing key, as
+described in <xref target="credentials"/>.</t>
+        <t>The <tt>capabilities</tt> field indicates the protocol features that the client
+supports, including protocol versions, cipher suites, credential types,
+non-default proposal types, and non-default extension types.  The following
+proposal and extension types are considered "default" and <bcp14>MUST NOT</bcp14> be
+listed:</t>
+        <ul spacing="normal">
+          <li>
+            <t>Proposal types:
+            </t>
+            <ul spacing="normal">
+              <li>0x0001 - <tt>add</tt></li>
+              <li>0x0002 - <tt>update</tt></li>
+              <li>0x0003 - <tt>remove</tt></li>
+              <li>0x0004 - <tt>psk</tt></li>
+              <li>0x0005 - <tt>reinit</tt></li>
+              <li>0x0006 - <tt>external_init</tt></li>
+              <li>0x0007 - <tt>group_context_extensions</tt></li>
+            </ul>
+          </li>
+          <li>
+            <t>Extension types:
+            </t>
+            <ul spacing="normal">
+              <li>0x0001 - <tt>application_id</tt></li>
+              <li>0x0002 - <tt>ratchet_tree</tt></li>
+              <li>0x0003 - <tt>required_capabilities</tt></li>
+              <li>0x0004 - <tt>external_pub</tt></li>
+              <li>0x0005 - <tt>external_senders</tt></li>
+            </ul>
+          </li>
+        </ul>
+        <t>There are no default values for the other fields of a capabilities object.  The
+client <bcp14>MUST</bcp14> list all values for the respective parameters that it supports.</t>
+        <t>The types of any non-default extensions that appear in the <tt>extensions</tt> field of a LeafNode
+<bcp14>MUST</bcp14> be included in the <tt>extensions</tt> field of the <tt>capabilities</tt> field, and the
+credential type used in the LeafNode <bcp14>MUST</bcp14> be included in the <tt>credentials</tt> field
+of the <tt>capabilities</tt> field.</t>
+        <t>As discussed in <xref target="extensibility"/>, unknown values
+in <tt>capabilities</tt> <bcp14>MUST</bcp14> be ignored, and the creator of a <tt>capabilities</tt> field
+<bcp14>SHOULD</bcp14> include some random GREASE values to help ensure that other clients correctly
+ignore unknown values.</t>
+        <t>The <tt>leaf_node_source</tt> field indicates how this LeafNode came to be added to the
+tree.  This signal tells other members of the group whether the leaf node is
+required to have a <tt>lifetime</tt> or <tt>parent_hash</tt>, and whether the <tt>group_id</tt> is
+added as context to the signature.
+These fields are included selectively because the client creating a LeafNode is
+not always able to compute all of them.
+For example, a KeyPackage is created before the client knows which group it will
+be used with, so its signature can't bind to a <tt>group_id</tt>.</t>
+        <t>In the case where the leaf was added to the tree based on a pre-published
+KeyPackage, the <tt>lifetime</tt> field represents the times between which clients will
+consider a LeafNode valid.  These times are represented as absolute times,
+measured in seconds since the Unix epoch (1970-01-01T00:00:00Z).  Applications
+<bcp14>MUST</bcp14> define a maximum total lifetime that is acceptable for a LeafNode, and
+reject any LeafNode where the total lifetime is longer than this duration. In
+order to avoid disagreements about whether a LeafNode has a valid lifetime, the
+clients in a group <bcp14>SHOULD</bcp14> maintain time synchronization (e.g., using the Network
+Time Protocol <xref target="RFC5905"/>).</t>
+        <t>In the case where the leaf node was inserted into the tree via a Commit message,
+the <tt>parent_hash</tt> field contains the parent hash for this leaf node (see
+<xref target="parent-hashes"/>).</t>
+        <t>The LeafNodeTBS structure covers the fields above the signature in the LeafNode.
+In addition, when the leaf node was created in the context of a group (the
+<tt>update</tt> and <tt>commit</tt> cases), the group ID of the group is added as context to the
+signature.</t>
+        <t>LeafNode objects stored in the group's ratchet tree
+are updated according to the evolution of the tree. Each modification of
+LeafNode content <bcp14>MUST</bcp14> be reflected by a change in its signature. This allows other
+members to verify the validity of the LeafNode at any time, particularly in the
+case of a newcomer joining the group.</t>
+      </section>
+      <section anchor="leaf-node-validation">
+        <name>Leaf Node Validation</name>
+        <t>The validity of a LeafNode needs to be verified at the following stages:</t>
+        <ul spacing="normal">
+          <li>When a LeafNode is downloaded in a KeyPackage, before it is used
+to add the client to the group</li>
+          <li>When a LeafNode is received by a group member in an Add, Update, or Commit
+message</li>
+          <li>When a client validates a ratchet tree, e.g., when joining a group or after
+processing a Commit</li>
+        </ul>
+        <t>The client verifies the validity of a LeafNode using the following steps:</t>
+        <ul spacing="normal">
+          <li>Verify that the credential in the LeafNode is valid, as described in
+<xref target="credential-validation"/>.</li>
+          <li>Verify that the signature on the LeafNode is valid using <tt>signature_key</tt>.</li>
+          <li>Verify that the LeafNode is compatible with the group's parameters.  If the
+GroupContext has a <tt>required_capabilities</tt> extension, then the required
+extensions, proposals, and credential types <bcp14>MUST</bcp14> be listed in the LeafNode's
+<tt>capabilities</tt> field.</li>
+          <li>Verify that the credential type is supported by all members of the group, as
+specified by the <tt>capabilities</tt> field of each member's LeafNode, and that the
+<tt>capabilities</tt> field of this LeafNode indicates support for all the credential
+types currently in use by other members.</li>
+          <li>
+            <t>Verify the <tt>lifetime</tt> field:
+            </t>
+            <ul spacing="normal">
+              <li>If the LeafNode appears in a message being sent by the client, e.g., a
+Proposal or a Commit, then the client <bcp14>MUST</bcp14> verify that the current time is within
+the range of the <tt>lifetime</tt> field.</li>
+              <li>If instead the LeafNode appears in a message being received by the client, e.g.,
+a Proposal, a Commit, or a ratchet tree of the group the client is joining, it is
+<bcp14>RECOMMENDED</bcp14> that the client verifies that the current time is within the range
+of the <tt>lifetime</tt> field.  (This check is not mandatory because the LeafNode
+might have expired in the time between when the message was sent and when it
+was received.)</li>
+            </ul>
+          </li>
+          <li>Verify that the extensions in the LeafNode are supported by checking that the
+ID for each extension in the <tt>extensions</tt> field is listed in the
+<tt>capabilities.extensions</tt> field of the LeafNode.</li>
+          <li>
+            <t>Verify the <tt>leaf_node_source</tt> field:
+            </t>
+            <ul spacing="normal">
+              <li>If the LeafNode appears in a KeyPackage, verify that <tt>leaf_node_source</tt> is
+set to <tt>key_package</tt>.</li>
+              <li>If the LeafNode appears in an Update proposal, verify that <tt>leaf_node_source</tt>
+is set to <tt>update</tt> and that <tt>encryption_key</tt> represents a different public
+key than the <tt>encryption_key</tt> in the leaf node being replaced by the Update
+proposal.</li>
+              <li>If the LeafNode appears in the <tt>leaf_node</tt> value of the UpdatePath in
+a Commit, verify that <tt>leaf_node_source</tt> is set to <tt>commit</tt>.</li>
+            </ul>
+          </li>
+          <li>
+            <t>Verify that the following fields are unique among the members of the group:
+            </t>
+            <ul spacing="normal">
+              <li>
+                <tt>signature_key</tt>
+              </li>
+              <li>
+                <tt>encryption_key</tt>
+              </li>
+            </ul>
+          </li>
+        </ul>
+      </section>
+      <section anchor="ratchet-tree-evolution">
+        <name>Ratchet Tree Evolution</name>
+        <t>Whenever a member initiates an epoch change (i.e., commits; see <xref target="commit"/>),
+they may need to refresh the key pairs of their leaf and of the nodes on their
+leaf's direct path in order to maintain forward secrecy and post-compromise
+security.</t>
+        <t>The member initiating the epoch change generates the fresh key pairs using the
+following procedure. The procedure is designed in a way that allows group members to
+efficiently communicate the fresh secret keys to other group members, as
+described in <xref target="update-paths"/>.</t>
+        <t>A member updates the nodes along its direct path as follows:</t>
+        <ul spacing="normal">
+          <li>Blank all the nodes on the direct path from the leaf to the root.</li>
+          <li>Generate a fresh HPKE key pair for the leaf.</li>
+          <li>Generate a sequence of path secrets, one for each node on the leaf's filtered direct
+path, as follows. In this setting, <tt>path_secret[0]</tt> refers to the first parent node
+in the filtered direct path, <tt>path_secret[1]</tt> to the second parent node, and so on.</li>
+        </ul>
+        <sourcecode type="pseudocode">
+path_secret[0] is sampled at random
+path_secret[n] = DeriveSecret(path_secret[n-1], "path")
+</sourcecode>
+        <ul spacing="normal">
+          <li>Compute the sequence of HPKE key pairs <tt>(node_priv,node_pub)</tt>, one for each
+node on the leaf's direct path, as follows.</li>
+        </ul>
+        <sourcecode type="pseudocode">
+node_secret[n] = DeriveSecret(path_secret[n], "node")
+node_priv[n], node_pub[n] = KEM.DeriveKeyPair(node_secret[n])
+</sourcecode>
+        <t>The node secret is derived as a temporary intermediate secret so that each
+secret is only used with one algorithm: The path secret is used as an input to
+DeriveSecret, and the node secret is used as an input to DeriveKeyPair.</t>
+        <t>For example, suppose there is a group with four members, with C an unmerged leaf
+at Z:</t>
+        <figure anchor="evolution-tree">
+          <name>A Full Tree with One Unmerged Leaf</name>
+          <artset>
+            <artwork type="svg">
+              <svg xmlns="http://www.w3.org/2000/svg" class="diagram" font-family="monospace" font-size="13px" height="176" text-anchor="middle" version="1.1" viewBox="0 0 120 176" width="120">
+                <path d="M 56,48 L 56,64" fill="none" stroke="black"/>
+                <path d="M 40,64 L 72,64" fill="none" stroke="black"/>
+                <path d="M 72,64 L 80,80" fill="none" stroke="black"/>
+                <path d="M 32,80 L 40,64" fill="none" stroke="black"/>
+                <g class="text">
+                  <text x="56" y="36">Y</text>
+                  <text x="24" y="100">X</text>
+                  <text x="100" y="100">Z[C]</text>
+                  <text x="16" y="116">/</text>
+                  <text x="32" y="116">\</text>
+                  <text x="80" y="116">/</text>
+                  <text x="96" y="116">\</text>
+                  <text x="8" y="132">A</text>
+                  <text x="40" y="132">B</text>
+                  <text x="72" y="132">C</text>
+                  <text x="104" y="132">D</text>
+                  <text x="8" y="164">0</text>
+                  <text x="40" y="164">1</text>
+                  <text x="72" y="164">2</text>
+                  <text x="104" y="164">3</text>
+                </g>
+              </svg>
+            </artwork>
+            <artwork type="ascii-art">
+      Y
+      |
+    .-+-.
+   /     \
+  X       Z[C]
+ / \     / \
+A   B   C   D
+
+0   1   2   3
+</artwork>
+          </artset>
+        </figure>
+        <t>If member B subsequently generates an UpdatePath based on a secret
+"leaf_secret", then it would generate the following sequence
+of path secrets:</t>
+        <figure>
+          <name>Derivation of Ratchet Tree Keys along a Direct Path</name>
+          <artset>
+            <artwork type="svg">
+              <svg xmlns="http://www.w3.org/2000/svg" class="diagram" font-family="monospace" font-size="13px" height="272" text-anchor="middle" version="1.1" viewBox="0 0 560 272" width="560">
+                <path d="M 48,64 L 48,96" fill="none" stroke="black"/>
+                <path d="M 48,144 L 48,176" fill="none" stroke="black"/>
+                <path d="M 128,32 L 152,32" fill="none" stroke="black"/>
+                <path d="M 288,32 L 344,32" fill="none" stroke="black"/>
+                <path d="M 128,112 L 152,112" fill="none" stroke="black"/>
+                <path d="M 288,112 L 344,112" fill="none" stroke="black"/>
+                <path d="M 104,192 L 152,192" fill="none" stroke="black"/>
+                <path d="M 304,192 L 344,192" fill="none" stroke="black"/>
+                <path d="M 368,224 L 416,224" fill="none" stroke="black"/>
+                <path d="M 448,224 L 496,224" fill="none" stroke="black"/>
+                <path d="M 368,224 C 359.16936,224 352,216.83064 352,208" fill="none" stroke="black"/>
+                <path d="M 416,224 C 424.83064,224 432,231.16936 432,240" fill="none" stroke="black"/>
+                <path d="M 448,224 C 439.16936,224 432,231.16936 432,240" fill="none" stroke="black"/>
+                <path d="M 496,224 C 504.83064,224 512,216.83064 512,208" fill="none" stroke="black"/>
+                <polygon class="arrowhead" fill="black" points="352,192 340,186.4 340,197.6" transform="rotate(0,344,192)"/>
+                <polygon class="arrowhead" fill="black" points="352,112 340,106.4 340,117.6" transform="rotate(0,344,112)"/>
+                <polygon class="arrowhead" fill="black" points="352,32 340,26.4 340,37.6" transform="rotate(0,344,32)"/>
+                <polygon class="arrowhead" fill="black" points="160,192 148,186.4 148,197.6" transform="rotate(0,152,192)"/>
+                <polygon class="arrowhead" fill="black" points="160,112 148,106.4 148,117.6" transform="rotate(0,152,112)"/>
+                <polygon class="arrowhead" fill="black" points="160,32 148,26.4 148,37.6" transform="rotate(0,152,32)"/>
+                <polygon class="arrowhead" fill="black" points="56,144 44,138.4 44,149.6" transform="rotate(270,48,144)"/>
+                <polygon class="arrowhead" fill="black" points="56,64 44,58.4 44,69.6" transform="rotate(270,48,64)"/>
+                <g class="text">
+                  <text x="60" y="36">path_secret[1]</text>
+                  <text x="220" y="36">node_secret[1]</text>
+                  <text x="408" y="36">node_priv[1],</text>
+                  <text x="512" y="36">node_pub[1]</text>
+                  <text x="60" y="116">path_secret[0]</text>
+                  <text x="220" y="116">node_secret[0]</text>
+                  <text x="408" y="116">node_priv[0],</text>
+                  <text x="512" y="116">node_pub[0]</text>
+                  <text x="48" y="196">leaf_secret</text>
+                  <text x="228" y="196">leaf_node_secret</text>
+                  <text x="396" y="196">leaf_priv,</text>
+                  <text x="476" y="196">leaf_pub</text>
+                  <text x="432" y="260">leaf_node</text>
+                </g>
+              </svg>
+            </artwork>
+            <artwork type="ascii-art">
+path_secret[1] ---&gt; node_secret[1] -------&gt; node_priv[1], node_pub[1]
+
+     ^
+     |
+     |
+path_secret[0] ---&gt; node_secret[0] -------&gt; node_priv[0], node_pub[0]
+
+     ^
+     |
+     |
+leaf_secret ------&gt; leaf_node_secret --+--&gt; leaf_priv, leaf_pub
+                                           |                   |
+                                            '-------. .-------'
+                                                     |
+                                                 leaf_node
+</artwork>
+          </artset>
+        </figure>
+        <t>After applying the UpdatePath, the tree will have the following structure:</t>
+        <figure>
+          <name>Placement of Keys in a Ratchet Tree</name>
+          <artset>
+            <artwork type="svg">
+              <svg xmlns="http://www.w3.org/2000/svg" class="diagram" font-family="monospace" font-size="13px" height="192" text-anchor="middle" version="1.1" viewBox="0 0 256 192" width="256">
+                <path d="M 176,144 L 176,160" fill="none" stroke="black"/>
+                <path d="M 192,48 L 192,64" fill="none" stroke="black"/>
+                <path d="M 112,32 L 176,32" fill="none" stroke="black"/>
+                <path d="M 176,64 L 208,64" fill="none" stroke="black"/>
+                <path d="M 112,96 L 144,96" fill="none" stroke="black"/>
+                <path d="M 88,160 L 176,160" fill="none" stroke="black"/>
+                <path d="M 208,64 L 216,80" fill="none" stroke="black"/>
+                <path d="M 168,80 L 176,64" fill="none" stroke="black"/>
+                <polygon class="arrowhead" fill="black" points="184,144 172,138.4 172,149.6" transform="rotate(270,176,144)"/>
+                <polygon class="arrowhead" fill="black" points="184,32 172,26.4 172,37.6" transform="rotate(0,176,32)"/>
+                <polygon class="arrowhead" fill="black" points="152,96 140,90.4 140,101.6" transform="rotate(0,144,96)"/>
+                <g class="text">
+                  <text x="52" y="36">node_priv[1]</text>
+                  <text x="196" y="36">Y'</text>
+                  <text x="52" y="100">node_priv[0]</text>
+                  <text x="164" y="100">X'</text>
+                  <text x="236" y="100">Z[C]</text>
+                  <text x="152" y="116">/</text>
+                  <text x="168" y="116">\</text>
+                  <text x="216" y="116">/</text>
+                  <text x="232" y="116">\</text>
+                  <text x="144" y="132">A</text>
+                  <text x="176" y="132">B</text>
+                  <text x="208" y="132">C</text>
+                  <text x="240" y="132">D</text>
+                  <text x="40" y="164">leaf_priv</text>
+                  <text x="144" y="180">0</text>
+                  <text x="176" y="180">1</text>
+                  <text x="208" y="180">2</text>
+                  <text x="240" y="180">3</text>
+                </g>
+              </svg>
+            </artwork>
+            <artwork type="ascii-art">
+node_priv[1] --------&gt; Y'
+                       |
+                     .-+-.
+                    /     \
+node_priv[0] ----&gt; X'      Z[C]
+                  / \     / \
+                 A   B   C   D
+                     ^
+leaf_priv -----------+
+                 0   1   2   3
+</artwork>
+          </artset>
+        </figure>
+      </section>
+      <section anchor="synchronizing-views-of-the-tree">
+        <name>Synchronizing Views of the Tree</name>
+        <t>After generating fresh key material and applying it to update their
+local tree state as described in <xref target="ratchet-tree-evolution"/>, the
+generator broadcasts
+this update to other members of the group in a Commit message, who
+apply it to keep their local views of the tree in
+sync with the sender's.  More specifically, when a member commits a change to
+the tree (e.g., to add or remove a member), it transmits an UpdatePath
+containing a set of public keys and encrypted path secrets
+for intermediate nodes in the filtered direct path of its leaf. The
+other members of the group use these values to update
+their view of the tree, aligning their copy of the tree to the
+sender's.</t>
+        <t>An UpdatePath contains
+the following information for each node in the filtered direct path of the
+sender's leaf, including the root:</t>
+        <ul spacing="normal">
+          <li>The public key for the node</li>
+          <li>One or more encrypted copies of the path secret corresponding to
+the node</li>
+        </ul>
+        <t>The path secret value for a given node is encrypted to the subtree
+rooted at the parent's non-updated child, i.e., the child
+on the copath of the sender's leaf node.
+There is one encryption of the path secret to each public key in the resolution
+of the non-updated child.</t>
+        <t>A member of the group <em>updates their direct path</em> by computing new values for
+their leaf node and the nodes along their filtered direct path as follows:</t>
+        <ol spacing="normal" type="1">
+          <li>Blank all nodes along the direct path of the sender's leaf.</li>
+          <li>
+            <t>Compute updated path secrets and public keys for the nodes on the sender's
+filtered direct path.
+            </t>
+            <ul spacing="normal">
+              <li>Generate a sequence of path secrets of the same length as the filtered
+direct path, as defined in <xref target="ratchet-tree-evolution"/>.</li>
+              <li>For each node in the filtered direct path, replace the node's public key
+with the <tt>node_pub[n]</tt> value derived from the corresponding path secret
+<tt>path_secret[n]</tt>.</li>
+            </ul>
+          </li>
+          <li>Compute the new parent hashes for the nodes along the filtered direct path
+and the sender's leaf node.</li>
+          <li>
+            <t>Update the leaf node for the sender.
+            </t>
+            <ul spacing="normal">
+              <li>Set the <tt>leaf_node_source</tt> to <tt>commit</tt>.</li>
+              <li>Set the <tt>encryption_key</tt> to the public key of a freshly sampled key pair.</li>
+              <li>Set the parent hash to the parent hash for the leaf.</li>
+              <li>Re-sign the leaf node with its new contents.</li>
+            </ul>
+          </li>
+        </ol>
+        <t>Since the new leaf node effectively updates an existing leaf node in the group,
+it <bcp14>MUST</bcp14> adhere to the same restrictions as LeafNodes used in Update proposals
+(aside from <tt>leaf_node_source</tt>). The application <bcp14>MAY</bcp14> specify other changes to
+the leaf node, e.g., providing a new signature key, updated capabilities, or
+different extensions.</t>
+        <t>The member then <em>encrypts path secrets to the group</em>.  For each node in the
+member's filtered direct path, the member takes the following steps:</t>
+        <ol spacing="normal" type="1">
+          <li>Compute the resolution of the node's child that is on the copath of the
+sender (the child that is not in the direct path of the sender).  Any new
+member (from an Add proposal) added in the same Commit <bcp14>MUST</bcp14> be excluded from
+this resolution.</li>
+          <li>For each node in the resolution, encrypt the path secret for the direct
+path node using the public key of the resolution node, as defined in
+<xref target="update-paths"/>.</li>
+        </ol>
+        <t>The recipient of an UpdatePath performs the corresponding steps. First, the
+recipient <em>merges UpdatePath into the tree</em>:</t>
+        <ol spacing="normal" type="1">
+          <li>Blank all nodes on the direct path of the sender's leaf.</li>
+          <li>
+            <t>For all nodes on the filtered direct path of the sender's leaf,
+            </t>
+            <ul spacing="normal">
+              <li>Set the public key to the public key in the UpdatePath.</li>
+              <li>Set the list of unmerged leaves to the empty list.</li>
+            </ul>
+          </li>
+          <li>
+            <t>Compute parent hashes for the nodes in the sender's filtered direct path,
+and verify that the <tt>parent_hash</tt> field of the leaf node matches the parent
+hash for the first node in its filtered direct path.
+            </t>
+            <ul spacing="normal">
+              <li>Note that these hashes are computed from root to leaf, so that
+each hash incorporates all the non-blank nodes above it. The root node
+always has a zero-length hash for its parent hash.</li>
+            </ul>
+          </li>
+        </ol>
+        <t>Second, the recipient <em>decrypts the path secrets</em>:</t>
+        <ol spacing="normal" type="1">
+          <li>Identify a node in the filtered direct path for which the recipient
+is in the subtree of the non-updated child.</li>
+          <li>Identify a node in the resolution of the copath node for
+which the recipient has a private key.</li>
+          <li>Decrypt the path secret for the parent of the copath node using
+the private key from the resolution node.</li>
+          <li>Derive path secrets for ancestors of that node in the sender's filtered
+direct path using the algorithm described above.</li>
+          <li>Derive the node secrets and node key pairs from the path secrets.</li>
+          <li>Verify that the derived public keys are the same as the corresponding public
+keys sent in the UpdatePath.</li>
+          <li>Store the derived private keys in the corresponding ratchet tree nodes.</li>
+        </ol>
+        <t>For example, in order to communicate the example update described in
+<xref target="ratchet-tree-evolution"/>, the member at node B would transmit the following
+values:</t>
+        <table>
+          <thead>
+            <tr>
+              <th align="left">Public Key</th>
+              <th align="left">Ciphertext(s)</th>
+            </tr>
+          </thead>
+          <tbody>
+            <tr>
+              <td align="left">
+                <tt>node_pub[1]</tt>
+              </td>
+              <td align="left"><tt>E(pk(Z), path_secret[1])</tt>, <tt>E(pk(C), path_secret[1]</tt>)</td>
+            </tr>
+            <tr>
+              <td align="left">
+                <tt>node_pub[0]</tt>
+              </td>
+              <td align="left">
+                <tt>E(pk(A), path_secret[0])</tt>
+              </td>
+            </tr>
+          </tbody>
+        </table>
+        <t>In this table, the value node_pub[i] represents the public key
+derived from node_secret[i], pk(X) represents the current public key
+of node X, and E(K, S) represents
+the public key encryption of the path secret S to the
+public key K (using HPKE).</t>
+        <t>A recipient at node A would decrypt <tt>E(pk(A), path_secret\[0\])</tt> to obtain
+<tt>path_secret\[0\]</tt>, then use it to derive <tt>path_secret[1]</tt> and the resulting
+node secrets and key pairs.  Thus, A would have the private keys to nodes X'
+and Y', in accordance with the tree invariant.</t>
+        <t>Similarly, a recipient at node D would decrypt <tt>E(pk(Z), path_secret[1])</tt> to
+obtain <tt>path_secret[1]</tt>, then use it to derive the node secret and key pair
+for the node Y'.  As required to maintain the tree invariant, node D does not
+receive the private key for the node X', since X' is not an ancestor of D.</t>
+        <t>After processing the update, each recipient <bcp14>MUST</bcp14> delete outdated key material,
+specifically:</t>
+        <ul spacing="normal">
+          <li>The path secrets and node secrets used to derive each updated node key pair.</li>
+          <li>Each outdated node key pair that was replaced by the update.</li>
+        </ul>
+      </section>
+      <section anchor="update-paths">
+        <name>Update Paths</name>
+        <t>As described in <xref target="commit"/>, each Commit message may optionally contain an
+UpdatePath, with a new LeafNode and set of parent nodes for the sender's
+filtered direct path. For each parent node, the UpdatePath contains a new
+public key and encrypted path secret. The parent nodes are kept in the same
+order as the filtered direct path.</t>
+        <sourcecode type="tls-presentation">
+struct {
+    opaque kem_output&lt;V&gt;;
+    opaque ciphertext&lt;V&gt;;
+} HPKECiphertext;
+
+struct {
+    HPKEPublicKey encryption_key;
+    HPKECiphertext encrypted_path_secret&lt;V&gt;;
+} UpdatePathNode;
+
+struct {
+    LeafNode leaf_node;
+    UpdatePathNode nodes&lt;V&gt;;
+} UpdatePath;
+</sourcecode>
+        <t>For each UpdatePathNode, the resolution of the corresponding copath node <bcp14>MUST</bcp14>
+exclude all new leaf nodes added as part of the current Commit. The length of
+the <tt>encrypted_path_secret</tt> vector <bcp14>MUST</bcp14> be equal to the length of the resolution
+of the copath node (excluding new leaf nodes), with each ciphertext being the
+encryption to the respective resolution node.</t>
+        <t>The HPKECiphertext values are encrypted and decrypted as follows:</t>
+        <sourcecode type="pseudocode">
+(kem_output, ciphertext) =
+  EncryptWithLabel(node_public_key, "UpdatePathNode",
+                   group_context, path_secret)
+
+path_secret =
+  DecryptWithLabel(node_private_key, "UpdatePathNode",
+                   group_context, kem_output, ciphertext)
+</sourcecode>
+        <t>Here <tt>node_public_key</tt> is the public key of the node for which the path secret is
+encrypted, <tt>group_context</tt> is the provisional GroupContext object for
+the group, and the <tt>EncryptWithLabel</tt> function is as defined in
+<xref target="public-key-encryption"/>.</t>
+      </section>
+      <section anchor="adding-and-removing-leaves">
+        <name>Adding and Removing Leaves</name>
+        <t>In addition to the path-based updates to the tree described above, it is also
+necessary to add and remove leaves of the tree in order to reflect changes to
+the membership of the group (see Sections <xref format="counter" target="add"/> and <xref format="counter" target="remove"/>).  Since the tree is
+always full, adding or removing leaves corresponds to increasing or decreasing
+the depth of the tree, resulting in the number of leaves being doubled or
+halved. These operations are also known as <em>extending</em> and <em>truncating</em> the
+tree.</t>
+        <t>Leaves are always added and removed at the right edge of the tree.  When the
+size of the tree needs to be increased, a new blank root node is added, whose
+left subtree is the existing tree and right subtree is a new all-blank subtree.
+This operation is typically done when adding a member to the group.</t>
+        <figure>
+          <name>Extending the Tree to Make Room for a Third Member</name>
+          <artwork type="ascii-art">
+                  _ &lt;-- new blank root                    _
+                __|__                                   __|__
+               /     \                                 /     \
+  X    ===&gt;   X       _ &lt;-- new blank subtree ===&gt;    X       _
+ / \         / \     / \                             / \     / \
+A   B       A   B   _   _                           A   B   C   _
+                                                            ^
+                                                            |
+                                               new member --+
+</artwork>
+        </figure>
+        <t>When the right subtree of the tree no longer has any non-blank nodes, it can be
+safely removed.  The root of the tree and the right subtree are discarded
+(whether or not the root node is blank). The left child of the root becomes the
+new root node, and the left subtree becomes the new tree.  This operation is
+typically done after removing a member from the group.</t>
+        <figure>
+          <name>Cleaning Up after Removing Member C</name>
+          <artwork type="ascii-art">
+               Y                  Y
+             __|__              __|__
+            /     \            /     \
+           X       _   ===&gt;   X       _   ==&gt;   X &lt;-- new root
+          / \     / \        / \     / \       / \
+         A   B   C   _      A   B   _   _     A   B
+                 ^
+                 |
+removed member --+
+</artwork>
+        </figure>
+        <t>Concrete algorithms for these operations on array-based and link-based trees are
+provided in Appendices <xref format="counter" target="array-based-trees"/> and <xref format="counter" target="link-based-trees"/>.  The concrete
+algorithms are non-normative.  An implementation may use any algorithm that
+produces the correct tree in its internal representation.</t>
+      </section>
+      <section anchor="tree-hashes">
+        <name>Tree Hashes</name>
+        <t>MLS hashes the contents of the tree in two ways to authenticate different
+properties of the tree.  <em>Tree hashes</em> are defined in this section, and <em>parent
+hashes</em> are defined in <xref target="parent-hashes"/>.</t>
+        <t>Each node in a ratchet tree has a tree hash that summarizes the subtree below
+that node.  The tree hash of the root is used in the GroupContext to confirm
+that the group agrees on the whole tree.  Tree hashes are computed recursively
+from the leaves up to the root.</t>
+        <figure>
+          <name>Composition of the Tree Hash</name>
+          <artset>
+            <artwork type="svg">
+              <svg xmlns="http://www.w3.org/2000/svg" class="diagram" font-family="monospace" font-size="13px" height="112" text-anchor="middle" version="1.1" viewBox="0 0 128 112" width="128">
+                <path d="M 24,32 L 40,32" fill="none" stroke="black"/>
+                <path d="M 72,48 L 88,80" fill="none" stroke="black"/>
+                <path d="M 40,80 L 56,48" fill="none" stroke="black"/>
+                <polygon class="arrowhead" fill="black" points="80,48 68,42.4 68,53.6" transform="rotate(243.43494882292202,72,48)"/>
+                <polygon class="arrowhead" fill="black" points="64,48 52,42.4 52,53.6" transform="rotate(296.565051177078,56,48)"/>
+                <polygon class="arrowhead" fill="black" points="48,32 36,26.4 36,37.6" transform="rotate(0,40,32)"/>
+                <g class="text">
+                  <text x="8" y="36">P</text>
+                  <text x="72" y="36">th(P)</text>
+                  <text x="24" y="100">th(L)</text>
+                  <text x="104" y="100">th(R)</text>
+                </g>
+              </svg>
+            </artwork>
+            <artwork type="ascii-art">
+P --&gt; th(P)
+      ^ ^
+     /   \
+    /     \
+th(L)     th(R)
+</artwork>
+          </artset>
+        </figure>
+        <t>The tree hash of an individual node is the hash of the node's TreeHashInput
+object, which may contain either a LeafNodeHashInput or a
+ParentNodeHashInput depending on the type of node. LeafNodeHashInput objects
+contain the <tt>leaf_index</tt> and the LeafNode (if any). ParentNodeHashInput
+objects contain the ParentNode (if any) and the tree hash of the node's left
+and right children.  For both parent and leaf nodes, the optional node value
+<bcp14>MUST</bcp14> be absent if the node is blank and present if the node contains a value.</t>
+        <sourcecode type="tls-presentation">
+enum {
+    reserved(0),
+    leaf(1),
+    parent(2),
+    (255)
+} NodeType;
+
+struct {
+  NodeType node_type;
+  select (TreeHashInput.node_type) {
+    case leaf:   LeafNodeHashInput leaf_node;
+    case parent: ParentNodeHashInput parent_node;
+  };
+} TreeHashInput;
+
+struct {
+    uint32 leaf_index;
+    optional&lt;LeafNode&gt; leaf_node;
+} LeafNodeHashInput;
+
+struct {
+    optional&lt;ParentNode&gt; parent_node;
+    opaque left_hash&lt;V&gt;;
+    opaque right_hash&lt;V&gt;;
+} ParentNodeHashInput;
+</sourcecode>
+        <t>The tree hash of an entire tree corresponds to the tree hash of the root node,
+which is computed recursively by starting at the leaf nodes and building up.</t>
+      </section>
+      <section anchor="parent-hashes">
+        <name>Parent Hashes</name>
+        <t>While tree hashes summarize the state of a tree at point in time, parent hashes
+capture information about how keys in the tree were populated.</t>
+        <t>When a client sends a Commit to change a group, it can include an UpdatePath to
+assign new keys to the nodes along its filtered direct path.  When a client
+computes an UpdatePath (as defined in <xref target="synchronizing-views-of-the-tree"/>), it
+computes and signs a parent hash that summarizes the state of the tree after the
+UpdatePath has been applied.  These summaries are constructed in a chain from
+the root to the member's leaf so that the part of the chain closer to the root
+can be overwritten as nodes set in one UpdatePath are reset by a later
+UpdatePath.</t>
+        <figure>
+          <name>Inputs to a Parent Hash</name>
+          <artset>
+            <artwork type="svg">
+              <svg xmlns="http://www.w3.org/2000/svg" class="diagram" font-family="monospace" font-size="13px" height="176" text-anchor="middle" version="1.1" viewBox="0 0 216 176" width="216">
+                <path d="M 112,96 L 128,96" fill="none" stroke="black"/>
+                <path d="M 160,112 L 176,144" fill="none" stroke="black"/>
+                <path d="M 128,144 L 144,112" fill="none" stroke="black"/>
+                <path d="M 160,80 L 176,48" fill="none" stroke="black"/>
+                <polygon class="arrowhead" fill="black" points="168,112 156,106.4 156,117.6" transform="rotate(243.43494882292202,160,112)"/>
+                <polygon class="arrowhead" fill="black" points="168,80 156,74.4 156,85.6" transform="rotate(116.56505117707799,160,80)"/>
+                <polygon class="arrowhead" fill="black" points="136,144 124,138.4 124,149.6" transform="rotate(116.56505117707799,128,144)"/>
+                <polygon class="arrowhead" fill="black" points="136,96 124,90.4 124,101.6" transform="rotate(0,128,96)"/>
+                <g class="text">
+                  <text x="192" y="36">ph(Q)</text>
+                  <text x="52" y="100">P.public_key</text>
+                  <text x="160" y="100">ph(P)</text>
+                  <text x="80" y="164">N.parent_hash</text>
+                  <text x="192" y="164">th(S)</text>
+                </g>
+              </svg>
+            </artwork>
+            <artwork type="ascii-art">
+                     ph(Q)
+                     /
+                    /
+                   V
+P.public_key --&gt; ph(P)
+                 / ^
+                /   \
+               V     \
+   N.parent_hash     th(S)
+</artwork>
+          </artset>
+        </figure>
+        <t>As a result, the signature over the parent hash in each member's leaf
+effectively signs the subtree of the tree that hasn't been changed since that
+leaf was last changed in an UpdatePath.  A new member joining the group uses
+these parent hashes to verify that the parent nodes in the tree were set by
+members of the group, not chosen by an external attacker.  For an example of how
+this works, see <xref target="ph-evolution"/>.</t>
+        <t>Consider a ratchet tree with a non-blank parent node P and children D and S (for
+"parent", "direct path", and "sibling"), with D and P in the direct path of a
+leaf node L (for "leaf"):</t>
+        <figure anchor="parent-hash-nodes">
+          <name>Nodes Involved in a Parent Hash Computation</name>
+          <artwork type="ascii-art">
+         ...
+         /
+        P
+      __|__
+     /     \
+    D       S
+   / \     / \
+ ... ... ... ...
+ /
+L
+</artwork>
+        </figure>
+        <t>The parent hash of P changes whenever an UpdatePath object is applied to
+the ratchet tree along a path from a leaf L traversing node D (and hence also
+P). The new "Parent hash of P (with copath child S)" is obtained by hashing P's
+ParentHashInput struct.</t>
+        <sourcecode type="tls-presentation">
+struct {
+    HPKEPublicKey encryption_key;
+    opaque parent_hash&lt;V&gt;;
+    opaque original_sibling_tree_hash&lt;V&gt;;
+} ParentHashInput;
+</sourcecode>
+        <t>The field <tt>encryption_key</tt> contains the HPKE public key of P. If P is the root,
+then the <tt>parent_hash</tt> field is set to a zero-length octet string. Otherwise,
+<tt>parent_hash</tt> is the parent hash of the next node after P on the filtered
+direct path of the leaf L. This way, P's parent hash fixes
+the new HPKE public key of each non-blank node on the path from P to the root. Note
+that the path from P to the root may contain some blank nodes that are not
+fixed by P's parent hash. However, for each node that has an HPKE key, this key
+is fixed by P's parent hash.</t>
+        <t>Finally, <tt>original_sibling_tree_hash</tt> is the tree hash of S in the ratchet tree
+modified as follows: For each leaf L in <tt>P.unmerged_leaves</tt>, blank L and remove
+it from the <tt>unmerged_leaves</tt> sets of all parent nodes.</t>
+        <t>Observe that <tt>original_sibling_tree_hash</tt> does not change between updates of P.
+This property is crucial for the correctness of the protocol.</t>
+        <t>Note that <tt>original_sibling_tree_hash</tt> is the tree hash of S, not the parent
+hash.  The <tt>parent_hash</tt> field in ParentHashInput captures information about the
+nodes above P. the <tt>original_sibling_tree_hash</tt> captures information about the
+subtree under S that is not being updated (and thus the subtree to which a path
+secret for P would be encrypted according to <xref target="synchronizing-views-of-the-tree"/>).</t>
+        <t>For example, in the following tree:</t>
+        <figure anchor="parent-hash-tree">
+          <name>A Tree Illustrating Parent Hash Computations</name>
+          <artwork type="ascii-art">
+              W [F]
+        ______|_____
+       /             \
+      U               Y [F]
+    __|__           __|__
+   /     \         /     \
+  T       _       _       _
+ / \     / \     / \     / \
+A   B   C   D   E   F   G   _
+</artwork>
+        </figure>
+        <t>With P = W and S = Y, <tt>original_sibling_tree_hash</tt> is the tree hash of the
+following tree:</t>
+        <artwork type="ascii-art">
+      Y
+    __|__
+   /     \
+  _       _
+ / \     / \
+E   _   G   _
+</artwork>
+        <t>Because <tt>W.unmerged_leaves</tt> includes F, F is blanked and removed from
+<tt>Y.unmerged_leaves</tt>.</t>
+        <t>Note that no recomputation is needed if the tree hash of S is unchanged since
+the last time P was updated. This is the case for computing or processing a
+Commit whose UpdatePath traverses P, since the Commit itself resets P. (In
+other words, it is only necessary to recompute the original sibling tree hash
+when validating a group's tree on joining.) More generally, if none of the entries
+in <tt>P.unmerged_leaves</tt> are in the subtree under S (and thus no leaves were blanked),
+then the original tree hash at S is the tree hash of S in the current tree.</t>
+        <t>If it is necessary to recompute the original tree hash of a node, the efficiency
+of recomputation can be improved by caching intermediate tree hashes, to avoid
+recomputing over the subtree when the subtree is included in multiple parent
+hashes.  A subtree hash can be reused as long as the intersection of the
+parent's unmerged leaves with the subtree is the same as in the earlier
+computation.</t>
+        <section anchor="using-parent-hashes">
+          <name>Using Parent Hashes</name>
+          <t>In ParentNode objects and LeafNode objects with <tt>leaf_node_source</tt> set to
+<tt>commit</tt>, the value of the <tt>parent_hash</tt> field is the parent hash of the next
+non-blank parent node above the node in question (the next node in the filtered
+direct path).  Using the node labels in <xref target="parent-hash-nodes"/>, the
+<tt>parent_hash</tt> field of D is equal to the parent hash of P with copath child S.
+This is the case even when the node D is a leaf node.</t>
+          <t>The <tt>parent_hash</tt> field of a LeafNode is signed by the member.  The signature of
+such a LeafNode thus attests to which keys the group member introduced into
+the ratchet tree and to whom the corresponding secret keys were sent, in
+addition to the other contents of the LeafNode. This
+prevents malicious insiders from constructing artificial ratchet trees with a
+node D whose HPKE secret key is known to the insider, yet where the insider isn't
+assigned a leaf in the subtree rooted at D. Indeed, such a ratchet tree would
+violate the tree invariant.</t>
+        </section>
+        <section anchor="verifying-parent-hashes">
+          <name>Verifying Parent Hashes</name>
+          <t>Parent hashes are verified at two points in the protocol: When joining a group
+and when processing a Commit.</t>
+          <t>The parent hash in a node D is valid with respect to a parent node P if the
+following criteria hold.  Here C and S are the children of P (for "child" and
+"sibling"), with C being the child that is on the direct path of D (possibly D
+itself) and S being the other child:</t>
+          <ul spacing="normal">
+            <li>D is a descendant of P in the tree.</li>
+            <li>The <tt>parent_hash</tt> field of D is equal to the parent hash of P with copath
+child S.</li>
+            <li>D is in the resolution of C, and the intersection of P's <tt>unmerged_leaves</tt>
+with the subtree under C is equal to the resolution of C with D removed.</li>
+          </ul>
+          <t>These checks verify that D and P were updated at the same time (in the same
+UpdatePath), and that they were neighbors in the UpdatePath because the nodes in
+between them would have omitted from the filtered direct path.</t>
+          <t>A parent node P is "parent-hash valid" if it can be chained back to a leaf node
+in this way.  That is, if there is leaf node L and a sequence of parent nodes
+P_1, ..., P_N such that P_N = P and each step in the chain is authenticated
+by a parent hash, then L's parent hash is valid with respect to P_1, P_1's parent
+hash is valid with respect to P_2, and so on.</t>
+          <t>When joining a group, the new member <bcp14>MUST</bcp14> authenticate that each non-blank
+parent node P is parent-hash valid.  This can be done "bottom up" by building
+chains up from leaves and verifying that all non-blank parent nodes are covered
+by exactly one such chain, or "top down" by verifying that there is exactly one
+descendant of each non-blank parent node for which the parent node is
+parent-hash valid.</t>
+          <t>When processing a Commit message that includes an UpdatePath, clients <bcp14>MUST</bcp14>
+recompute the expected value of <tt>parent_hash</tt> for the committer's new leaf and
+verify that it matches the <tt>parent_hash</tt> value in the supplied <tt>leaf_node</tt>.
+After being merged into the tree, the nodes in the UpdatePath form a parent-hash
+chain from the committer's leaf to the root.</t>
+        </section>
+      </section>
+    </section>
+    <section anchor="key-schedule">
+      <name>Key Schedule</name>
+      <t>Group keys are derived using the <tt>Extract</tt> and <tt>Expand</tt> functions from the KDF
+for the group's cipher suite, as well as the functions defined below:</t>
+      <sourcecode type="pseudocode">
+ExpandWithLabel(Secret, Label, Context, Length) =
+    KDF.Expand(Secret, KDFLabel, Length)
+
+DeriveSecret(Secret, Label) =
+    ExpandWithLabel(Secret, Label, "", KDF.Nh)
+</sourcecode>
+      <t>Where KDFLabel is specified as:</t>
+      <sourcecode type="tls-presentation">
+struct {
+    uint16 length;
+    opaque label&lt;V&gt;;
+    opaque context&lt;V&gt;;
+} KDFLabel;
+</sourcecode>
+      <t>And its fields are set to:</t>
+      <sourcecode type="pseudocode">
+length = Length;
+label = "MLS 1.0 " + Label;
+context = Context;
+</sourcecode>
+      <t>The value <tt>KDF.Nh</tt> is the size of an output from <tt>KDF.Extract</tt>, in bytes.  In
+the below diagram:</t>
+      <ul spacing="normal">
+        <li>KDF.Extract takes its salt argument from the top and its Input
+Keying Material (IKM) argument from the left.</li>
+        <li>DeriveSecret takes its Secret argument from the incoming arrow.</li>
+        <li><tt>0</tt> represents an all-zero byte string of length <tt>KDF.Nh</tt>.</li>
+      </ul>
+      <t>When processing a handshake message, a client combines the
+following information to derive new epoch secrets:</t>
+      <ul spacing="normal">
+        <li>The init secret from the previous epoch</li>
+        <li>The commit secret for the current epoch</li>
+        <li>The GroupContext object for current epoch</li>
+      </ul>
+      <t>Given these inputs, the derivation of secrets for an epoch
+proceeds as shown in the following diagram:</t>
+      <figure>
+        <name>The MLS Key Schedule</name>
+        <artset>
+          <artwork type="svg">
+            <svg xmlns="http://www.w3.org/2000/svg" class="diagram" font-family="monospace" font-size="13px" height="656" text-anchor="middle" version="1.1" viewBox="0 0 584 656" width="584">
+              <path d="M 216,48 L 216,80" fill="none" stroke="black"/>
+              <path d="M 216,112 L 216,144" fill="none" stroke="black"/>
+              <path d="M 216,176 L 216,208" fill="none" stroke="black"/>
+              <path d="M 216,232 L 216,272" fill="none" stroke="black"/>
+              <path d="M 216,304 L 216,384" fill="none" stroke="black"/>
+              <path d="M 216,416 L 216,448" fill="none" stroke="black"/>
+              <path d="M 216,472 L 216,560" fill="none" stroke="black"/>
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+              <g class="text">
+                <text x="232" y="36">init_secret_[n-1]</text>
+                <text x="88" y="100">commit_secret</text>
+                <text x="224" y="100">KDF.Extract</text>
+                <text x="220" y="164">ExpandWithLabel(.,</text>
+                <text x="336" y="164">"joiner",</text>
+                <text x="448" y="164">GroupContext_[n],</text>
+                <text x="552" y="164">KDF.Nh)</text>
+                <text x="224" y="228">joiner_secret</text>
+                <text x="44" y="292">psk_secret</text>
+                <text x="104" y="292">(or</text>
+                <text x="132" y="292">0)</text>
+                <text x="224" y="292">KDF.Extract</text>
+                <text x="312" y="340">DeriveSecret(.,</text>
+                <text x="420" y="340">"welcome")</text>
+                <text x="256" y="356">=</text>
+                <text x="324" y="356">welcome_secret</text>
+                <text x="220" y="404">ExpandWithLabel(.,</text>
+                <text x="332" y="404">"epoch",</text>
+                <text x="440" y="404">GroupContext_[n],</text>
+                <text x="544" y="404">KDF.Nh)</text>
+                <text x="220" y="468">epoch_secret</text>
+                <text x="312" y="516">DeriveSecret(.,</text>
+                <text x="412" y="516">&lt;label&gt;)</text>
+                <text x="256" y="532">=</text>
+                <text x="300" y="532">&lt;secret&gt;</text>
+                <text x="224" y="580">DeriveSecret(.,</text>
+                <text x="320" y="580">"init")</text>
+                <text x="224" y="644">init_secret_[n]</text>
+              </g>
+            </svg>
+          </artwork>
+          <artwork type="ascii-art">
+                    init_secret_[n-1]
+                          |
+                          |
+                          V
+    commit_secret --&gt; KDF.Extract
+                          |
+                          |
+                          V
+                  ExpandWithLabel(., "joiner", GroupContext_[n], KDF.Nh)
+                          |
+                          |
+                          V
+                     joiner_secret
+                          |
+                          |
+                          V
+psk_secret (or 0) --&gt; KDF.Extract
+                          |
+                          |
+                          +--&gt; DeriveSecret(., "welcome")
+                          |    = welcome_secret
+                          |
+                          V
+                  ExpandWithLabel(., "epoch", GroupContext_[n], KDF.Nh)
+                          |
+                          |
+                          V
+                     epoch_secret
+                          |
+                          |
+                          +--&gt; DeriveSecret(., &lt;label&gt;)
+                          |    = &lt;secret&gt;
+                          |
+                          V
+                    DeriveSecret(., "init")
+                          |
+                          |
+                          V
+                    init_secret_[n]
+</artwork>
+        </artset>
+      </figure>
+      <t>A number of values are derived from the epoch secret for different purposes:</t>
+      <table anchor="epoch-derived-secrets">
+        <name>Epoch-Derived Secrets</name>
+        <thead>
+          <tr>
+            <th align="left">Label</th>
+            <th align="left">Secret</th>
+            <th align="left">Purpose</th>
+          </tr>
+        </thead>
+        <tbody>
+          <tr>
+            <td align="left">"sender data"</td>
+            <td align="left">
+              <tt>sender_data_secret</tt>
+            </td>
+            <td align="left">Deriving keys to encrypt sender data</td>
+          </tr>
+          <tr>
+            <td align="left">"encryption"</td>
+            <td align="left">
+              <tt>encryption_secret</tt>
+            </td>
+            <td align="left">Deriving message encryption keys (via the secret tree)</td>
+          </tr>
+          <tr>
+            <td align="left">"exporter"</td>
+            <td align="left">
+              <tt>exporter_secret</tt>
+            </td>
+            <td align="left">Deriving exported secrets</td>
+          </tr>
+          <tr>
+            <td align="left">"external"</td>
+            <td align="left">
+              <tt>external_secret</tt>
+            </td>
+            <td align="left">Deriving the external init key</td>
+          </tr>
+          <tr>
+            <td align="left">"confirm"</td>
+            <td align="left">
+              <tt>confirmation_key</tt>
+            </td>
+            <td align="left">Computing the confirmation MAC for an epoch</td>
+          </tr>
+          <tr>
+            <td align="left">"membership"</td>
+            <td align="left">
+              <tt>membership_key</tt>
+            </td>
+            <td align="left">Computing the membership MAC for a PublicMessage</td>
+          </tr>
+          <tr>
+            <td align="left">"resumption"</td>
+            <td align="left">
+              <tt>resumption_psk</tt>
+            </td>
+            <td align="left">Proving membership in this epoch (via a PSK injected later)</td>
+          </tr>
+          <tr>
+            <td align="left">"authentication"</td>
+            <td align="left">
+              <tt>epoch_authenticator</tt>
+            </td>
+            <td align="left">Confirming that two clients have the same view of the group</td>
+          </tr>
+        </tbody>
+      </table>
+      <t>The <tt>external_secret</tt> is used to derive an HPKE key pair whose private key is
+held by the entire group:</t>
+      <sourcecode type="pseudocode">
+external_priv, external_pub = KEM.DeriveKeyPair(external_secret)
+</sourcecode>
+      <t>The public key <tt>external_pub</tt> can be published as part of the GroupInfo struct
+in order to allow non-members to join the group using an external Commit.</t>
+      <section anchor="group-context">
+        <name>Group Context</name>
+        <t>Each member of the group maintains a GroupContext object that
+summarizes the state of the group:</t>
+        <sourcecode type="tls-presentation">
+struct {
+    ProtocolVersion version = mls10;
+    CipherSuite cipher_suite;
+    opaque group_id&lt;V&gt;;
+    uint64 epoch;
+    opaque tree_hash&lt;V&gt;;
+    opaque confirmed_transcript_hash&lt;V&gt;;
+    Extension extensions&lt;V&gt;;
+} GroupContext;
+</sourcecode>
+        <t>The fields in this state have the following semantics:</t>
+        <ul spacing="normal">
+          <li>The <tt>cipher_suite</tt> is the cipher suite used by the group.</li>
+          <li>The <tt>group_id</tt> field is an application-defined identifier for the
+group.</li>
+          <li>The <tt>epoch</tt> field represents the current version of the group.</li>
+          <li>The <tt>tree_hash</tt> field contains a commitment to the contents of the
+group's ratchet tree and the credentials for the members of the
+group, as described in <xref target="tree-hashes"/>.</li>
+          <li>The <tt>confirmed_transcript_hash</tt> field contains a running hash over
+the messages that led to this state.</li>
+          <li>The <tt>extensions</tt> field contains the details of any protocol extensions that
+apply to the group.</li>
+        </ul>
+        <t>When a new member is added to the group, an existing member of the
+group provides the new member with a Welcome message.  The Welcome
+message provides the information the new member needs to initialize
+its GroupContext.</t>
+        <t>Different changes to the group will have different effects on the group state.
+These effects are described in their respective subsections of <xref target="proposals"/>.
+The following general rules apply:</t>
+        <ul spacing="normal">
+          <li>The <tt>group_id</tt> field is constant.</li>
+          <li>The <tt>epoch</tt> field increments by one for each Commit message that
+is processed.</li>
+          <li>The <tt>tree_hash</tt> is updated to represent the current tree and
+credentials.</li>
+          <li>The <tt>confirmed_transcript_hash</tt> field is updated with the data for an
+AuthenticatedContent encoding a Commit message, as described below.</li>
+          <li>The <tt>extensions</tt> field changes when a GroupContextExtensions proposal is
+committed.</li>
+        </ul>
+      </section>
+      <section anchor="transcript-hashes">
+        <name>Transcript Hashes</name>
+        <t>The transcript hashes computed in MLS represent a running hash over all Proposal
+and Commit messages that have ever been sent in a group.  Commit messages are
+included directly. Proposal messages are indirectly included via the Commit that
+applied them. Messages of both types are included by hashing the AuthenticatedContent
+object in which they were sent.</t>
+        <t>The transcript hash comprises two individual hashes:</t>
+        <ul spacing="normal">
+          <li>A <tt>confirmed_transcript_hash</tt> that represents a transcript over the whole
+history of Commit messages, up to and including the signature of the most
+recent Commit.</li>
+          <li>An <tt>interim_transcript_hash</tt> that covers the confirmed transcript hash plus
+the <tt>confirmation_tag</tt> of the most recent Commit.</li>
+        </ul>
+        <t>New members compute the interim transcript hash using the <tt>confirmation_tag</tt>
+field of the GroupInfo struct, while existing members can compute it directly.</t>
+        <t>Each Commit message updates these hashes by way of its enclosing
+AuthenticatedContent.  The AuthenticatedContent struct is split into
+ConfirmedTranscriptHashInput and InterimTranscriptHashInput. The former is used to
+update the confirmed transcript hash and the latter is used to update the interim
+transcript hash.</t>
+        <sourcecode type="tls-presentation">
+struct {
+    WireFormat wire_format;
+    FramedContent content; /* with content_type == commit */
+    opaque signature&lt;V&gt;;
+} ConfirmedTranscriptHashInput;
+
+struct {
+    MAC confirmation_tag;
+} InterimTranscriptHashInput;
+</sourcecode>
+        <sourcecode type="pseudocode">
+confirmed_transcript_hash_[0] = ""; /* zero-length octet string */
+interim_transcript_hash_[0] = ""; /* zero-length octet string */
+
+confirmed_transcript_hash_[epoch] =
+    Hash(interim_transcript_hash_[epoch - 1] ||
+        ConfirmedTranscriptHashInput_[epoch]);
+
+interim_transcript_hash_[epoch] =
+    Hash(confirmed_transcript_hash_[epoch] ||
+        InterimTranscriptHashInput_[epoch]);
+</sourcecode>
+        <t>In this notation, <tt>ConfirmedTranscriptHashInput_[epoch]</tt> and
+<tt>InterimTranscriptHashInput_[epoch]</tt> are based on the Commit that initiated the
+epoch with epoch number <tt>epoch.  (Note that the </tt>epoch<tt> field in this
+Commit will be set to </tt>epoch - 1`, since it is sent within the previous epoch.)</t>
+        <t>The transcript hash <tt>ConfirmedTranscriptHashInput_[epoch]</tt> is used as the
+<tt>confirmed_transcript_hash</tt> input to the <tt>confirmation_tag</tt> field for this
+Commit. Each Commit thus confirms the whole transcript of Commits up to that
+point, except for the latest Commit's confirmation tag.</t>
+        <figure>
+          <name>Evolution of the Transcript Hashes through Two Epoch Changes</name>
+          <artset>
+            <artwork type="svg">
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+                  <text x="496" y="132">interim_[N-1]</text>
+                  <text x="80" y="196">Ratchet</text>
+                  <text x="132" y="196">Tree</text>
+                  <text x="296" y="196">wire_format</text>
+                  <text x="64" y="212">Key</text>
+                  <text x="116" y="212">Schedule</text>
+                  <text x="280" y="212">content</text>
+                  <text x="288" y="228">epoch</text>
+                  <text x="320" y="228">=</text>
+                  <text x="344" y="228">N-1</text>
+                  <text x="292" y="244">commit</text>
+                  <text x="288" y="260">signature</text>
+                  <text x="108" y="292">confirmation_key_[N]</text>
+                  <text x="316" y="292">confirmation_tag</text>
+                  <text x="496" y="292">confirmed_[N]</text>
+                  <text x="496" y="372">interim_[N]</text>
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+                  <text x="496" y="692">...</text>
+                </g>
+              </svg>
+            </artwork>
+            <artwork type="ascii-art">
+                                                             ...
+
+                                                              |
+                                                              |
+                                                              V
+                                                     +-----------------+
+                                                     |  interim_[N-1]  |
+                                                     +--------+--------+
+                                                              |
+     .--------------.         +------------------+            |
+    |  Ratchet Tree  |        | wire_format      |            |
+    |  Key Schedule  |&lt;-------+ content          |            |
+     '-------+------'         |   epoch = N-1    +------------+
+             |                |   commit         |            |
+             V                | signature        |            V
+ +------------------------+   +------------------+   +-----------------+
+ |  confirmation_key_[N]  +--&gt;| confirmation_tag |&lt;--+  confirmed_[N]  |
+ +------------------------+   +--------+---------+   +--------+--------+
+                                       |                      |
+                                       |                      V
+                                       |             +-----------------+
+                                       +------------&gt;|   interim_[N]   |
+                                                     +--------+--------+
+                                                              |
+     .--------------.         +------------------+            |
+    |  Ratchet Tree  |        | wire_format      |            |
+    |  Key Schedule  |&lt;-------+ content          |            |
+     '-------+------'         |   epoch = N      +------------+
+             |                |   commit         |            |
+             V                | signature        |            V
+ +------------------------+   +------------------+   +-----------------+
+ | confirmation_key_[N+1] +--&gt;| confirmation_tag |&lt;--+ confirmed_[N+1] |
+ +------------------------+   +--------+---------+   +--------+--------+
+                                       |                      |
+                                       |                      V
+                                       |             +-----------------+
+                                       +------------&gt;|  interim_[N+1]  |
+                                                     +--------+--------+
+                                                              |
+                                                              V
+
+                                                             ...
+</artwork>
+          </artset>
+        </figure>
+      </section>
+      <section anchor="external-initialization">
+        <name>External Initialization</name>
+        <t>In addition to initializing a new epoch via KDF invocations as described above,
+an MLS group can also initialize a new epoch via an asymmetric interaction using
+the external key pair for the previous epoch.  This is done when a new member
+is joining via an external commit.</t>
+        <t>In this process, the joiner sends a new <tt>init_secret</tt> value to the group using
+the HPKE export method.  The joiner then uses that <tt>init_secret</tt> with
+information provided in the GroupInfo and an external Commit to initialize
+their copy of the key schedule for the new epoch.</t>
+        <sourcecode type="pseudocode">
+kem_output, context = SetupBaseS(external_pub, "")
+init_secret = context.export("MLS 1.0 external init secret", KDF.Nh)
+</sourcecode>
+        <t>Members of the group receive the <tt>kem_output</tt> in an ExternalInit proposal and
+perform the corresponding calculation to retrieve the <tt>init_secret</tt> value.</t>
+        <sourcecode type="pseudocode">
+context = SetupBaseR(kem_output, external_priv, "")
+init_secret = context.export("MLS 1.0 external init secret", KDF.Nh)
+</sourcecode>
+      </section>
+      <section anchor="pre-shared-keys">
+        <name>Pre-Shared Keys</name>
+        <t>Groups that already have an out-of-band mechanism to generate
+shared group secrets can inject them into the MLS key schedule to
+incorporate this external entropy in the computation of MLS group secrets.</t>
+        <t>Injecting an external PSK can improve security in the case
+where having a full run of Updates across members is too expensive, or if
+the external group key establishment mechanism provides
+stronger security against classical or quantum adversaries.</t>
+        <t>Note that, as a PSK may have a different lifetime than an Update, it does not
+necessarily provide the same forward secrecy or post-compromise security
+guarantees as a Commit message.  Unlike the key pairs populated in the
+tree by an Update or Commit, which are always freshly generated, PSKs may be
+pre-distributed and stored. This creates the risk that a PSK may be compromised
+in the process of distribution and storage. The security that the group gets
+from injecting a PSK thus depends on both the entropy of the PSK and the risk of
+compromise.  These factors are outside of the scope of this document, but they should
+be considered by application designers relying on PSKs.</t>
+        <t>Each PSK in MLS has a type that designates how it was provisioned.
+External PSKs are provided by the application, while resumption PSKs
+are derived from the MLS key schedule and used in cases where it is
+necessary to authenticate a member's participation in a prior epoch.</t>
+        <t>The injection of one or more PSKs into the key schedule is signaled in two ways:
+Existing members are informed via PreSharedKey proposals covered by a Commit,
+and new members added in the Commit are informed by the GroupSecrets object in the
+Welcome message corresponding to the Commit.  To ensure that existing and new
+members compute the same PSK input to the key schedule, the Commit and
+GroupSecrets objects <bcp14>MUST</bcp14> indicate the same set of PSKs, in the same order.</t>
+        <sourcecode type="tls-presentation">
+enum {
+  reserved(0),
+  external(1),
+  resumption(2),
+  (255)
+} PSKType;
+
+enum {
+  reserved(0),
+  application(1),
+  reinit(2),
+  branch(3),
+  (255)
+} ResumptionPSKUsage;
+
+struct {
+  PSKType psktype;
+  select (PreSharedKeyID.psktype) {
+    case external:
+      opaque psk_id&lt;V&gt;;
+
+    case resumption:
+      ResumptionPSKUsage usage;
+      opaque psk_group_id&lt;V&gt;;
+      uint64 psk_epoch;
+  };
+  opaque psk_nonce&lt;V&gt;;
+} PreSharedKeyID;
+</sourcecode>
+        <t>Each time a client injects a PSK into a group, the <tt>psk_nonce</tt> of its
+PreSharedKeyID <bcp14>MUST</bcp14> be set to a fresh random value of length <tt>KDF.Nh</tt>, where
+<tt>KDF</tt> is the KDF for the cipher suite of the group into which the PSK is being
+injected. This ensures that even when a PSK is used multiple times, the value
+used as an input into the key schedule is different each time.</t>
+        <t>Upon receiving a Commit with a PreSharedKey proposal or a GroupSecrets object
+with the <tt>psks</tt> field set, the receiving client includes them in the key
+schedule in the order listed in the Commit, or in the <tt>psks</tt> field, respectively.
+For resumption PSKs, the PSK is defined as the <tt>resumption_psk</tt> of the group and
+epoch specified in the PreSharedKeyID object. Specifically, <tt>psk_secret</tt> is
+computed as follows:</t>
+        <sourcecode type="tls-presentation">
+struct {
+    PreSharedKeyID id;
+    uint16 index;
+    uint16 count;
+} PSKLabel;
+</sourcecode>
+        <sourcecode type="pseudocode">
+psk_extracted_[i] = KDF.Extract(0, psk_[i])
+psk_input_[i] = ExpandWithLabel(psk_extracted_[i], "derived psk",
+                  PSKLabel, KDF.Nh)
+
+psk_secret_[0] = 0
+psk_secret_[i] = KDF.Extract(psk_input_[i-1], psk_secret_[i-1])
+psk_secret     = psk_secret_[n]
+</sourcecode>
+        <t>Here <tt>0</tt> represents the all-zero vector of length <tt>KDF.Nh</tt>. The <tt>index</tt> field in
+PSKLabel corresponds to the index of the PSK in the <tt>psk</tt> array, while the
+<tt>count</tt> field contains the total number of PSKs.  In other words, the PSKs are
+chained together with KDF.Extract invocations (labeled "Extract" for brevity
+in the diagram), as follows:</t>
+        <figure>
+          <name>Computation of a PSK Secret from a Set of PSKs</name>
+          <artset>
+            <artwork type="svg">
+              <svg xmlns="http://www.w3.org/2000/svg" class="diagram" font-family="monospace" font-size="13px" height="256" text-anchor="middle" version="1.1" viewBox="0 0 568 256" width="568">
+                <path d="M 400,96 L 400,144" fill="none" stroke="black"/>
+                <path d="M 400,192 L 400,224" fill="none" stroke="black"/>
+                <path d="M 88,80 L 104,80" fill="none" stroke="black"/>
+                <path d="M 184,80 L 200,80" fill="none" stroke="black"/>
+                <path d="M 344,80 L 360,80" fill="none" stroke="black"/>
+                <path d="M 88,160 L 104,160" fill="none" stroke="black"/>
+                <path d="M 184,160 L 200,160" fill="none" stroke="black"/>
+                <path d="M 344,160 L 360,160" fill="none" stroke="black"/>
+                <path d="M 88,240 L 104,240" fill="none" stroke="black"/>
+                <path d="M 184,240 L 200,240" fill="none" stroke="black"/>
+                <path d="M 344,240 L 360,240" fill="none" stroke="black"/>
+                <polygon class="arrowhead" fill="black" points="408,224 396,218.4 396,229.6" transform="rotate(90,400,224)"/>
+                <polygon class="arrowhead" fill="black" points="408,144 396,138.4 396,149.6" transform="rotate(90,400,144)"/>
+                <polygon class="arrowhead" fill="black" points="368,240 356,234.4 356,245.6" transform="rotate(0,360,240)"/>
+                <polygon class="arrowhead" fill="black" points="368,160 356,154.4 356,165.6" transform="rotate(0,360,160)"/>
+                <polygon class="arrowhead" fill="black" points="368,80 356,74.4 356,85.6" transform="rotate(0,360,80)"/>
+                <polygon class="arrowhead" fill="black" points="208,240 196,234.4 196,245.6" transform="rotate(0,200,240)"/>
+                <polygon class="arrowhead" fill="black" points="208,160 196,154.4 196,165.6" transform="rotate(0,200,160)"/>
+                <polygon class="arrowhead" fill="black" points="208,80 196,74.4 196,85.6" transform="rotate(0,200,80)"/>
+                <polygon class="arrowhead" fill="black" points="112,240 100,234.4 100,245.6" transform="rotate(0,104,240)"/>
+                <polygon class="arrowhead" fill="black" points="112,160 100,154.4 100,165.6" transform="rotate(0,104,160)"/>
+                <polygon class="arrowhead" fill="black" points="112,80 100,74.4 100,85.6" transform="rotate(0,104,80)"/>
+                <g class="text">
+                  <text x="144" y="36">0</text>
+                  <text x="400" y="36">0</text>
+                  <text x="440" y="36">=</text>
+                  <text x="508" y="36">psk_secret_[0]</text>
+                  <text x="144" y="52">|</text>
+                  <text x="400" y="52">|</text>
+                  <text x="32" y="84">psk_[0]</text>
+                  <text x="144" y="84">Extract</text>
+                  <text x="272" y="84">ExpandWithLabel</text>
+                  <text x="400" y="84">Extract</text>
+                  <text x="440" y="84">=</text>
+                  <text x="508" y="84">psk_secret_[1]</text>
+                  <text x="144" y="116">0</text>
+                  <text x="144" y="132">|</text>
+                  <text x="32" y="164">psk_[1]</text>
+                  <text x="144" y="164">Extract</text>
+                  <text x="272" y="164">ExpandWithLabel</text>
+                  <text x="400" y="164">Extract</text>
+                  <text x="440" y="164">=</text>
+                  <text x="508" y="164">psk_secret_[2]</text>
+                  <text x="400" y="180">|</text>
+                  <text x="144" y="196">0</text>
+                  <text x="392" y="196">.</text>
+                  <text x="408" y="196">.</text>
+                  <text x="144" y="212">|</text>
+                  <text x="40" y="244">psk_[n-1]</text>
+                  <text x="144" y="244">Extract</text>
+                  <text x="272" y="244">ExpandWithLabel</text>
+                  <text x="400" y="244">Extract</text>
+                  <text x="440" y="244">=</text>
+                  <text x="508" y="244">psk_secret_[n]</text>
+                </g>
+              </svg>
+            </artwork>
+            <artwork type="ascii-art">
+                 0                               0    = psk_secret_[0]
+                 |                               |
+                 V                               V
+psk_[0]   --&gt; Extract --&gt; ExpandWithLabel --&gt; Extract = psk_secret_[1]
+                                                 |
+                 0                               |
+                 |                               |
+                 V                               V
+psk_[1]   --&gt; Extract --&gt; ExpandWithLabel --&gt; Extract = psk_secret_[2]
+                                                 |
+                 0                              ...
+                 |                               |
+                 V                               V
+psk_[n-1] --&gt; Extract --&gt; ExpandWithLabel --&gt; Extract = psk_secret_[n]
+</artwork>
+          </artset>
+        </figure>
+        <t>In particular, if there are no PreSharedKey proposals in a given Commit, then
+the resulting <tt>psk_secret</tt> is <tt>psk_secret_[0]</tt>, the all-zero vector.</t>
+      </section>
+      <section anchor="exporters">
+        <name>Exporters</name>
+        <t>The main MLS key schedule provides an <tt>exporter_secret</tt> that can
+be used by an application to derive new secrets for use outside of MLS.</t>
+        <sourcecode type="pseudocode">
+MLS-Exporter(Label, Context, Length) =
+       ExpandWithLabel(DeriveSecret(exporter_secret, Label),
+                         "exported", Hash(Context), Length)
+</sourcecode>
+        <t>Applications <bcp14>SHOULD</bcp14> provide a unique label to <tt>MLS-Exporter</tt> that
+identifies the secret's intended purpose. This is to help prevent the same
+secret from being generated and used in two different places. To help avoid
+the same label being used in different applications, an IANA registry for these
+labels has been defined in <xref target="mls-exporter-labels"/>.</t>
+        <t>The exported values are bound to the group epoch from which the
+<tt>exporter_secret</tt> is derived, and hence reflect a particular state of
+the group.</t>
+        <t>It is <bcp14>RECOMMENDED</bcp14> for the application generating exported values
+to refresh those values after a Commit is processed.</t>
+      </section>
+      <section anchor="resumption-psk">
+        <name>Resumption PSK</name>
+        <t>The main MLS key schedule provides a <tt>resumption_psk</tt> that is used as a PSK
+to inject entropy from one epoch into another.  This functionality is used in the
+reinitialization and branching processes described in Sections <xref format="counter" target="reinitialization"/> and
+<xref format="counter" target="subgroup-branching"/>, but it may be used by applications for other purposes.</t>
+        <t>Some uses of resumption PSKs might call for the use of PSKs from historical
+epochs. The application <bcp14>SHOULD</bcp14> specify an upper limit on the number of past
+epochs for which the <tt>resumption_psk</tt> may be stored.</t>
+      </section>
+      <section anchor="epoch-authenticators">
+        <name>Epoch Authenticators</name>
+        <t>The main MLS key schedule provides a per-epoch <tt>epoch_authenticator</tt>. If one
+member of the group is being impersonated by an active attacker, the
+<tt>epoch_authenticator</tt> computed by their client will differ from those computed
+by the other group members.</t>
+        <t>This property can be used to construct defenses against impersonation attacks
+that are effective even if members' signature keys are compromised. As a trivial
+example, if the users of the clients in an MLS group were to meet in person and
+reliably confirm that their epoch authenticator values were equal (using some
+suitable user interface), then each user would be assured that the others were
+not being impersonated in the current epoch. As soon as the epoch changed,
+though, they would need to redo this confirmation. The state of the group would
+have changed, possibly introducing an attacker.</t>
+        <t>More generally, in order for the members of an MLS group to obtain concrete
+authentication protections using the <tt>epoch_authenticator</tt>, they will need to
+use it in some secondary protocol (such as the face-to-face protocol above).
+The details of that protocol will then determine the specific authentication
+protections provided to the MLS group.</t>
+      </section>
+    </section>
+    <section anchor="secret-tree">
+      <name>Secret Tree</name>
+      <t>For the generation of encryption keys and nonces, the key schedule begins with
+the <tt>encryption_secret</tt> at the root and derives a tree of secrets with the same
+structure as the group's ratchet tree. Each leaf in the secret tree is
+associated with the same group member as the corresponding leaf in the ratchet
+tree.</t>
+      <t>If N is a parent node in the secret tree, then the secrets of the children of N
+are defined as follows (where left(N) and right(N) denote the children of N):</t>
+      <figure>
+        <name>Derivation of Secrets from Parent to Children within a Secret Tree</name>
+        <artset>
+          <artwork type="svg">
+            <svg xmlns="http://www.w3.org/2000/svg" class="diagram" font-family="monospace" font-size="13px" height="160" text-anchor="middle" version="1.1" viewBox="0 0 456 160" width="456">
+              <path d="M 72,40 L 72,128" fill="none" stroke="black"/>
+              <path d="M 248,80 L 248,88" fill="none" stroke="black"/>
+              <path d="M 72,80 L 96,80" fill="none" stroke="black"/>
+              <path d="M 72,128 L 96,128" fill="none" stroke="black"/>
+              <polygon class="arrowhead" fill="black" points="104,128 92,122.4 92,133.6" transform="rotate(0,96,128)"/>
+              <polygon class="arrowhead" fill="black" points="104,80 92,74.4 92,85.6" transform="rotate(0,96,80)"/>
+              <g class="text">
+                <text x="84" y="36">tree_node_[N]_secret</text>
+                <text x="176" y="84">ExpandWithLabel(.</text>
+                <text x="288" y="84">"tree",</text>
+                <text x="352" y="84">"left",</text>
+                <text x="416" y="84">KDF.Nh)</text>
+                <text x="112" y="100">=</text>
+                <text x="228" y="100">tree_node_[left(N)]_secret</text>
+                <text x="180" y="132">ExpandWithLabel(.,</text>
+                <text x="288" y="132">"tree",</text>
+                <text x="356" y="132">"right",</text>
+                <text x="424" y="132">KDF.Nh)</text>
+                <text x="112" y="148">=</text>
+                <text x="232" y="148">tree_node_[right(N)]_secret</text>
+              </g>
+            </svg>
+          </artwork>
+          <artwork type="ascii-art">
+tree_node_[N]_secret
+        |
+        |
+        +--&gt; ExpandWithLabel(., "tree", "left", KDF.Nh)
+        |    = tree_node_[left(N)]_secret
+        |
+        +--&gt; ExpandWithLabel(., "tree", "right", KDF.Nh)
+             = tree_node_[right(N)]_secret
+</artwork>
+        </artset>
+      </figure>
+      <t>The secret in the leaf of the secret tree is used to initiate two symmetric hash
+ratchets, from which a sequence of single-use keys and nonces are derived, as
+described in <xref target="encryption-keys"/>. The root of each ratchet is computed as:</t>
+      <figure>
+        <name>Initialization of the Hash Ratchets from the Leaves of a Secret Tree</name>
+        <artset>
+          <artwork type="svg">
+            <svg xmlns="http://www.w3.org/2000/svg" class="diagram" font-family="monospace" font-size="13px" height="160" text-anchor="middle" version="1.1" viewBox="0 0 472 160" width="472">
+              <path d="M 72,40 L 72,128" fill="none" stroke="black"/>
+              <path d="M 72,80 L 96,80" fill="none" stroke="black"/>
+              <path d="M 72,128 L 96,128" fill="none" stroke="black"/>
+              <polygon class="arrowhead" fill="black" points="104,128 92,122.4 92,133.6" transform="rotate(0,96,128)"/>
+              <polygon class="arrowhead" fill="black" points="104,80 92,74.4 92,85.6" transform="rotate(0,96,80)"/>
+              <g class="text">
+                <text x="84" y="36">tree_node_[N]_secret</text>
+                <text x="180" y="84">ExpandWithLabel(.,</text>
+                <text x="308" y="84">"handshake",</text>
+                <text x="376" y="84">"",</text>
+                <text x="424" y="84">KDF.Nh)</text>
+                <text x="112" y="100">=</text>
+                <text x="252" y="100">handshake_ratchet_secret_[N]_[0]</text>
+                <text x="180" y="132">ExpandWithLabel(.,</text>
+                <text x="316" y="132">"application",</text>
+                <text x="392" y="132">"",</text>
+                <text x="440" y="132">KDF.Nh)</text>
+                <text x="112" y="148">=</text>
+                <text x="260" y="148">application_ratchet_secret_[N]_[0]</text>
+              </g>
+            </svg>
+          </artwork>
+          <artwork type="ascii-art">
+tree_node_[N]_secret
+        |
+        |
+        +--&gt; ExpandWithLabel(., "handshake", "", KDF.Nh)
+        |    = handshake_ratchet_secret_[N]_[0]
+        |
+        +--&gt; ExpandWithLabel(., "application", "", KDF.Nh)
+             = application_ratchet_secret_[N]_[0]
+</artwork>
+        </artset>
+      </figure>
+      <section anchor="encryption-keys">
+        <name>Encryption Keys</name>
+        <t>As described in <xref target="message-framing"/>, MLS encrypts three different
+types of information:</t>
+        <ul spacing="normal">
+          <li>Metadata (sender information)</li>
+          <li>Handshake messages (Proposal and Commit)</li>
+          <li>Application messages</li>
+        </ul>
+        <t>The sender information used to look up the key for content encryption is
+encrypted with an AEAD where the key and nonce are derived from both
+<tt>sender_data_secret</tt> and a sample of the encrypted message content.</t>
+        <t>For handshake and application messages, a sequence of keys is derived via a
+"sender ratchet".  Each sender has their own sender ratchet, and each step along
+the ratchet is called a "generation".</t>
+        <t>The following figure shows a secret tree for a four-member group, with the
+handshake and application ratchets that member D will use for sending and the
+first two application keys and nonces.</t>
+        <figure anchor="secret-tree-example">
+          <name>Secret Tree for a Four-Member Group</name>
+          <artset>
+            <artwork type="svg">
+              <svg xmlns="http://www.w3.org/2000/svg" class="diagram" font-family="monospace" font-size="13px" height="320" text-anchor="middle" version="1.1" viewBox="0 0 200 320" width="200">
+                <path d="M 56,48 L 56,64" fill="none" stroke="black"/>
+                <path d="M 128,176 L 128,208" fill="none" stroke="black"/>
+                <path d="M 128,240 L 128,272" fill="none" stroke="black"/>
+                <path d="M 160,160 L 160,192" fill="none" stroke="black"/>
+                <path d="M 160,224 L 160,256" fill="none" stroke="black"/>
+                <path d="M 40,64 L 72,64" fill="none" stroke="black"/>
+                <path d="M 144,160 L 176,160" fill="none" stroke="black"/>
+                <path d="M 160,192 L 176,192" fill="none" stroke="black"/>
+                <path d="M 144,224 L 176,224" fill="none" stroke="black"/>
+                <path d="M 160,256 L 176,256" fill="none" stroke="black"/>
+                <path d="M 72,64 L 80,80" fill="none" stroke="black"/>
+                <path d="M 32,80 L 40,64" fill="none" stroke="black"/>
+                <g class="text">
+                  <text x="56" y="36">G</text>
+                  <text x="24" y="100">E</text>
+                  <text x="88" y="100">F</text>
+                  <text x="16" y="116">/</text>
+                  <text x="32" y="116">\</text>
+                  <text x="80" y="116">/</text>
+                  <text x="96" y="116">\</text>
+                  <text x="8" y="132">A</text>
+                  <text x="40" y="132">B</text>
+                  <text x="72" y="132">C</text>
+                  <text x="104" y="132">D</text>
+                  <text x="96" y="148">/</text>
+                  <text x="112" y="148">\</text>
+                  <text x="88" y="164">HR0</text>
+                  <text x="128" y="164">AR0</text>
+                  <text x="188" y="164">K0</text>
+                  <text x="188" y="196">N0</text>
+                  <text x="128" y="228">AR1</text>
+                  <text x="188" y="228">K1</text>
+                  <text x="188" y="260">N1</text>
+                  <text x="128" y="292">AR2</text>
+                </g>
+              </svg>
+            </artwork>
+            <artwork type="ascii-art">
+       G
+       |
+     .-+-.
+    /     \
+   E       F
+  / \     / \
+ A   B   C   D
+            / \
+          HR0  AR0--+--K0
+                |   |
+                |   +--N0
+                |
+               AR1--+--K1
+                |   |
+                |   +--N1
+                |
+               AR2
+</artwork>
+          </artset>
+        </figure>
+        <t>A sender ratchet starts from a per-sender base secret derived from a Secret
+Tree, as described in <xref target="secret-tree"/>. The base secret initiates a symmetric
+hash ratchet, which generates a sequence of keys and nonces. The sender uses the
+j-th key/nonce pair in the sequence to encrypt (using the AEAD) the j-th message
+they send during that epoch. Each key/nonce pair <bcp14>MUST NOT</bcp14> be used to encrypt
+more than one message.</t>
+        <t>Keys, nonces, and the secrets in ratchets are derived using
+DeriveTreeSecret. The context in a given call consists of the current position
+in the ratchet.</t>
+        <sourcecode type="pseudocode">
+DeriveTreeSecret(Secret, Label, Generation, Length) =
+    ExpandWithLabel(Secret, Label, Generation, Length)
+</sourcecode>
+        <t>Where <tt>Generation</tt> is encoded as a big endian uint32.</t>
+        <artset>
+          <artwork type="svg">
+            <svg xmlns="http://www.w3.org/2000/svg" class="diagram" font-family="monospace" font-size="13px" height="208" text-anchor="middle" version="1.1" viewBox="0 0 416 208" width="416">
+              <path d="M 56,40 L 56,160" fill="none" stroke="black"/>
+              <path d="M 56,64 L 80,64" fill="none" stroke="black"/>
+              <path d="M 56,112 L 80,112" fill="none" stroke="black"/>
+              <polygon class="arrowhead" fill="black" points="88,112 76,106.4 76,117.6" transform="rotate(0,80,112)"/>
+              <polygon class="arrowhead" fill="black" points="88,64 76,58.4 76,69.6" transform="rotate(0,80,64)"/>
+              <polygon class="arrowhead" fill="black" points="64,160 52,154.4 52,165.6" transform="rotate(90,56,160)"/>
+              <g class="text">
+                <text x="92" y="36">ratchet_secret_[N]_[j]</text>
+                <text x="168" y="68">DeriveTreeSecret(.,</text>
+                <text x="284" y="68">"nonce",</text>
+                <text x="332" y="68">j,</text>
+                <text x="380" y="68">AEAD.Nn)</text>
+                <text x="96" y="84">=</text>
+                <text x="192" y="84">ratchet_nonce_[N]_[j]</text>
+                <text x="168" y="116">DeriveTreeSecret(.,</text>
+                <text x="276" y="116">"key",</text>
+                <text x="316" y="116">j,</text>
+                <text x="372" y="116">AEAD.Nk)</text>
+                <text x="96" y="132">=</text>
+                <text x="184" y="132">ratchet_key_[N]_[j]</text>
+                <text x="80" y="180">DeriveTreeSecret(.,</text>
+                <text x="200" y="180">"secret",</text>
+                <text x="252" y="180">j,</text>
+                <text x="296" y="180">KDF.Nh)</text>
+                <text x="8" y="196">=</text>
+                <text x="116" y="196">ratchet_secret_[N]_[j+1]</text>
+              </g>
+            </svg>
+          </artwork>
+          <artwork type="ascii-art">
+ratchet_secret_[N]_[j]
+      |
+      +--&gt; DeriveTreeSecret(., "nonce", j, AEAD.Nn)
+      |    = ratchet_nonce_[N]_[j]
+      |
+      +--&gt; DeriveTreeSecret(., "key", j,  AEAD.Nk)
+      |    = ratchet_key_[N]_[j]
+      |
+      V
+DeriveTreeSecret(., "secret", j, KDF.Nh)
+= ratchet_secret_[N]_[j+1]
+</artwork>
+        </artset>
+        <t>Here <tt>AEAD.Nn</tt> and <tt>AEAD.Nk</tt> denote the lengths
+in bytes of the nonce and key for the AEAD scheme defined by
+the cipher suite.</t>
+      </section>
+      <section anchor="deletion-schedule">
+        <name>Deletion Schedule</name>
+        <t>It is important to delete all security-sensitive values as soon as they are
+<em>consumed</em>. A sensitive value S is said to be <em>consumed</em> if:</t>
+        <ul spacing="normal">
+          <li>S was used to encrypt or (successfully) decrypt a message, or</li>
+          <li>a key, nonce, or secret derived from S has been consumed. (This goes for
+values derived via DeriveSecret as well as ExpandWithLabel.)</li>
+        </ul>
+        <t>Here S may be the <tt>init_secret</tt>, <tt>commit_secret</tt>, <tt>epoch_secret</tt>, or
+<tt>encryption_secret</tt> as well as any secret in a secret tree or one of the
+ratchets.</t>
+        <t>As soon as a group member consumes a value, they <bcp14>MUST</bcp14> immediately delete
+(all representations of) that value. This is crucial to ensuring
+forward secrecy for past messages. Members <bcp14>MAY</bcp14> keep unconsumed values around
+for some reasonable amount of time to handle out-of-order message delivery.</t>
+        <t>For example, suppose a group member encrypts or (successfully) decrypts an
+application message using the j-th key and nonce in the ratchet of leaf node
+L in some epoch n. Then, for that member, at least the following
+values have been consumed and <bcp14>MUST</bcp14> be deleted:</t>
+        <ul spacing="normal">
+          <li>the <tt>commit_secret</tt>, <tt>joiner_secret</tt>, <tt>epoch_secret</tt>, and <tt>encryption_secret</tt> of
+that epoch n as well as the <tt>init_secret</tt> of the previous epoch n-1,</li>
+          <li>all node secrets in the secret tree on the path from the root to the leaf with
+node L,</li>
+          <li>the first j secrets in the application data ratchet of node L, and</li>
+          <li><tt>application_ratchet_nonce_[L]_[j]</tt> and <tt>application_ratchet_key_[L]_[j]</tt>.</li>
+        </ul>
+        <t>Concretely, consider the secret tree shown in <xref target="secret-tree-example"/>.  Client
+A, B, or C would generate the illustrated values on receiving a message from D
+with generation equal to 1, having not received a message with generation 0
+(e.g., due to out-of-order delivery).  In such a case, the following values
+would be consumed:</t>
+        <ul spacing="normal">
+          <li>The key K1 and nonce N1 used to decrypt the message</li>
+          <li>The application ratchet secrets AR1 and AR0</li>
+          <li>The tree secrets D, F, and G (recall that G is the <tt>encryption_secret</tt> for the
+epoch)</li>
+          <li>The <tt>epoch_secret</tt>, <tt>commit_secret</tt>, <tt>psk_secret</tt>, and <tt>joiner_secret</tt> for the
+current epoch</li>
+        </ul>
+        <t>Other values may be retained (not consumed):</t>
+        <ul spacing="normal">
+          <li>K0 and N0 for decryption of an out-of-order message with generation 0</li>
+          <li>AR2 for derivation of further message decryption keys and nonces</li>
+          <li>HR0 for protection of handshake messages from D</li>
+          <li>E and C for deriving secrets used by senders A, B, and C</li>
+        </ul>
+      </section>
+    </section>
+    <section anchor="key-packages">
+      <name>Key Packages</name>
+      <t>In order to facilitate the asynchronous addition of clients to a
+group, clients can pre-publish KeyPackage objects that
+provide some public information about a user. A KeyPackage object specifies:</t>
+      <ol spacing="normal" type="1">
+        <li>a protocol version and cipher suite that the client supports,</li>
+        <li>a public key that others can use to encrypt a Welcome message to this client
+(an "init key"), and</li>
+        <li>the content of the leaf node that should be added to the tree to represent
+this client.</li>
+      </ol>
+      <t>KeyPackages are intended to be used only once and <bcp14>SHOULD NOT</bcp14>
+be reused except in the case of a "last resort" KeyPackage (see <xref target="keypackage-reuse"/>).
+Clients <bcp14>MAY</bcp14> generate and publish multiple KeyPackages to
+support multiple cipher suites.</t>
+      <t>The value for <tt>init_key</tt> <bcp14>MUST</bcp14> be a public key for the asymmetric encryption
+scheme defined by <tt>cipher_suite</tt>, and it <bcp14>MUST</bcp14> be unique among the set of
+KeyPackages created by this client.  Likewise, the <tt>leaf_node</tt> field <bcp14>MUST</bcp14> be
+valid for the cipher suite, including both the <tt>encryption_key</tt> and
+<tt>signature_key</tt> fields.  The whole structure is signed using the client's
+signature key. A KeyPackage object with an invalid signature field <bcp14>MUST</bcp14> be
+considered malformed.</t>
+      <t>The signature is computed by the function <tt>SignWithLabel</tt> with a label
+<tt>"KeyPackageTBS"</tt> and a <tt>Content</tt> input comprising all of the fields except for the
+signature field.</t>
+      <sourcecode type="tls-presentation">
+struct {
+    ProtocolVersion version;
+    CipherSuite cipher_suite;
+    HPKEPublicKey init_key;
+    LeafNode leaf_node;
+    Extension extensions&lt;V&gt;;
+    /* SignWithLabel(., "KeyPackageTBS", KeyPackageTBS) */
+    opaque signature&lt;V&gt;;
+} KeyPackage;
+
+struct {
+    ProtocolVersion version;
+    CipherSuite cipher_suite;
+    HPKEPublicKey init_key;
+    LeafNode leaf_node;
+    Extension extensions&lt;V&gt;;
+} KeyPackageTBS;
+</sourcecode>
+      <t>If a client receives a KeyPackage carried within an MLSMessage object, then it
+<bcp14>MUST</bcp14> verify that the <tt>version</tt> field of the KeyPackage has the same value as the
+<tt>version</tt> field of the MLSMessage.  The <tt>version</tt> field in the KeyPackage
+provides an explicit signal of the intended version to the other members of
+group when they receive the KeyPackage in an Add proposal.</t>
+      <t>The field <tt>leaf_node.capabilities</tt> indicates what protocol versions,
+cipher suites, credential types, and non-default proposal/extension types are supported
+by the client.  (As discussed in <xref target="leaf-node-contents"/>, some proposal and extension types defined in this document are considered
+"default" and thus are not listed.)  This information allows MLS session
+establishment to be safe from downgrade attacks on the parameters described (as
+discussed in <xref target="group-creation"/>), while still only advertising one version and
+one cipher suite per KeyPackage.</t>
+      <t>The field <tt>leaf_node.leaf_node_source</tt> of the LeafNode in a KeyPackage <bcp14>MUST</bcp14> be
+set to <tt>key_package</tt>.</t>
+      <t>Extensions included in the <tt>extensions</tt> or <tt>leaf_node.extensions</tt> fields <bcp14>MUST</bcp14>
+be included in the <tt>leaf_node.capabilities</tt> field.  As discussed in
+<xref target="extensibility"/>, unknown extensions in <tt>KeyPackage.extensions</tt> <bcp14>MUST</bcp14> be
+ignored, and the creator of a KeyPackage object <bcp14>SHOULD</bcp14> include some random GREASE
+extensions to help ensure that other clients correctly ignore unknown
+extensions.</t>
+      <section anchor="keypackage-validation">
+        <name>KeyPackage Validation</name>
+        <t>The validity of a KeyPackage needs to be verified at a few stages:</t>
+        <ul spacing="normal">
+          <li>When a KeyPackage is downloaded by a group member, before it is used
+to add the client to the group</li>
+          <li>When a KeyPackage is received by a group member in an Add message</li>
+        </ul>
+        <t>The client verifies the validity of a KeyPackage using the following steps:</t>
+        <ul spacing="normal">
+          <li>Verify that the cipher suite and protocol version of the KeyPackage match
+those in the GroupContext.</li>
+          <li>Verify that the <tt>leaf_node</tt> of the KeyPackage is valid for a KeyPackage
+according to <xref target="leaf-node-validation"/>.</li>
+          <li>Verify that the signature on the KeyPackage is valid using the public key
+in <tt>leaf_node.credential</tt>.</li>
+          <li>Verify that the value of <tt>leaf_node.encryption_key</tt> is different from the value of
+the <tt>init_key</tt> field.</li>
+        </ul>
+      </section>
+    </section>
+    <section anchor="group-creation">
+      <name>Group Creation</name>
+      <t>A group is always created with a single member, the "creator".  Other members
+are then added to the group using the usual Add/Commit mechanism.</t>
+      <t>The creator of a group is responsible for setting the group ID, cipher suite, and
+initial extensions for the group.  If the creator intends to add other members
+at the time of creation, then it <bcp14>SHOULD</bcp14> fetch KeyPackages for the members to be
+added, and select a cipher suite and extensions according to the capabilities of
+the members.  To protect against downgrade attacks, the creator <bcp14>MUST</bcp14> use the
+<tt>capabilities</tt> information in these KeyPackages to verify that the chosen
+version and cipher suite is the best option supported by all members.</t>
+      <t>Group IDs <bcp14>SHOULD</bcp14> be constructed in such a way that there is an overwhelmingly low
+probability of honest group creators generating the same group ID, even without
+assistance from the Delivery Service. This can be done, for example, by making the group ID a
+freshly generated random value of size <tt>KDF.Nh</tt>. The Delivery Service <bcp14>MAY</bcp14>
+attempt to ensure that group IDs are globally unique by rejecting the creation
+of new groups with a previously used ID.</t>
+      <t>To initialize a group, the creator of the group <bcp14>MUST</bcp14> take the
+following steps:</t>
+      <ul spacing="normal">
+        <li>
+          <t>Initialize a one-member group with the following initial values:
+          </t>
+          <ul spacing="normal">
+            <li>Ratchet tree: A tree with a single node, a leaf node containing an HPKE public
+key and credential for the creator</li>
+            <li>Group ID: A value set by the creator</li>
+            <li>Epoch: 0</li>
+            <li>Tree hash: The root hash of the above ratchet tree</li>
+            <li>Confirmed transcript hash: The zero-length octet string</li>
+            <li>Epoch secret: A fresh random value of size <tt>KDF.Nh</tt></li>
+            <li>Extensions: Any values of the creator's choosing</li>
+          </ul>
+        </li>
+        <li>
+          <t>Calculate the interim transcript hash:
+          </t>
+          <ul spacing="normal">
+            <li>Derive the <tt>confirmation_key</tt> for the epoch as described in
+<xref target="key-schedule"/>.</li>
+            <li>Compute a <tt>confirmation_tag</tt> over the empty <tt>confirmed_transcript_hash</tt>
+using the <tt>confirmation_key</tt> as described in <xref target="content-authentication"/>.</li>
+            <li>Compute the updated <tt>interim_transcript_hash</tt> from the
+<tt>confirmed_transcript_hash</tt> and the <tt>confirmation_tag</tt> as described in
+<xref target="transcript-hashes"/>.</li>
+          </ul>
+        </li>
+      </ul>
+      <t>At this point, the creator's state represents a one-member group with a fully
+initialized key schedule, transcript hashes, etc.  Proposals and Commits can be
+generated for this group state just like any other state of the group, such as
+Add proposals and Commits to add other members to the group.  A GroupInfo object
+for this group state can also be published to facilitate external joins.</t>
+      <t>Members other than the creator join either by being sent a Welcome message (as
+described in <xref target="joining-via-welcome-message"/>) or by sending an external Commit
+(see <xref target="joining-via-external-commits"/>).</t>
+      <t>In principle, the above process could be streamlined by having the
+creator directly create a tree and choose a random value for first
+epoch's epoch secret.  We follow the steps above because it removes
+unnecessary choices, by which, for example, bad randomness could be
+introduced.  The only choices the creator makes here are its own
+KeyPackage and the leaf secret from which the Commit is built.</t>
+      <section anchor="required-capabilities">
+        <name>Required Capabilities</name>
+        <t>The configuration of a group imposes certain requirements on clients in the
+group.  At a minimum, all members of the group need to support the cipher suite
+and protocol version in use.  Additional requirements can be imposed by
+including a <tt>required_capabilities</tt> extension in the GroupContext.</t>
+        <sourcecode type="tls-presentation">
+struct {
+    ExtensionType extension_types&lt;V&gt;;
+    ProposalType proposal_types&lt;V&gt;;
+    CredentialType credential_types&lt;V&gt;;
+} RequiredCapabilities;
+</sourcecode>
+        <t>This extension lists the extensions, proposals, and credential types that must be supported by
+all members of the group. The "default" proposal and extension types defined in this
+document are assumed to be implemented by all clients, and need not be listed in
+RequiredCapabilities in order to be safely used. Note that this is not true for
+credential types.</t>
+        <t>For new members, support for required capabilities is enforced by existing
+members during the application of Add commits.  Existing members should of
+course be in compliance already.  In order to ensure this continues to be the
+case even as the group's extensions are updated, a GroupContextExtensions
+proposal is deemed invalid if it contains a <tt>required_capabilities</tt> extension that
+requires non-default capabilities not supported by all current members.</t>
+      </section>
+      <section anchor="reinitialization">
+        <name>Reinitialization</name>
+        <t>A group may be reinitialized by creating a new group with the same membership
+and different parameters, and linking it to the old group via a resumption PSK.
+The members of a group reinitialize it using the following steps:</t>
+        <ol spacing="normal" type="1">
+          <li>A member of the old group sends a ReInit proposal (see <xref target="reinit"/>).</li>
+          <li>A member of the old group sends a Commit covering the ReInit proposal.</li>
+          <li>
+            <t>A member of the old group creates an initial Commit that sets up a new group
+that matches the ReInit and sends a Welcome message:
+            </t>
+            <ul spacing="normal">
+              <li>The <tt>version</tt>, <tt>cipher_suite</tt>, <tt>group_id</tt>, and <tt>extensions</tt> fields of the GroupContext object in the Welcome
+message <bcp14>MUST</bcp14> be the same as the corresponding fields in the ReInit
+proposal. The <tt>epoch</tt> in the Welcome message <bcp14>MUST</bcp14> be 1.</li>
+              <li>The Welcome message <bcp14>MUST</bcp14> specify a PreSharedKeyID of type <tt>resumption</tt> with usage
+<tt>reinit</tt>, where the <tt>group_id</tt> field matches the old group and the <tt>epoch</tt>
+field indicates the epoch after the Commit covering the ReInit.</li>
+            </ul>
+          </li>
+        </ol>
+        <t>Note that these three steps may be done by the same group member or different
+members.  For example, if a group member sends a Commit with an inline ReInit
+proposal (steps 1 and 2) but then goes offline, another group member may
+recreate the group instead.  This flexibility avoids situations where a group
+gets stuck between steps 2 and 3.</t>
+        <t>Resumption PSKs with usage <tt>reinit</tt> <bcp14>MUST NOT</bcp14> be used in other contexts.  A
+PreSharedKey proposal with type <tt>resumption</tt> and usage <tt>reinit</tt> <bcp14>MUST</bcp14> be
+considered invalid.</t>
+      </section>
+      <section anchor="subgroup-branching">
+        <name>Subgroup Branching</name>
+        <t>A new group can be formed from a subset of an existing group's members, using
+the same parameters as the old group.</t>
+        <t>A member can create a subgroup by performing the following steps:</t>
+        <ol spacing="normal" type="1">
+          <li>Fetch a new KeyPackage for each group member that should be included in the
+subgroup.</li>
+          <li>Create an initial Commit message that sets up the new group and contains a
+PreSharedKey proposal of type <tt>resumption</tt> with usage <tt>branch</tt>. To avoid key
+reuse, the <tt>psk_nonce</tt> included in the PreSharedKeyID object <bcp14>MUST</bcp14> be a
+randomly sampled nonce of length <tt>KDF.Nh</tt>.</li>
+          <li>Send the corresponding Welcome message to the subgroup members.</li>
+        </ol>
+        <t>A client receiving a Welcome message including a PreSharedKey of type <tt>resumption</tt> with
+usage <tt>branch</tt> <bcp14>MUST</bcp14> verify that the new group reflects a subgroup branched from
+the referenced group by checking that:</t>
+        <ul spacing="normal">
+          <li>The <tt>version</tt> and <tt>cipher_suite</tt> values in the Welcome message are the same as
+those used by the old group.</li>
+          <li>The <tt>epoch</tt> in the Welcome message <bcp14>MUST</bcp14> be 1.</li>
+          <li>Each LeafNode in a new subgroup <bcp14>MUST</bcp14> match some LeafNode in the original
+group. In this context, a pair of LeafNodes is said to "match" if the
+identifiers presented by their respective credentials are considered
+equivalent by the application.</li>
+        </ul>
+        <t>Resumption PSKs with usage <tt>branch</tt> <bcp14>MUST NOT</bcp14> be used in other contexts.  A
+PreSharedKey proposal with type <tt>resumption</tt> and usage <tt>branch</tt> <bcp14>MUST</bcp14> be
+considered invalid.</t>
+      </section>
+    </section>
+    <section anchor="group-evolution">
+      <name>Group Evolution</name>
+      <t>Over the lifetime of a group, its membership can change, and existing members
+might want to change their keys in order to achieve post-compromise security.
+In MLS, each such change is accomplished by a two-step process:</t>
+      <ol spacing="normal" type="1">
+        <li>A proposal to make the change is broadcast to the group in a Proposal
+message.</li>
+        <li>A member of the group or a new member broadcasts a Commit message that causes
+one or more proposed changes to enter into effect.</li>
+      </ol>
+      <t>In cases where the Proposal and Commit are sent by the same member, these two steps
+can be combined by sending the proposals in the commit.</t>
+      <t>The group thus evolves from one cryptographic state to another each time a
+Commit message is sent and processed.  These states are referred to as "epochs"
+and are uniquely identified among states of the group by eight-octet epoch values.
+When a new group is initialized, its initial state epoch is 0x0000000000000000.  Each time
+a state transition occurs, the epoch number is incremented by one.</t>
+      <section anchor="proposals">
+        <name>Proposals</name>
+        <t>Proposals are included in a FramedContent by way of a Proposal structure
+that indicates their type:</t>
+        <sourcecode type="tls-presentation">
+// See the "MLS Proposal Types" IANA registry for values
+uint16 ProposalType;
+
+struct {
+    ProposalType proposal_type;
+    select (Proposal.proposal_type) {
+        case add:                      Add;
+        case update:                   Update;
+        case remove:                   Remove;
+        case psk:                      PreSharedKey;
+        case reinit:                   ReInit;
+        case external_init:            ExternalInit;
+        case group_context_extensions: GroupContextExtensions;
+    };
+} Proposal;
+</sourcecode>
+        <t>On receiving a FramedContent containing a Proposal, a client <bcp14>MUST</bcp14> verify the
+signature inside FramedContentAuthData and that the <tt>epoch</tt> field of the enclosing
+FramedContent is equal to the <tt>epoch</tt> field of the current GroupContext object.
+If the verification is successful, then the Proposal should be cached in such a way
+that it can be retrieved by hash (as a ProposalOrRef object) in a later Commit message.</t>
+        <section anchor="add">
+          <name>Add</name>
+          <t>An Add proposal requests that a client with a specified KeyPackage be added
+to the group.</t>
+          <sourcecode type="tls-presentation">
+struct {
+    KeyPackage key_package;
+} Add;
+</sourcecode>
+          <t>An Add proposal is invalid if the KeyPackage is invalid according to
+<xref target="keypackage-validation"/>.</t>
+          <t>An Add is applied after being included in a Commit message.  The position of the
+Add in the list of proposals determines the leaf node where the new member will
+be added.  For the first Add in the Commit, the corresponding new member will be
+placed in the leftmost empty leaf in the tree, for the second Add, the next
+empty leaf to the right, etc. If no empty leaf exists, the tree is extended to
+the right.</t>
+          <ul spacing="normal">
+            <li>Identify the leaf L for the new member: if there are empty leaves in the tree,
+L is the leftmost empty leaf. Otherwise, the tree is extended to the right as
+described in <xref target="adding-and-removing-leaves"/>, and L is assigned the leftmost new
+blank leaf.</li>
+            <li>For each non-blank intermediate node along the path from the leaf L
+to the root, add L's leaf index to the <tt>unmerged_leaves</tt> list for the node.</li>
+            <li>Set the leaf node L to a new node containing the LeafNode object carried in
+the <tt>leaf_node</tt> field of the KeyPackage in the Add.</li>
+          </ul>
+        </section>
+        <section anchor="update">
+          <name>Update</name>
+          <t>An Update proposal is a similar mechanism to Add with the distinction
+that it replaces the sender's LeafNode in the tree instead of adding a new leaf
+to the tree.</t>
+          <sourcecode type="tls-presentation">
+struct {
+    LeafNode leaf_node;
+} Update;
+</sourcecode>
+          <t>An Update proposal is invalid if the LeafNode is invalid for an Update
+proposal according to <xref target="leaf-node-validation"/>.</t>
+          <t>A member of the group applies an Update message by taking the following steps:</t>
+          <ul spacing="normal">
+            <li>Replace the sender's LeafNode with the one contained in the Update proposal.</li>
+            <li>Blank the intermediate nodes along the path from the sender's leaf to the
+root.</li>
+          </ul>
+        </section>
+        <section anchor="remove">
+          <name>Remove</name>
+          <t>A Remove proposal requests that the member with the leaf index <tt>removed</tt> be removed
+from the group.</t>
+          <sourcecode type="tls-presentation">
+struct {
+    uint32 removed;
+} Remove;
+</sourcecode>
+          <t>A Remove proposal is invalid if the <tt>removed</tt> field does not identify a non-blank
+leaf node.</t>
+          <t>A member of the group applies a Remove message by taking the following steps:</t>
+          <ul spacing="normal">
+            <li>Identify the leaf node matching <tt>removed</tt>.  Let L be this leaf node.</li>
+            <li>Replace the leaf node L with a blank node.</li>
+            <li>Blank the intermediate nodes along the path from L to the root.</li>
+            <li>Truncate the tree by removing the right subtree until there is at least one
+non-blank leaf node in the right subtree.  If the rightmost non-blank leaf has
+index L, then this will result in the tree having 2<sup>d</sup> leaves, where <tt>d</tt> is
+the smallest value such that 2<sup>d</sup> &gt; <tt>L</tt>.</li>
+          </ul>
+        </section>
+        <section anchor="presharedkey">
+          <name>PreSharedKey</name>
+          <t>A PreSharedKey proposal can be used to request that a pre-shared key be
+injected into the key schedule in the process of advancing the epoch.</t>
+          <sourcecode type="tls-presentation">
+struct {
+    PreSharedKeyID psk;
+} PreSharedKey;
+</sourcecode>
+          <t>A PreSharedKey proposal is invalid if any of the following is true:</t>
+          <ul spacing="normal">
+            <li>The PreSharedKey proposal is not being processed as part of a reinitialization
+of the group (see <xref target="reinitialization"/>), and the PreSharedKeyID has <tt>psktype</tt>
+set to <tt>resumption</tt> and <tt>usage</tt> set to <tt>reinit</tt>.</li>
+            <li>The PreSharedKey proposal is not being processed as part of a subgroup
+branching operation (see <xref target="subgroup-branching"/>), and the PreSharedKeyID has
+<tt>psktype</tt> set to <tt>resumption</tt> and <tt>usage</tt> set to <tt>branch</tt>.</li>
+            <li>The <tt>psk_nonce</tt> is not of length <tt>KDF.Nh</tt>.</li>
+          </ul>
+          <t>The <tt>psk_nonce</tt> <bcp14>MUST</bcp14> be randomly sampled. When processing
+a Commit message that includes one or more PreSharedKey proposals, group
+members derive <tt>psk_secret</tt> as described in <xref target="pre-shared-keys"/>, where the
+order of the PSKs corresponds to the order of the PreSharedKey proposals
+in the Commit.</t>
+        </section>
+        <section anchor="reinit">
+          <name>ReInit</name>
+          <t>A ReInit proposal represents a request to reinitialize the group with different
+parameters, for example, to increase the version number or to change the
+cipher suite. The reinitialization is done by creating a completely new group
+and shutting down the old one.</t>
+          <sourcecode type="tls-presentation">
+struct {
+    opaque group_id&lt;V&gt;;
+    ProtocolVersion version;
+    CipherSuite cipher_suite;
+    Extension extensions&lt;V&gt;;
+} ReInit;
+</sourcecode>
+          <t>A ReInit proposal is invalid if the <tt>version</tt> field is less than the version
+for the current group.</t>
+          <t>A member of the group applies a ReInit proposal by waiting for the committer to
+send the Welcome message that matches the ReInit, according to the criteria in
+<xref target="reinitialization"/>.</t>
+        </section>
+        <section anchor="externalinit">
+          <name>ExternalInit</name>
+          <t>An ExternalInit proposal is used by new members that want to join a group by
+using an external commit. This proposal can only be used in that context.</t>
+          <sourcecode type="tls-presentation">
+struct {
+  opaque kem_output&lt;V&gt;;
+} ExternalInit;
+</sourcecode>
+          <t>A member of the group applies an ExternalInit message by initializing the next
+epoch using an init secret computed as described in <xref target="external-initialization"/>.
+The <tt>kem_output</tt> field contains the required KEM output.</t>
+        </section>
+        <section anchor="groupcontextextensions">
+          <name>GroupContextExtensions</name>
+          <t>A GroupContextExtensions proposal is used to update the list of extensions in
+the GroupContext for the group.</t>
+          <sourcecode type="tls-presentation">
+struct {
+  Extension extensions&lt;V&gt;;
+} GroupContextExtensions;
+</sourcecode>
+          <t>A GroupContextExtensions proposal is invalid if it includes a
+<tt>required_capabilities</tt> extension and some members of the group do not support
+some of the required capabilities (including those added in the same Commit,
+and excluding those removed).</t>
+          <t>A member of the group applies a GroupContextExtensions proposal with the
+following steps:</t>
+          <ul spacing="normal">
+            <li>Remove all of the existing extensions from the GroupContext object for the
+group and replace them with the list of extensions in the proposal.  (This
+is a wholesale replacement, not a merge. An extension is only carried over if
+the sender of the proposal includes it in the new list.)</li>
+          </ul>
+          <t>Note that once the GroupContext is updated, its inclusion in the
+<tt>confirmation_tag</tt> by way of the key schedule will confirm that all members of the
+group agree on the extensions in use.</t>
+        </section>
+        <section anchor="external-proposals">
+          <name>External Proposals</name>
+          <t>Proposals can be constructed and sent to the group by a party
+that is outside the group in two cases. One case, indicated by the <tt>external</tt> SenderType,
+allows an entity outside the group to submit proposals to the group.
+For example, an automated service might propose
+removing a member of a group who has been inactive for a long time, or propose adding
+a newly hired staff member to a group representing a real-world team.
+An <tt>external</tt> sender might send a ReInit proposal to enforce a changed policy
+regarding MLS versions or cipher suites.</t>
+          <t>The <tt>external</tt> SenderType requires that signers are pre-provisioned
+to the clients within a group and can only be used if the
+<tt>external_senders</tt> extension is present in the group's GroupContext.</t>
+          <t>The other case, indicated by the <tt>new_member_proposal</tt> SenderType, is useful
+when existing members of the group can independenly verify that an Add proposal
+sent by the new joiner itself (not an existing member) is authorized. External
+proposals that are not authorized are considered invalid.</t>
+          <t>An external proposal <bcp14>MUST</bcp14> be sent as a PublicMessage object, since the sender
+will not have the keys necessary to construct a PrivateMessage object.</t>
+          <t>Proposals of some types cannot be sent by an <tt>external</tt> sender.  Among the
+proposal types defined in this document, only the following types may be sent by
+an <tt>external</tt> sender:</t>
+          <ul spacing="normal">
+            <li>
+              <tt>add</tt>
+            </li>
+            <li>
+              <tt>remove</tt>
+            </li>
+            <li>
+              <tt>psk</tt>
+            </li>
+            <li>
+              <tt>reinit</tt>
+            </li>
+            <li>
+              <tt>group_context_extensions</tt>
+            </li>
+          </ul>
+          <t>Messages from <tt>external</tt> senders containing proposal types other than the above
+<bcp14>MUST</bcp14> be rejected as malformed.  New proposal types defined in the future <bcp14>MUST</bcp14>
+define whether they may be sent by <tt>external</tt> senders.  The "Ext" column in
+the "MLS Proposal Types" registry (<xref target="mls-proposal-types"/>) reflects this property.</t>
+          <section anchor="external-senders-extension">
+            <name>External Senders Extension</name>
+            <t>The <tt>external_senders</tt> extension is a group context extension that contains
+the credentials and signature keys of senders that are permitted to send
+external proposals to the group.</t>
+            <sourcecode type="tls-presentation">
+struct {
+  SignaturePublicKey signature_key;
+  Credential credential;
+} ExternalSender;
+
+ExternalSender external_senders&lt;V&gt;;
+</sourcecode>
+          </section>
+        </section>
+      </section>
+      <section anchor="proposal-list-validation">
+        <name>Proposal List Validation</name>
+        <t>A group member creating a Commit and a group member processing a Commit
+<bcp14>MUST</bcp14> verify that the list of committed proposals is valid using one of the following
+procedures, depending on whether the Commit is external or not.  If the list of
+proposals is invalid, then the Commit message <bcp14>MUST</bcp14> be rejected as invalid.</t>
+        <t>For a regular, i.e., not external, Commit, the list is invalid if any of the
+following occurs:</t>
+        <ul spacing="normal">
+          <li>It contains an individual proposal that is invalid as specified in <xref target="proposals"/>.</li>
+          <li>It contains an Update proposal generated by the committer.</li>
+          <li>It contains a Remove proposal that removes the committer.</li>
+          <li>It contains multiple Update and/or Remove proposals that apply to the same leaf.
+If the committer has received multiple such proposals they <bcp14>SHOULD</bcp14> prefer any Remove
+received, or the most recent Update if there are no Removes.</li>
+          <li>It contains multiple Add proposals that contain KeyPackages that represent the same
+client according to the application (for example, identical signature keys).</li>
+          <li>It contains an Add proposal with a KeyPackage that represents a client already
+in the group according to the application, unless there is a Remove proposal
+in the list removing the matching client from the group.</li>
+          <li>It contains multiple PreSharedKey proposals that reference the same PreSharedKeyID.</li>
+          <li>It contains multiple GroupContextExtensions proposals.</li>
+          <li>It contains a ReInit proposal together with any other proposal. If the committer has
+received other proposals during the epoch, they <bcp14>SHOULD</bcp14> prefer them over the
+ReInit proposal, allowing the ReInit to be resent and applied in a subsequent
+epoch.</li>
+          <li>It contains an ExternalInit proposal.</li>
+          <li>It contains a Proposal with a non-default proposal type that is not supported by some
+members of the group that will process the Commit (i.e., members being added
+or removed by the Commit do not need to support the proposal type).</li>
+          <li>After processing the Commit the ratchet tree is invalid, in particular, if it
+contains any leaf node that is invalid according to <xref target="leaf-node-validation"/>.</li>
+        </ul>
+        <t>An application may extend the above procedure by additional rules, for example,
+requiring application-level permissions to add members, or rules concerning
+non-default proposal types.</t>
+        <t>For an external Commit, the list is valid if it contains only the following proposals
+(not necessarily in this order):</t>
+        <ul spacing="normal">
+          <li>Exactly one ExternalInit</li>
+          <li>At most one Remove proposal, with which the joiner removes an
+old version of themselves. If a Remove proposal is present, then the LeafNode in the
+<tt>path</tt> field of the external Commit <bcp14>MUST</bcp14> meet the same criteria as would the LeafNode
+in an Update for the removed leaf (see <xref target="update"/>). In particular, the <tt>credential</tt>
+in the LeafNode <bcp14>MUST</bcp14> present a set of identifiers that is acceptable to the
+application for the removed participant.</li>
+          <li>Zero or more PreSharedKey proposals</li>
+          <li>No other proposals</li>
+        </ul>
+        <t>Proposal types defined in the future may make updates to the above validation
+logic to incorporate considerations related to proposals of the new type.</t>
+      </section>
+      <section anchor="applying-a-proposal-list">
+        <name>Applying a Proposal List</name>
+        <t>The sections above defining each proposal type describe how each individual
+proposal is applied.  When creating or processing a Commit, a client applies a
+list of proposals to the ratchet tree and GroupContext. The client <bcp14>MUST</bcp14> apply
+the proposals in the list in the following order:</t>
+        <ul spacing="normal">
+          <li>If there is a GroupContextExtensions proposal, replace the <tt>extensions</tt> field
+of the GroupContext for the group with the contents of the proposal.  The
+new <tt>extensions</tt> <bcp14>MUST</bcp14> be used when evaluating other proposals in this list. For
+example, if a GroupContextExtensions proposal adds a <tt>required_capabilities</tt>
+extension, then any Add proposals need to indicate support for those
+capabilities.</li>
+          <li>Apply any Update proposals to the ratchet tree, in any order.</li>
+          <li>Apply any Remove proposals to the ratchet tree, in any order.</li>
+          <li>Apply any Add proposals to the ratchet tree, in the order they appear in the list.</li>
+          <li>Look up the PSK secrets for any PreSharedKey proposals, in the order they
+appear in the list.  These secrets are then used to advance the key schedule
+later in Commit processing.</li>
+          <li>If there is an ExternalInit proposal, use it to derive the <tt>init_secret</tt> for
+use later in Commit processing.</li>
+          <li>If there is a ReInit proposal, note its parameters for application later in
+Commit processing.</li>
+        </ul>
+        <t>Proposal types defined in the future <bcp14>MUST</bcp14> specify how the above steps are to be
+adjusted to accommodate the application of proposals of the new type.</t>
+      </section>
+      <section anchor="commit">
+        <name>Commit</name>
+        <t>A Commit message initiates a new epoch for the group, based on a collection of
+Proposals. It instructs group members to update their representation of the
+state of the group by applying the proposals and advancing the key schedule.</t>
+        <t>Each proposal covered by the Commit is included by a ProposalOrRef value, which
+identifies the proposal to be applied by value or by reference.  Commits that
+refer to new Proposals from the committer can be included by value. Commits
+for previously sent proposals from anyone (including the committer) can be sent
+by reference.  Proposals sent by reference are specified by including the hash of
+the AuthenticatedContent object in which the proposal was sent (see <xref target="hash-based-identifiers"/>).</t>
+        <sourcecode type="tls-presentation">
+enum {
+  reserved(0),
+  proposal(1),
+  reference(2),
+  (255)
+} ProposalOrRefType;
+
+struct {
+  ProposalOrRefType type;
+  select (ProposalOrRef.type) {
+    case proposal:  Proposal proposal;
+    case reference: ProposalRef reference;
+  };
+} ProposalOrRef;
+
+struct {
+    ProposalOrRef proposals&lt;V&gt;;
+    optional&lt;UpdatePath&gt; path;
+} Commit;
+</sourcecode>
+        <t>A group member that has observed one or more valid proposals within an epoch <bcp14>MUST</bcp14> send
+a Commit message before sending application data. This ensures, for example,
+that any members whose removal was proposed during the epoch are actually
+removed before any application data is transmitted.</t>
+        <t>A sender and a receiver of a Commit <bcp14>MUST</bcp14> verify that the committed list of
+proposals is valid as specified in <xref target="proposal-list-validation"/>. A list is invalid if, for example,
+it includes an Update and a Remove for the same member, or an Add when the sender does not have
+the application-level permission to add new users.</t>
+        <t>The sender of a Commit <bcp14>SHOULD</bcp14> include all proposals that it has received
+during the current epoch that are valid according to the rules for their
+proposal types and according to application policy, as long as this results in
+a valid proposal list.</t>
+        <t>Due to the asynchronous nature of proposals, receivers of a Commit <bcp14>SHOULD NOT</bcp14> enforce
+that all valid proposals sent within the current epoch are referenced by the next
+Commit. In the event that a valid proposal is omitted from the next Commit, and
+that proposal is still valid in the current epoch, the sender of the proposal
+<bcp14>MAY</bcp14> resend it after updating it to reflect the current epoch.</t>
+        <t>A member of the group <bcp14>MAY</bcp14> send a Commit that references no proposals at all,
+which would thus have an empty <tt>proposals</tt> vector.  Such
+a Commit resets the sender's leaf and the nodes along its direct path, and
+provides forward secrecy and post-compromise security with regard to the sender
+of the Commit.  An Update proposal can be regarded as a "lazy" version of this
+operation, where only the leaf changes and intermediate nodes are blanked out.</t>
+        <t>By default, the <tt>path</tt> field of a Commit <bcp14>MUST</bcp14> be populated.  The <tt>path</tt> field
+<bcp14>MAY</bcp14> be omitted if (a) it covers at least one proposal and (b) none of the proposals
+covered by the Commit are of "path required" types.  A proposal type requires a
+path if it cannot change the group membership in a way that requires the forward
+secrecy and post-compromise security guarantees that an UpdatePath provides.
+The only proposal types defined in this document that do not require a path are:</t>
+        <ul spacing="normal">
+          <li>
+            <tt>add</tt>
+          </li>
+          <li>
+            <tt>psk</tt>
+          </li>
+          <li>
+            <tt>reinit</tt>
+          </li>
+        </ul>
+        <t>New proposal types <bcp14>MUST</bcp14> state whether they require a path. If any instance of a
+proposal type requires a path, then the proposal type requires a path. This
+attribute of a proposal type is reflected in the "Path Required" field of the
+"MLS Proposal Types" registry defined in <xref target="mls-proposal-types"/>.</t>
+        <t>Update and Remove proposals are the clearest examples of proposals that require
+a path.  An UpdatePath is required to evict the removed member or the old
+appearance of the updated member.</t>
+        <t>In pseudocode, the logic for validating the <tt>path</tt> field of a Commit is as
+follows:</t>
+        <sourcecode type="pseudocode">
+pathRequiredTypes = [
+    update,
+    remove,
+    external_init,
+    group_context_extensions
+]
+
+pathRequired = false
+
+for proposal in commit.proposals:
+    pathRequired = pathRequired ||
+                   (proposal.msg_type in pathRequiredTypes)
+
+if len(commit.proposals) == 0 || pathRequired:
+    assert(commit.path != null)
+</sourcecode>
+        <t>To summarize, a Commit can have three different configurations, with different
+uses:</t>
+        <ol spacing="normal" type="1">
+          <li>An "empty" Commit that references no proposals, which updates the committer's
+contribution to the group and provides PCS with regard to the committer.</li>
+          <li>A "partial" Commit that references proposals that do not require a path, and
+where the path is empty. Such a Commit doesn't provide PCS with regard to the
+committer.</li>
+          <li>A "full" Commit that references proposals of any type, which provides FS with
+regard to any removed members and PCS for the committer and any updated
+members.</li>
+        </ol>
+        <section anchor="creating-a-commit">
+          <name>Creating a Commit</name>
+          <t>When creating or processing a Commit, a client updates the ratchet tree and
+GroupContext for the group.  These values advance from an "old" state reflecting
+the current epoch to a "new" state reflecting the new epoch initiated by the
+Commit.  When the Commit includes an UpdatePath, a "provisional" group context
+is constructed that reflects changes due to the proposals and UpdatePath, but
+with the old confirmed transcript hash.</t>
+          <t>A member of the group creates a Commit message and the corresponding Welcome
+message at the same time, by taking the following steps:</t>
+          <ul spacing="normal">
+            <li>Verify that the list of proposals to be committed is valid as specified in
+<xref target="proposal-list-validation"/>.</li>
+            <li>Construct an initial Commit object with the <tt>proposals</tt> field populated from
+Proposals received during the current epoch, and with the <tt>path</tt> field empty.</li>
+            <li>Create the new ratchet tree and GroupContext by applying the list of proposals
+to the old ratchet tree and GroupContext, as defined in
+<xref target="applying-a-proposal-list"/>.</li>
+            <li>Decide whether to populate the <tt>path</tt> field: If the <tt>path</tt> field is required
+based on the proposals that are in the Commit (see above), then it <bcp14>MUST</bcp14> be
+populated.  Otherwise, the sender <bcp14>MAY</bcp14> omit the <tt>path</tt> field at its discretion.</li>
+            <li>
+              <t>If populating the <tt>path</tt> field:  </t>
+              <ul spacing="normal">
+                <li>If this is an external Commit, assign the sender the leftmost blank leaf
+node in the new ratchet tree.  If there are no blank leaf nodes in the new
+ratchet tree, expand the tree to the right as defined in
+<xref target="adding-and-removing-leaves"/> and assign the leftmost new blank leaf to the
+sender.</li>
+                <li>Update the sender's direct path in the ratchet tree as described in
+<xref target="synchronizing-views-of-the-tree"/>.  Define
+<tt>commit_secret</tt> as the value <tt>path_secret[n+1]</tt> derived from the
+last path secret value (<tt>path_secret[n]</tt>) derived for the UpdatePath.</li>
+                <li>
+                  <t>Construct a provisional GroupContext object containing the following values:
+                  </t>
+                  <ul spacing="normal">
+                    <li><tt>group_id</tt>: Same as the old GroupContext</li>
+                    <li><tt>epoch</tt>: The epoch number for the new epoch</li>
+                    <li><tt>tree_hash</tt>: The tree hash of the new ratchet tree</li>
+                    <li><tt>confirmed_transcript_hash</tt>: Same as the old GroupContext</li>
+                    <li><tt>extensions</tt>: The new GroupContext extensions (possibly updated by a
+GroupContextExtensions proposal)</li>
+                  </ul>
+                </li>
+                <li>Encrypt the path secrets resulting from the tree update to the group as
+described in <xref target="synchronizing-views-of-the-tree"/>, using the provisional
+group context as the context for HPKE encryption.</li>
+                <li>Create an UpdatePath containing the sender's new leaf node and the new
+public keys and encrypted path secrets along the sender's filtered direct
+path.  Assign this UpdatePath to the <tt>path</tt> field in the Commit.</li>
+              </ul>
+            </li>
+            <li>If not populating the <tt>path</tt> field: Set the <tt>path</tt> field in the Commit to the
+null optional.  Define <tt>commit_secret</tt> as the all-zero vector of length
+<tt>KDF.Nh</tt> (the same length as a <tt>path_secret</tt> value would be).</li>
+            <li>Derive the <tt>psk_secret</tt> as specified in <xref target="pre-shared-keys"/>, where the order
+of PSKs in the derivation corresponds to the order of PreSharedKey proposals
+in the <tt>proposals</tt> vector.</li>
+            <li>
+              <t>Construct a FramedContent object containing the Commit object. Sign the
+FramedContent using the old GroupContext as context.
+              </t>
+              <ul spacing="normal">
+                <li>Use the FramedContent to update the confirmed transcript hash and update
+the new GroupContext.</li>
+                <li>Use the <tt>init_secret</tt> from the previous epoch, the <tt>commit_secret</tt> and
+<tt>psk_secret</tt> defined in the previous steps, and the new GroupContext to
+compute the new <tt>joiner_secret</tt>, <tt>welcome_secret</tt>, <tt>epoch_secret</tt>, and
+derived secrets for the new epoch.</li>
+                <li>Use the <tt>confirmation_key</tt> for the new epoch to compute the
+<tt>confirmation_tag</tt> value.</li>
+                <li>Calculate the interim transcript hash using the new confirmed transcript
+hash and the <tt>confirmation_tag</tt> from the FramedContentAuthData.</li>
+              </ul>
+            </li>
+            <li>
+              <t>Protect the AuthenticatedContent object using keys from the old epoch:
+              </t>
+              <ul spacing="normal">
+                <li>If encoding as PublicMessage, compute the <tt>membership_tag</tt> value using the
+<tt>membership_key</tt>.</li>
+                <li>If encoding as a PrivateMessage, encrypt the message using the
+<tt>sender_data_secret</tt> and the next (key, nonce) pair from the sender's
+handshake ratchet.</li>
+              </ul>
+            </li>
+            <li>
+              <t>Construct a GroupInfo reflecting the new state:
+              </t>
+              <ul spacing="normal">
+                <li>Set the <tt>group_id</tt>, <tt>epoch</tt>, <tt>tree</tt>, <tt>confirmed_transcript_hash</tt>,
+<tt>interim_transcript_hash</tt>, and <tt>group_context_extensions</tt> fields to reflect
+the new state.</li>
+                <li>Set the <tt>confirmation_tag</tt> field to the value of the corresponding field in
+the FramedContentAuthData object.</li>
+                <li>Add any other extensions as defined by the application.</li>
+                <li>Optionally derive an external key pair as described in <xref target="key-schedule"/>.
+(required for external Commits, see <xref target="joining-via-external-commits"/>).</li>
+                <li>Sign the GroupInfo using the member's private signing key.</li>
+                <li>Encrypt the GroupInfo using the key and nonce derived from the <tt>joiner_secret</tt>.
+for the new epoch (see <xref target="joining-via-welcome-message"/>).</li>
+              </ul>
+            </li>
+            <li>
+              <t>For each new member in the group:
+              </t>
+              <ul spacing="normal">
+                <li>Identify the lowest common ancestor in the tree of the new member's
+leaf node and the member sending the Commit.</li>
+                <li>If the <tt>path</tt> field was populated above: Compute the path secret
+corresponding to the common ancestor node.</li>
+                <li>Compute an EncryptedGroupSecrets object that encapsulates the <tt>init_secret</tt>
+for the current epoch and the path secret (if present).</li>
+              </ul>
+            </li>
+            <li>Construct one or more Welcome messages from the encrypted GroupInfo object,
+the encrypted key packages, and any PSKs for which a proposal was included in
+the Commit. The order of the <tt>psks</tt> <bcp14>MUST</bcp14> be the same as the order of
+PreSharedKey proposals in the <tt>proposals</tt> vector.  As discussed in
+<xref target="joining-via-welcome-message"/>, the committer is free to choose how many
+Welcome messages to construct.  However, the set of Welcome messages produced
+in this step <bcp14>MUST</bcp14> cover every new member added in the Commit.</li>
+            <li>
+              <t>If a ReInit proposal was part of the Commit, the committer <bcp14>MUST</bcp14> create a new
+group with the parameters specified in the ReInit proposal,
+and with the same members as the original group.
+The Welcome message <bcp14>MUST</bcp14> include a PreSharedKeyID with the following
+parameters:
+              </t>
+              <ul spacing="normal">
+                <li><tt>psktype</tt>: <tt>resumption</tt></li>
+                <li><tt>usage</tt>: <tt>reinit</tt></li>
+                <li><tt>group_id</tt>: The group ID for the current group</li>
+                <li><tt>epoch</tt>: The epoch that the group will be in after this Commit</li>
+              </ul>
+            </li>
+          </ul>
+        </section>
+        <section anchor="processing-a-commit">
+          <name>Processing a Commit</name>
+          <t>A member of the group applies a Commit message by taking the following steps:</t>
+          <ul spacing="normal">
+            <li>Verify that the <tt>epoch</tt> field of the enclosing FramedContent is equal
+to the <tt>epoch</tt> field of the current GroupContext object.</li>
+            <li>
+              <t>Unprotect the Commit using the keys from the current epoch:
+              </t>
+              <ul spacing="normal">
+                <li>If the message is encoded as PublicMessage, verify the membership MAC using
+the <tt>membership_key</tt>.</li>
+                <li>If the message is encoded as PrivateMessage, decrypt the message using the
+<tt>sender_data_secret</tt> and the (key, nonce) pair from the step on the sender's
+hash ratchet indicated by the <tt>generation</tt> field.</li>
+              </ul>
+            </li>
+            <li>Verify the signature on the FramedContent message as described in
+<xref target="content-authentication"/>.</li>
+            <li>Verify that the <tt>proposals</tt> vector is valid according to the rules in
+<xref target="proposal-list-validation"/>.</li>
+            <li>Verify that all PreSharedKey proposals in the <tt>proposals</tt> vector are available.</li>
+            <li>Create the new ratchet tree and GroupContext by applying the list of proposals
+to the old ratchet tree and GroupContext, as defined in
+<xref target="applying-a-proposal-list"/>.</li>
+            <li>Verify that the <tt>path</tt> value is populated if the <tt>proposals</tt> vector contains
+any Update or Remove proposals, or if it's empty. Otherwise, the <tt>path</tt> value
+<bcp14>MAY</bcp14> be omitted.</li>
+            <li>
+              <t>If the <tt>path</tt> value is populated, validate it and apply it to the tree:  </t>
+              <ul spacing="normal">
+                <li>If this is an external Commit, assign the sender the leftmost blank leaf
+node in the new ratchet tree.  If there are no blank leaf nodes in the new
+ratchet tree, add a blank leaf to the right side of the new ratchet tree and
+assign it to the sender.</li>
+                <li>Validate the LeafNode as specified in <xref target="leaf-node-validation"/>.  The
+<tt>leaf_node_source</tt> field <bcp14>MUST</bcp14> be set to <tt>commit</tt>.</li>
+                <li>Verify that the <tt>encryption_key</tt> value in the LeafNode is different from the
+committer's current leaf node.</li>
+                <li>Verify that none of the public keys in the UpdatePath appear in any node of
+the new ratchet tree.</li>
+                <li>Merge the UpdatePath into the new ratchet tree, as described in
+<xref target="synchronizing-views-of-the-tree"/>.</li>
+                <li>
+                  <t>Construct a provisional GroupContext object containing the following values:
+                  </t>
+                  <ul spacing="normal">
+                    <li><tt>group_id</tt>: Same as the old GroupContext</li>
+                    <li><tt>epoch</tt>: The epoch number for the new epoch</li>
+                    <li><tt>tree_hash</tt>: The tree hash of the new ratchet tree</li>
+                    <li><tt>confirmed_transcript_hash</tt>: Same as the old GroupContext</li>
+                    <li><tt>extensions</tt>: The new GroupContext extensions (possibly updated by a
+GroupContextExtensions proposal)</li>
+                  </ul>
+                </li>
+                <li>Decrypt the path secrets for UpdatePath as described in
+<xref target="synchronizing-views-of-the-tree"/>, using the provisional GroupContext as
+the context for HPKE decryption.</li>
+                <li>Define <tt>commit_secret</tt> as the value <tt>path_secret[n+1]</tt> derived from the
+last path secret value (<tt>path_secret[n]</tt>) derived for the UpdatePath.</li>
+              </ul>
+            </li>
+            <li>If the <tt>path</tt> value is not populated, define <tt>commit_secret</tt> as the all-zero
+vector of length <tt>KDF.Nh</tt> (the same length as a <tt>path_secret</tt> value would be).</li>
+            <li>Update the confirmed and interim transcript hashes using the new Commit, and
+generate the new GroupContext.</li>
+            <li>Derive the <tt>psk_secret</tt> as specified in <xref target="pre-shared-keys"/>, where the order
+of PSKs in the derivation corresponds to the order of PreSharedKey proposals
+in the <tt>proposals</tt> vector.</li>
+            <li>Use the <tt>init_secret</tt> from the previous epoch, the <tt>commit_secret</tt> and
+<tt>psk_secret</tt> defined in the previous steps, and the new GroupContext to
+compute the new <tt>joiner_secret</tt>, <tt>welcome_secret</tt>, <tt>epoch_secret</tt>, and
+derived secrets for the new epoch.</li>
+            <li>Use the <tt>confirmation_key</tt> for the new epoch to compute the confirmation tag
+for this message, as described below, and verify that it is the same as the
+<tt>confirmation_tag</tt> field in the FramedContentAuthData object.</li>
+            <li>If the above checks are successful, consider the new GroupContext object
+as the current state of the group.</li>
+            <li>If the Commit included a ReInit proposal, the client <bcp14>MUST NOT</bcp14> use the group to
+send messages anymore. Instead, it <bcp14>MUST</bcp14> wait for a Welcome message from the committer
+meeting the requirements of <xref target="reinitialization"/>.</li>
+          </ul>
+          <t>Note that clients need to be prepared to receive a valid Commit message that removes
+them from the group. In this case, the client cannot send any more messages in the
+group and <bcp14>SHOULD</bcp14> promptly delete its group state and secret tree. (A client might keep
+the secret tree for a short time to decrypt late messages in the previous epoch.)</t>
+        </section>
+        <section anchor="adding-members-to-the-group">
+          <name>Adding Members to the Group</name>
+          <t>New members can join the group in two ways: by being added by a group
+member or by adding themselves through an external Commit. In both cases, the
+new members need information to bootstrap their local group state.</t>
+          <sourcecode type="tls-presentation">
+struct {
+    GroupContext group_context;
+    Extension extensions&lt;V&gt;;
+    MAC confirmation_tag;
+    uint32 signer;
+    /* SignWithLabel(., "GroupInfoTBS", GroupInfoTBS) */
+    opaque signature&lt;V&gt;;
+} GroupInfo;
+</sourcecode>
+          <t>The <tt>group_context</tt> field represents the current state of the group.  The
+<tt>extensions</tt> field allows the sender to provide additional data that might be
+useful to new joiners.  The <tt>confirmation_tag</tt> represents the  confirmation tag
+from the Commit that initiated the current epoch, or for epoch 0, the
+confirmation tag computed in the creation of the group (see <xref target="group-creation"/>).
+(In either case, the creator of a GroupInfo may recompute the confirmation tag
+as <tt>MAC(confirmation_key, confirmed_transcript_hash)</tt>.)</t>
+          <t>As discussed in <xref target="extensibility"/>, unknown extensions in <tt>GroupInfo.extensions</tt> <bcp14>MUST</bcp14> be ignored, and the creator of a GroupInfo object <bcp14>SHOULD</bcp14> include some
+random GREASE extensions to help ensure that other clients correctly ignore unknown
+extensions.  Extensions in <tt>GroupInfo.group_context.extensions</tt>, however, <bcp14>MUST</bcp14>
+be supported by the new joiner.</t>
+          <t>New members <bcp14>MUST</bcp14> verify that <tt>group_id</tt> is unique among the groups they are
+currently participating in.</t>
+          <t>New members also <bcp14>MUST</bcp14> verify the <tt>signature</tt> using the public key taken from the
+leaf node of the ratchet tree with leaf index <tt>signer</tt>. The
+signature covers the following structure, comprising all the fields in the
+GroupInfo above <tt>signature</tt>:</t>
+          <sourcecode type="tls-presentation">
+struct {
+    GroupContext group_context;
+    Extension extensions&lt;V&gt;;
+    MAC confirmation_tag;
+    uint32 signer;
+} GroupInfoTBS;
+</sourcecode>
+          <section anchor="joining-via-welcome-message">
+            <name>Joining via Welcome Message</name>
+            <t>The sender of a Commit message is responsible for sending a Welcome message to
+each new member added via Add proposals.  The format of the Welcome message
+allows a single Welcome message to be encrypted for multiple new members.  It is
+up to the committer to decide how many Welcome messages to create for a given
+Commit. The committer could create one Welcome that is encrypted for all new
+members, a different Welcome for each new member, or Welcome messages for
+batches of new members (according to some batching scheme that works well for
+the application).  The processes for creating and processing the Welcome are the
+same in all cases, aside from the set of new members for whom a given Welcome is
+encrypted.</t>
+            <t>The Welcome message provides the new
+members with the current state of the group after the application of the Commit
+message.  The new members will not be able to decrypt or verify the Commit
+message, but they will have the secrets they need to participate in the epoch
+initiated by the Commit message.</t>
+            <t>In order to allow the same Welcome message to be sent to multiple new members,
+information describing the group is encrypted with a symmetric key and nonce
+derived from the <tt>joiner_secret</tt> for the new epoch.  The <tt>joiner_secret</tt> is
+then encrypted to each new member using HPKE.  In the same encrypted package,
+the committer transmits the path secret for the lowest (closest to the leaf) node
+that is contained in the direct paths of both the committer and the new member.
+This allows the new
+member to compute private keys for nodes in its direct path that are being
+reset by the corresponding Commit.</t>
+            <t>If the sender of the Welcome message wants the receiving member to include a PSK
+in the derivation of the <tt>epoch_secret</tt>, they can populate the <tt>psks</tt> field
+indicating which PSK to use.</t>
+            <sourcecode type="tls-presentation">
+struct {
+  opaque path_secret&lt;V&gt;;
+} PathSecret;
+
+struct {
+  opaque joiner_secret&lt;V&gt;;
+  optional&lt;PathSecret&gt; path_secret;
+  PreSharedKeyID psks&lt;V&gt;;
+} GroupSecrets;
+
+struct {
+  KeyPackageRef new_member;
+  HPKECiphertext encrypted_group_secrets;
+} EncryptedGroupSecrets;
+
+struct {
+  CipherSuite cipher_suite;
+  EncryptedGroupSecrets secrets&lt;V&gt;;
+  opaque encrypted_group_info&lt;V&gt;;
+} Welcome;
+</sourcecode>
+            <t>The client processing a Welcome message will need to have a copy of the group's
+ratchet tree.  The tree can be provided in the Welcome message, in an extension
+of type <tt>ratchet_tree</tt>.  If it is sent otherwise (e.g., provided by a caching
+service on the Delivery Service), then the client <bcp14>MUST</bcp14> download the tree before
+processing the Welcome.</t>
+            <t>On receiving a Welcome message, a client processes it using the following steps:</t>
+            <ul spacing="normal">
+              <li>Identify an entry in the <tt>secrets</tt> array where the <tt>new_member</tt>
+value corresponds to one of this client's KeyPackages, using the hash
+indicated by the <tt>cipher_suite</tt> field. If no such field exists, or if the
+cipher suite indicated in the KeyPackage does not match the one in the
+Welcome message, return an error.</li>
+              <li>Decrypt the <tt>encrypted_group_secrets</tt> value with the algorithms indicated by
+the cipher suite and the private key <tt>init_key_priv</tt> corresponding to
+<tt>init_key</tt> in the referenced KeyPackage.</li>
+            </ul>
+            <sourcecode type="pseudocode">
+encrypted_group_secrets =
+  EncryptWithLabel(init_key, "Welcome",
+                   encrypted_group_info, group_secrets)
+
+group_secrets =
+  DecryptWithLabel(init_key_priv, "Welcome",
+                   encrypted_group_info, kem_output, ciphertext)
+</sourcecode>
+            <ul spacing="normal">
+              <li>If a PreSharedKeyID is part of the GroupSecrets and the client is not in
+possession of the corresponding PSK, return an error. Additionally, if a
+PreSharedKeyID has type <tt>resumption</tt> with usage <tt>reinit</tt> or <tt>branch</tt>, verify
+that it is the only such PSK.</li>
+              <li>From the <tt>joiner_secret</tt> in the decrypted GroupSecrets object and the PSKs
+specified in the GroupSecrets, derive the <tt>welcome_secret</tt> and then
+the <tt>welcome_key</tt> and <tt>welcome_nonce</tt>. Use the key and nonce to decrypt the
+<tt>encrypted_group_info</tt> field.</li>
+            </ul>
+            <sourcecode type="pseudocode">
+welcome_nonce = ExpandWithLabel(welcome_secret, "nonce", "", AEAD.Nn)
+welcome_key = ExpandWithLabel(welcome_secret, "key", "", AEAD.Nk)
+</sourcecode>
+            <ul spacing="normal">
+              <li>Verify the signature on the GroupInfo object. The signature input comprises
+all of the fields in the GroupInfo object except the signature field. The
+public key is taken from the LeafNode of the
+ratchet tree with leaf index <tt>signer</tt>. If the node is blank or if
+signature verification fails, return an error.</li>
+              <li>Verify that the <tt>group_id</tt> is unique among the groups that the client is
+currently participating in.</li>
+              <li>Verify that the <tt>cipher_suite</tt> in the GroupInfo matches the <tt>cipher_suite</tt> in
+the KeyPackage.</li>
+              <li>
+                <t>Verify the integrity of the ratchet tree.  </t>
+                <ul spacing="normal">
+                  <li>Verify that the tree hash of the ratchet tree matches the <tt>tree_hash</tt> field
+in GroupInfo.</li>
+                  <li>For each non-empty parent node, verify that it is "parent-hash valid",
+as described in <xref target="verifying-parent-hashes"/>.</li>
+                  <li>For each non-empty leaf node, validate the LeafNode as described in
+<xref target="leaf-node-validation"/>.</li>
+                  <li>
+                    <t>For each non-empty parent node and each entry in the node's
+<tt>unmerged_leaves</tt> field:      </t>
+                    <ul spacing="normal">
+                      <li>Verify that the entry represents a non-blank leaf node that is a
+descendant of the parent node.</li>
+                      <li>Verify that every non-blank intermediate node between the leaf node and the
+parent node also has an entry for the leaf node in its <tt>unmerged_leaves</tt>.</li>
+                      <li>Verify that the encryption key in the parent node does not appear in any
+other node of the tree.</li>
+                    </ul>
+                  </li>
+                </ul>
+              </li>
+              <li>Identify a leaf whose LeafNode is
+identical to the one in the KeyPackage.  If no such field exists, return an
+error.  Let <tt>my_leaf</tt> represent this leaf in the tree.</li>
+              <li>
+                <t>Construct a new group state using the information in the GroupInfo object.  </t>
+                <ul spacing="normal">
+                  <li>Initialize the GroupContext for the group from the <tt>group_context</tt> field
+from the GroupInfo object.</li>
+                  <li>Update the leaf <tt>my_leaf</tt> with the private key corresponding to the
+public key in the node, where <tt>my_leaf</tt> is the new member's leaf node in
+the ratchet tree, as defined above.</li>
+                  <li>If the <tt>path_secret</tt> value is set in the GroupSecrets object: Identify the
+lowest common ancestor of the leaf node <tt>my_leaf</tt> and of the node of
+the member with leaf index <tt>GroupInfo.signer</tt>. Set the private key for
+this node to the private key derived from the <tt>path_secret</tt>.</li>
+                  <li>For each parent of the common ancestor, up to the root of the tree, derive
+a new path secret, and set the private key for the node to the private key
+derived from the path secret.  The private key <bcp14>MUST</bcp14> be the private key
+that corresponds to the public key in the node.</li>
+                </ul>
+              </li>
+              <li>Use the <tt>joiner_secret</tt> from the GroupSecrets object to generate the epoch secret
+and other derived secrets for the current epoch.</li>
+              <li>Set the confirmed transcript hash in the new state to the value of the
+<tt>confirmed_transcript_hash</tt> in the GroupInfo.</li>
+              <li>Verify the confirmation tag in the GroupInfo using the derived confirmation
+key and the <tt>confirmed_transcript_hash</tt> from the GroupInfo.</li>
+              <li>Use the confirmed transcript hash and confirmation tag to compute the interim
+transcript hash in the new state.</li>
+              <li>
+                <t>If a PreSharedKeyID was used that has type <tt>resumption</tt> with usage <tt>reinit</tt>
+or <tt>branch</tt>, verify that the <tt>epoch</tt> field in the GroupInfo is equal to 1.  </t>
+                <ul spacing="normal">
+                  <li>For usage <tt>reinit</tt>, verify that the last Commit to the referenced group
+contains a ReInit proposal and that the <tt>group_id</tt>, <tt>version</tt>,
+<tt>cipher_suite</tt>, and <tt>group_context.extensions</tt> fields of the GroupInfo match
+the ReInit proposal. Additionally, verify that all the members of the old
+group are also members of the new group, according to the application.</li>
+                  <li>For usage <tt>branch</tt>, verify that the <tt>version</tt> and <tt>cipher_suite</tt> of the new
+group match those of the old group, and that the members of the new group
+compose a subset of the members of the old group, according to the
+application.</li>
+                </ul>
+              </li>
+            </ul>
+          </section>
+          <section anchor="joining-via-external-commits">
+            <name>Joining via External Commits</name>
+            <t>External Commits are a mechanism for new members (external parties that want to
+become members of the group) to add themselves to a group, without requiring
+that an existing member has to come online to issue a Commit that references an
+Add proposal.</t>
+            <t>Whether existing members of the group will accept or reject an external Commit
+follows the same rules that are applied to other handshake messages.</t>
+            <t>New members can create and issue an external Commit if they have access to the
+following information for the group's current epoch:</t>
+            <ul spacing="normal">
+              <li>group ID</li>
+              <li>epoch ID</li>
+              <li>cipher suite</li>
+              <li>public tree hash</li>
+              <li>confirmed transcript hash</li>
+              <li>confirmation tag of the most recent Commit</li>
+              <li>group extensions</li>
+              <li>external public key</li>
+            </ul>
+            <t>In other words, to join a group via an external Commit, a new member needs a
+GroupInfo with an <tt>external_pub</tt> extension present in its <tt>extensions</tt> field.</t>
+            <sourcecode type="tls-presentation">
+struct {
+    HPKEPublicKey external_pub;
+} ExternalPub;
+</sourcecode>
+            <t>Thus, a member of the group can enable new clients to join by making a GroupInfo
+object available to them. Note that because a GroupInfo object is specific to an
+epoch, it will need to be updated as the group advances. In particular, each
+GroupInfo object can be used for one external join, since that external join
+will cause the epoch to change.</t>
+            <t>Note that the <tt>tree_hash</tt> field is used the same way as in the Welcome message.
+The full tree can be included via the <tt>ratchet_tree</tt> extension
+(see <xref target="ratchet-tree-extension"/>).</t>
+            <t>The information in a GroupInfo is not generally public information, but applications
+can choose to make it available to new members in order to allow External
+Commits.</t>
+            <t>In principle, external Commits work like regular Commits. However, their content
+has to meet a specific set of requirements:</t>
+            <ul spacing="normal">
+              <li>External Commits <bcp14>MUST</bcp14> contain a <tt>path</tt> field (and is therefore a "full"
+Commit).  The joiner is added at the leftmost free leaf node (just as if they
+were added with an Add proposal), and the path is calculated relative to that
+leaf node.</li>
+              <li>The Commit <bcp14>MUST NOT</bcp14> include any proposals by reference, since an external
+joiner cannot determine the validity of proposals sent within the group.</li>
+              <li>External Commits <bcp14>MUST</bcp14> be signed by the new member.  In particular, the
+signature on the enclosing AuthenticatedContent <bcp14>MUST</bcp14> verify using the public key for
+the credential in the <tt>leaf_node</tt> of the <tt>path</tt> field.</li>
+              <li>When processing a Commit, both existing and new members <bcp14>MUST</bcp14> use the external
+init secret as described in <xref target="external-initialization"/>.</li>
+              <li>The sender type for the AuthenticatedContent encapsulating the external Commit <bcp14>MUST</bcp14> be
+<tt>new_member_commit</tt>.</li>
+            </ul>
+            <t>External Commits come in two "flavors" -- a "join" Commit that
+adds the sender to the group or a "resync" Commit that replaces a member's prior
+appearance with a new one.</t>
+            <t>Note that the "resync" operation allows an attacker that has compromised a
+member's signature private key to introduce themselves into the group and remove the
+prior, legitimate member in a single Commit.  Without resync, this
+can still be done, but it requires two operations: the external Commit to join and
+a second Commit to remove the old appearance.  Applications for whom this
+distinction is salient can choose to disallow external commits that contain a
+Remove, or to allow such resync commits only if they contain a "reinit" PSK
+proposal that demonstrates the joining member's presence in a prior epoch of the
+group.  With the latter approach, the attacker would need to compromise the PSK
+as well as the signing key, but the application will need to ensure that
+continuing, non-resynchronizing members have the required PSK.</t>
+          </section>
+          <section anchor="ratchet-tree-extension">
+            <name>Ratchet Tree Extension</name>
+            <t>By default, a GroupInfo message only provides the joiner with a hash of
+the group's ratchet tree.  In order to process or generate handshake
+messages, the joiner will need to get a copy of the ratchet tree from some other
+source.  (For example, the DS might provide a cached copy.)  The inclusion of
+the tree hash in the GroupInfo message means that the source of the ratchet
+tree need not be trusted to maintain the integrity of the tree.</t>
+            <t>In cases where the application does not wish to provide such an external source,
+the whole public state of the ratchet tree can be provided in an extension of
+type <tt>ratchet_tree</tt>, containing a <tt>ratchet_tree</tt> object of the following form:</t>
+            <sourcecode type="tls-presentation">
+struct {
+    NodeType node_type;
+    select (Node.node_type) {
+        case leaf:   LeafNode leaf_node;
+        case parent: ParentNode parent_node;
+    };
+} Node;
+
+optional&lt;Node&gt; ratchet_tree&lt;V&gt;;
+</sourcecode>
+            <t>Each entry in the <tt>ratchet_tree</tt> vector provides the value for a node in the
+tree, or the null optional for a blank node.</t>
+            <t>The nodes are listed in the order specified by a left-to-right in-order
+traversal of the ratchet tree. Each node is listed between its left subtree and
+its right subtree.  (This is the same ordering as specified for the array-based
+trees outlined in <xref target="array-based-trees"/>.)</t>
+            <t>If the tree has 2<sup>d</sup> leaves, then it has 2<sup>d+1</sup> - 1 nodes.  The
+<tt>ratchet_tree</tt> vector logically has this number of entries, but the sender
+<bcp14>MUST NOT</bcp14> include blank nodes after the last non-blank node.  The receiver <bcp14>MUST</bcp14>
+check that the last node in <tt>ratchet_tree</tt> is non-blank, and then extend the tree to the
+right until it has a length of the form 2<sup>d+1</sup> - 1, adding the minimum number
+of blank values possible.  (Obviously, this may be done "virtually", by
+synthesizing blank nodes when required, as opposed to actually changing the
+structure in memory.)</t>
+            <t>The leaves of the tree are stored in even-numbered entries in the array (the
+leaf with index <tt>L</tt> in array position <tt>2*L</tt>). The root node of the tree is at
+position 2<sup>d</sup> - 1 of the array. Intermediate parent nodes can be identified by
+performing the same calculation to the subarrays to the left and right of the
+root, following something like the following algorithm:</t>
+            <sourcecode type="python">
+# Assuming a class Node that has left and right members
+def subtree_root(nodes):
+    # If there is only one node in the array, return it
+    if len(nodes) == 1:
+        return Node(nodes[0])
+
+    # Otherwise, the length of the array MUST be odd
+    if len(nodes) % 2 == 0:
+        raise Exception("Malformed node array {}", len(nodes))
+
+    # Identify the root of the subtree
+    d = 0
+    while (2**(d+1)) &lt; len(nodes):
+       d += 1
+    R = 2**d - 1
+    root = Node(nodes[R])
+    root.left = subtree_root(nodes[:R])
+    root.right = subtree_root(nodes[(R+1):])
+    return root
+</sourcecode>
+            <t>(Note that this is the same ordering of nodes as in the array-based tree representation
+described in <xref target="array-based-trees"/>.  The algorithms in that section may be used to
+simplify decoding this extension into other representations.)</t>
+            <t>For example, the following tree with six non-blank leaves would be represented
+as an array of eleven elements, <tt>[A, W, B, X, C, _, D, Y, E, Z, F]</tt>.  The above
+decoding procedure would identify the subtree roots as follows (using R to
+represent a subtree root):</t>
+            <figure>
+              <name>Left-to-Right In-Order Traversal of a Six-Member Tree</name>
+              <artset>
+                <artwork type="svg">
+                  <svg xmlns="http://www.w3.org/2000/svg" class="diagram" font-family="monospace" font-size="13px" height="304" text-anchor="middle" version="1.1" viewBox="0 0 240 304" width="240">
+                    <path d="M 56,112 L 56,128" fill="none" stroke="black"/>
+                    <path d="M 120,48 L 120,64" fill="none" stroke="black"/>
+                    <path d="M 184,104 L 184,128" fill="none" stroke="black"/>
+                    <path d="M 72,64 L 168,64" fill="none" stroke="black"/>
+                    <path d="M 40,128 L 72,128" fill="none" stroke="black"/>
+                    <path d="M 168,128 L 200,128" fill="none" stroke="black"/>
+                    <path d="M 8,256 L 104,256" fill="none" stroke="black"/>
+                    <path d="M 136,256 L 232,256" fill="none" stroke="black"/>
+                    <path d="M 8,272 L 40,272" fill="none" stroke="black"/>
+                    <path d="M 72,272 L 104,272" fill="none" stroke="black"/>
+                    <path d="M 136,272 L 168,272" fill="none" stroke="black"/>
+                    <path d="M 200,272 L 232,272" fill="none" stroke="black"/>
+                    <path d="M 72,128 L 80,144" fill="none" stroke="black"/>
+                    <path d="M 92,168 L 96,176" fill="none" stroke="black"/>
+                    <path d="M 168,64 L 176,80" fill="none" stroke="black"/>
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+            <t>The presence of a <tt>ratchet_tree</tt> extension in a GroupInfo message does not
+result in any changes to the GroupContext extensions for the group.  The ratchet
+tree provided is simply stored by the client and used for MLS operations.</t>
+            <t>If this extension is not provided in a Welcome message, then the client will
+need to fetch the ratchet tree over some other channel before it can generate or
+process Commit messages.  Applications should ensure that this out-of-band
+channel is provided with security protections equivalent to the protections that
+are afforded to Proposal and Commit messages.  For example, an application that
+encrypts Proposal and Commit messages might distribute ratchet trees encrypted
+using a key exchanged over the MLS channel.</t>
+            <t>Regardless of how the client obtains the tree, the client <bcp14>MUST</bcp14> verify that the
+root hash of the ratchet tree matches the <tt>tree_hash</tt> of the GroupContext before
+using the tree for MLS operations.</t>
+          </section>
+        </section>
+      </section>
+    </section>
+    <section anchor="extensibility">
+      <name>Extensibility</name>
+      <t>The base MLS protocol can be extended in a few ways.  New cipher suites can be
+added to enable the use of new cryptographic algorithms.  New types of proposals
+can be used to perform new actions within an epoch.  Extension fields can be
+used to add additional information to the protocol.  In this section, we discuss
+some constraints on these extensibility mechanisms that are necessary to ensure
+broad interoperability.</t>
+      <section anchor="additional-cipher-suites">
+        <name>Additional Cipher Suites</name>
+        <t>As discussed in <xref target="cipher-suites"/>, MLS allows the participants in a group to
+negotiate the cryptographic algorithms used within the group.  This
+extensibility is important for maintaining the security of the protocol over
+time <xref target="RFC7696"/>.  It also creates a risk of interoperability failure due to
+clients not supporting a common cipher suite.</t>
+        <t>The cipher suite registry defined in <xref target="mls-cipher-suites"/> attempts to strike a
+balance on this point.  On the one hand, the base policy for the registry is
+Specification Required, a fairly low bar designed to avoid the need for
+standards work in cases where different ciphers are needed for niche
+applications.  On the other hand, there is a higher bar (Standards Action) for ciphers to set the
+Recommended field in the registry.  This higher bar is there in part to ensure
+that the interoperability implications of new cipher suites are considered.</t>
+        <t>MLS cipher suites are defined independent of MLS versions, so that in principle,
+the same cipher suite can be used across versions.  Standards work defining new
+versions of MLS should consider whether it is desirable for the new version to
+be compatible with existing cipher suites, or whether the new version should rule
+out some cipher suites. For example, a new version could follow the example of
+HTTP/2, which restricted the set of allowed TLS ciphers (see <xref section="9.2.2" sectionFormat="of" target="RFC9113"/>).</t>
+      </section>
+      <section anchor="proposals-1">
+        <name>Proposals</name>
+        <t>Commit messages do not have an extension field because the set of proposals is
+extensible.  As discussed in <xref target="commit"/>, Proposals with a non-default proposal
+type <bcp14>MUST NOT</bcp14> be included in a commit unless the proposal type is supported by
+all the members of the group that will process the Commit.</t>
+      </section>
+      <section anchor="credential-extensibility">
+        <name>Credential Extensibility</name>
+        <t>In order to ensure that MLS provides meaningful authentication, it is important
+that each member is able to authenticate some identity information for each
+other member.  Identity information is encoded in Credentials, so this property
+is provided by ensuring that members use compatible credential types.</t>
+        <t>The only types of credential that may be used in a group are those that all
+members of the group support, as specified by the <tt>capabilities</tt> field of each
+LeafNode in the ratchet tree. An application can introduce new credential types
+by choosing an unallocated identifier from the registry in
+<xref target="mls-credential-types"/> and indicating support for the credential type in
+published LeafNodes, whether in Update proposals to existing groups or
+KeyPackages that are added to new groups. Once all members in a group indicate
+support for the credential type, members can start using LeafNodes with the new
+credential. Application may enforce that certain credential types always remain
+supported by adding a <tt>required_capabilities</tt> extension to the group's
+GroupContext, which would prevent any member from being added to the group that
+doesn't support them.</t>
+        <t>In future extensions to MLS, it may be useful to allow a member to present more
+than one credential.  For example, such credentials might present different
+attributes attested by different authorities.  To be consistent with the general
+principle stated at the beginning of this section, such an extension would need
+to ensure that each member can authenticate some identity for each other member.
+For each pair of members (Alice, Bob), Alice would need to present at least one
+credential of a type that Bob supports.</t>
+      </section>
+      <section anchor="extensions">
+        <name>Extensions</name>
+        <t>This protocol includes a mechanism for negotiating extension parameters similar
+to the one in TLS <xref target="RFC8446"/>.  In TLS, extension negotiation is one-to-one: The
+client offers extensions in its ClientHello message, and the server expresses
+its choices for the session with extensions in its ServerHello and
+EncryptedExtensions messages.  In MLS, extensions appear in the following
+places:</t>
+        <ul spacing="normal">
+          <li>In KeyPackages, to describe additional information related to the client</li>
+          <li>In LeafNodes, to describe additional information about the client or its
+participation in the group (once in the ratchet tree)</li>
+          <li>In the GroupInfo, to tell new members of a group what parameters are
+being used by the group, and to provide any additional details required to
+join the group</li>
+          <li>In the GroupContext object, to ensure that all members of the group have the
+same view of the parameters in use</li>
+        </ul>
+        <t>In other words, an application can use GroupContext extensions to ensure that
+all members of the group agree on a set of parameters. Clients indicate their
+support for parameters in the <tt>capabilities</tt> field of their LeafNode. New
+members of a group are informed of the group's GroupContext extensions via the
+<tt>extensions</tt> field in the <tt>group_context</tt> field of the GroupInfo object. The
+<tt>extensions</tt> field in a GroupInfo object (outside of the <tt>group_context</tt> field)
+can be used to provide additional parameters to new joiners that are used to
+join the group.</t>
+        <t>This extension mechanism is designed to allow for the secure and forward-compatible
+negotiation of extensions.  For this to work, implementations <bcp14>MUST</bcp14> correctly
+handle extensible fields:</t>
+        <ul spacing="normal">
+          <li>A client that posts a KeyPackage <bcp14>MUST</bcp14> support all parameters advertised in
+it. Otherwise, another client might fail to interoperate by selecting one of
+those parameters.</li>
+          <li>A client processing a KeyPackage object <bcp14>MUST</bcp14> ignore all unrecognized values
+in the <tt>capabilities</tt> field of the LeafNode and all unknown extensions in
+the <tt>extensions</tt> and <tt>leaf_node.extensions</tt> fields.  Otherwise, it could fail
+to interoperate with newer clients.</li>
+          <li>A client processing a GroupInfo object <bcp14>MUST</bcp14> ignore all unrecognized
+extensions in the <tt>extensions</tt> field.</li>
+          <li>Any field containing a list of extensions <bcp14>MUST NOT</bcp14> have more than one
+extension of any given type.</li>
+          <li>A client adding a new member to a group <bcp14>MUST</bcp14> verify that the LeafNode for the
+new member is compatible with the group's extensions.  The <tt>capabilities</tt>
+field <bcp14>MUST</bcp14> indicate support for each extension in the GroupContext.</li>
+          <li>A client joining a group <bcp14>MUST</bcp14> verify that it supports every extension in the
+GroupContext for the group.  Otherwise, it <bcp14>MUST</bcp14> treat the enclosing
+GroupInfo message as invalid and not join the group.</li>
+        </ul>
+        <t>Note that the latter two requirements mean that all MLS GroupContext extensions
+are mandatory, in the sense that an extension in use by the group <bcp14>MUST</bcp14> be
+supported by all members of the group.</t>
+        <t>The parameters of a group may be changed by sending a GroupContextExtensions
+proposal to enable additional extensions (<xref target="groupcontextextensions"/>), or
+by reinitializing the group (<xref target="reinitialization"/>).</t>
+      </section>
+      <section anchor="grease">
+        <name>GREASE</name>
+        <t>As described in <xref target="extensions"/>, clients are required to ignore unknown values
+for certain parameters.  To help ensure that other clients implement this
+behavior, a client can follow the "Generate Random Extensions And Sustain
+Extensibility" or GREASE approach described in <xref target="RFC8701"/>.  In the context of
+MLS, this means that a client generating a KeyPackage, LeafNode, or GroupInfo object includes
+random values in certain fields which would be ignored by a
+correctly implemented client processing the message.  A client that incorrectly
+rejects unknown code points will fail to process such a message, providing a
+signal to its implementer that the client needs to be fixed.</t>
+        <t>When generating the following fields, an MLS client <bcp14>SHOULD</bcp14> include a random
+selection of values chosen from these GREASE values:</t>
+        <ul spacing="normal">
+          <li>
+            <tt>LeafNode.capabilities.cipher_suites</tt>
+          </li>
+          <li>
+            <tt>LeafNode.capabilities.extensions</tt>
+          </li>
+          <li>
+            <tt>LeafNode.capabilities.proposals</tt>
+          </li>
+          <li>
+            <tt>LeafNode.capabilities.credentials</tt>
+          </li>
+          <li>
+            <tt>LeafNode.extensions</tt>
+          </li>
+          <li>
+            <tt>KeyPackage.extensions</tt>
+          </li>
+          <li>
+            <tt>GroupInfo.extensions</tt>
+          </li>
+        </ul>
+        <t>For the KeyPackage and GroupInfo extensions, the <tt>extension_data</tt> for GREASE
+extensions <bcp14>MAY</bcp14> have any contents selected by the sender, since they will be
+ignored by a correctly implemented receiver.  For example, a sender might
+populate these extensions with a randomly sized amount of random data.</t>
+        <t>Note that any GREASE values added to <tt>LeafNode.extensions</tt> need to be reflected
+in <tt>LeafNode.capabilities.extensions</tt>, since the LeafNode validation process
+described in <xref target="leaf-node-validation"/> requires that these two fields be
+consistent.</t>
+        <t>GREASE values <bcp14>MUST NOT</bcp14> be sent in the following fields, because an unsupported
+value in one these fields (including a GREASE value) will cause the enclosing
+message to be rejected:</t>
+        <ul spacing="normal">
+          <li>
+            <tt>Proposal.proposal_type</tt>
+          </li>
+          <li>
+            <tt>Credential.credential_type</tt>
+          </li>
+          <li>
+            <tt>GroupContext.extensions</tt>
+          </li>
+          <li>
+            <tt>GroupContextExtensions.extensions</tt>
+          </li>
+        </ul>
+        <t>Values reserved for GREASE have been registered in the various
+registries in <xref target="iana-considerations"/>.  This prevents conflict between GREASE
+and real future values.  The following values are reserved in each registry:
+<tt>0x0A0A</tt>, <tt>0x1A1A</tt>, <tt>0x2A2A</tt>, <tt>0x3A3A</tt>, <tt>0x4A4A</tt>, <tt>0x5A5A</tt>, <tt>0x6A6A</tt>, <tt>0x7A7A</tt>,
+<tt>0x8A8A</tt>, <tt>0x9A9A</tt>, <tt>0xAAAA</tt>, <tt>0xBABA</tt>, <tt>0xCACA</tt>, <tt>0xDADA</tt>, and <tt>0xEAEA</tt>.  (The
+value <tt>0xFAFA</tt> falls within the private use range.) These values <bcp14>MUST</bcp14> only
+appear in the fields listed above, and not, for example, in the <tt>proposal_type</tt>
+field of a Proposal.  Clients <bcp14>MUST NOT</bcp14> implement any special processing rules
+for how to handle these values when receiving them, since this negates their
+utility for detecting extensibility failures.</t>
+        <t>GREASE values <bcp14>MUST</bcp14> be handled using normal logic for processing unsupported
+values.  When comparing lists of capabilities to identify mutually supported
+capabilities, clients <bcp14>MUST</bcp14> represent their own capabilities with a list
+containing only the capabilities actually supported, without any GREASE values.
+In other words, lists including GREASE values are only sent to other clients;
+representations of a client's own capabilities <bcp14>MUST NOT</bcp14> contain GREASE values.</t>
+      </section>
+    </section>
+    <section anchor="sequencing">
+      <name>Sequencing of State Changes</name>
+      <t>Each Commit message is premised on a given starting state,
+indicated by the <tt>epoch</tt> field of the enclosing FramedContent.
+If the changes implied by a Commit message are made
+starting from a different state, the results will be incorrect.</t>
+      <t>This need for sequencing is not a problem as long as each time a
+group member sends a Commit message, it is based on the most
+current state of the group.  In practice, however, there is a risk
+that two members will generate Commit messages simultaneously
+based on the same state.</t>
+      <t>Applications <bcp14>MUST</bcp14> have an established way to resolve conflicting Commit messages
+for the same epoch. They can do this either by preventing conflicting messages
+from occurring in the first place, or by developing rules for deciding which
+Commit out of several sent in an epoch will be canonical. The approach chosen
+<bcp14>MUST</bcp14> minimize the amount of time that forked or previous group states are kept
+in memory, and promptly delete them once they're no longer necessary to ensure
+forward secrecy.</t>
+      <t>The generation of Commit messages <bcp14>MUST NOT</bcp14> modify a client's state, since the
+client doesn't know at that time whether the changes implied by the Commit
+message will conflict with another Commit or not. Similarly, the Welcome
+message corresponding to a Commit <bcp14>MUST NOT</bcp14> be delivered to a new
+joiner until it's clear that the Commit has been accepted.</t>
+      <t>Regardless of how messages are kept in sequence, there is a risk that
+in a sufficiently busy group, a given member may never
+be able to send a Commit message because they always lose to other
+members. The degree to which this is a practical problem will depend
+on the dynamics of the application.</t>
+    </section>
+    <section anchor="application-messages">
+      <name>Application Messages</name>
+      <t>The primary purpose of handshake messages is to provide an authenticated group
+key exchange to clients. In order to protect application messages sent among the
+members of a group, the <tt>encryption_secret</tt> provided by the key schedule is used
+to derive a sequence of nonces and keys for message encryption. Every epoch
+moves the key schedule forward, which triggers the creation of a new secret
+tree, as described in <xref target="secret-tree"/>, along with a new set of symmetric
+ratchets of nonces and keys for each member.</t>
+      <t>Each client maintains their own local copy of the key
+schedule for each epoch during which they are a group member. They
+derive new keys, nonces, and secrets as needed while deleting old
+ones as soon as they have been used.</t>
+      <t>The group identifier and epoch allow a recipient to know which group secrets
+should be used and from which <tt>epoch_secret</tt> to start computing other secrets.
+The sender identifier and content type are used to identify which
+symmetric ratchet to use from the secret tree. The
+<tt>generation</tt> counter determines how far into the ratchet to iterate in
+order to produce the required nonce and key for encryption or decryption.</t>
+      <section anchor="padding">
+        <name>Padding</name>
+        <t>Application messages <bcp14>MAY</bcp14> be padded to provide some resistance
+against traffic analysis techniques over encrypted traffic
+<xref target="CLINIC"/> <xref target="HCJ16"/>.
+While MLS might deliver the same payload less frequently across
+a lot of ciphertexts than traditional web servers, it might still provide
+the attacker enough information to mount an attack. If Alice asks Bob
+"When are we going to the movie?", then the answer "Wednesday" could be leaked
+to an adversary solely by the ciphertext length.</t>
+        <t>The length of the <tt>padding</tt> field in PrivateMessageContent can be
+chosen by the sender at the time of message encryption. Senders may use padding
+to reduce the ability of attackers outside the group to infer the size of the
+encrypted content.  Note, however, that the transports used to carry MLS
+messages may have maximum message sizes, so padding schemes <bcp14>SHOULD</bcp14> avoid
+increasing message size beyond any such limits that exist in a given
+deployment scenario.</t>
+      </section>
+      <section anchor="restrictions">
+        <name>Restrictions</name>
+        <t>During each epoch, senders <bcp14>MUST NOT</bcp14> encrypt more data than permitted by the
+security bounds of the AEAD scheme used <xref target="I-D.irtf-cfrg-aead-limits"/>.</t>
+        <t>Note that each change to the group through a handshake message will also set a
+new <tt>encryption_secret</tt>. Hence this change <bcp14>MUST</bcp14> be applied before encrypting
+any new application message. This is required both to ensure that any users
+removed from the group can no longer receive messages and to (potentially)
+recover confidentiality and authenticity for future messages despite a past
+state compromise.</t>
+      </section>
+      <section anchor="delayed-and-reordered-application-messages">
+        <name>Delayed and Reordered Application Messages</name>
+        <t>Since each application message contains the group identifier, the epoch, and a
+generation counter, a client can receive messages out of order. When messages
+are received out of order, the client moves the sender ratchet forward to match
+the received generation counter. Any unused nonce and key pairs from the ratchet
+are potentially stored so that they can be used to decrypt the messages that
+were delayed or reordered.</t>
+        <t>Applications <bcp14>SHOULD</bcp14> define a policy on how long to keep unused nonce and key
+pairs for a sender, and the maximum number to keep. This is in addition to
+ensuring that these secrets are deleted according to the deletion schedule
+defined in <xref target="deletion-schedule"/>. Applications <bcp14>SHOULD</bcp14> also define a policy
+limiting the maximum number of steps that clients will move the ratchet forward
+in response to a new message.  Messages received with a generation counter
+that is too much higher than the last message received would then be rejected.
+This avoids causing a denial-of-service attack by requiring the recipient to
+perform an excessive number of key derivations. For example, a malicious group
+member could send a message with <tt>generation = 0xffffffff</tt> at the beginning of a
+new epoch, forcing recipients to perform billions of key derivations unless they
+apply limits of the type discussed above.</t>
+      </section>
+    </section>
+    <section anchor="security-considerations">
+      <name>Security Considerations</name>
+      <t>The security goals of MLS are described in <xref target="I-D.ietf-mls-architecture"/>.
+We describe here how the protocol achieves its goals at a high level,
+though a complete security analysis is outside of the scope of this
+document.  The Security Considerations section of <xref target="I-D.ietf-mls-architecture"/>
+provides some citations to detailed security analyses.</t>
+      <section anchor="transport-security">
+        <name>Transport Security</name>
+        <t>Because MLS messages are protected at the message level, the
+confidentiality and integrity of the group state do not depend on
+those messages being protected in transit. However, an attacker who
+can observe those messages in transit will be able to learn about the
+group state, including potentially the group membership (see
+<xref target="group-membership"/> below). Such an attacker might also be able to
+mount denial-of-service attacks on the group or exclude new members by
+selectively removing messages in transit. In order to prevent this
+form of attack, it is <bcp14>RECOMMENDED</bcp14> that all MLS messages be carried
+over a secure transport such as TLS <xref target="RFC8446"/> or QUIC <xref target="RFC9000"/>.</t>
+      </section>
+      <section anchor="confidentiality-of-group-secrets">
+        <name>Confidentiality of Group Secrets</name>
+        <t>Group secrets are partly derived from the output of a ratchet tree. Ratchet
+trees work by assigning each member of the group to a leaf in the tree and
+maintaining the following property: the private key of a node in the tree is
+known only to members of the group that are assigned a leaf in the node's
+subtree. This is called the <em>tree invariant</em>, and it makes it possible to
+encrypt to all group members except one, with a number of ciphertexts that is
+logarithmic in the number of group members.</t>
+        <t>The ability to efficiently encrypt to all members except one allows members to
+be securely removed from a group. It also allows a member to rotate their
+key pair such that the old private key can no longer be used to decrypt new
+messages.</t>
+      </section>
+      <section anchor="confidentiality-of-sender-data">
+        <name>Confidentiality of Sender Data</name>
+        <t>The PrivateMessage framing encrypts "sender data" that identifies which group
+member sent an encrypted message, as described in <xref target="sender-data-encryption"/>.
+As with the QUIC header protection scheme <xref section="5.4" sectionFormat="comma" target="RFC9001"/>, this scheme
+is a variant of the HN1 construction analyzed in <xref target="NAN"/>.  A sample of the
+ciphertext is combined with a <tt>sender_data_secret</tt> to derive a key and nonce
+that are used for AEAD encryption of the sender data.</t>
+        <sourcecode type="pseudocode">
+(key, nonce) = PRF(sender_data_secret, sample)
+encrypted_sender_data =
+  AEAD.Seal(key, nonce, sender_data_aad, sender_data)
+</sourcecode>
+        <t>The only differences between this construction and HN1 as described in <xref target="NAN"/>
+are that it (1) uses authenticated encryption instead of unauthenticated
+encryption and (2) protects information used to derive a nonce instead of the
+nonce itself.</t>
+        <t>Since the <tt>sender_data_secret</tt> is distinct from the content encryption key, it
+follows that the sender data encryption scheme achieves AE2 security as defined
+in <xref target="NAN"/>, and therefore guarantees the confidentiality of the sender data.</t>
+        <t>Use of the same <tt>sender_data_secret</tt> and ciphertext sample more than once risks
+compromising sender data protection by reusing an AEAD (key, nonce) pair.  For
+example, in many AEAD schemes, reusing a key and nonce reveals the exclusive OR
+of the two plaintexts. Assuming the ciphertext output of the AEAD algorithm is
+indistinguishable from random data (i.e., the AEAD is AE1-secure in the phrasing
+of <xref target="NAN"/>), the odds of two ciphertext samples being identical is roughly
+2<sup>-L/2</sup>, i.e., the birthday bound.</t>
+        <t>The AEAD algorithms for cipher suites defined in this document all provide this
+property. The size of the sample depends on the cipher suite's hash function, but
+in all cases, the probability of collision is no more than 2<sup>-128</sup>.
+Any future cipher suite <bcp14>MUST</bcp14> use an AE1-secure AEAD algorithm.</t>
+      </section>
+      <section anchor="confidentiality-of-group-metadata">
+        <name>Confidentiality of Group Metadata</name>
+        <t>MLS does not provide confidentiality protection to some messages and fields
+within messages:</t>
+        <ul spacing="normal">
+          <li>KeyPackage messages</li>
+          <li>GroupInfo messages</li>
+          <li>The unencrypted portion of a Welcome message</li>
+          <li>Any Proposal or Commit messages sent as PublicMessage messages</li>
+          <li>The unencrypted header fields in PrivateMessage messages</li>
+          <li>The lengths of encrypted Welcome and PrivateMessage messages</li>
+        </ul>
+        <t>The only mechanism MLS provides for confidentially distributing a group's
+ratchet tree to new members is to send it in a Welcome message as a
+<tt>ratchet_tree</tt> extension.  If an application distributes the tree in some other
+way, its security will depend on that application mechanism.</t>
+        <t>A party observing these fields might be able to infer certain properties of the
+group:</t>
+        <ul spacing="normal">
+          <li>Group ID</li>
+          <li>Current epoch and frequency of epoch changes</li>
+          <li>Frequency of messages within an epoch</li>
+          <li>Group extensions</li>
+          <li>Group membership</li>
+        </ul>
+        <t>The amount of metadata exposed to parties outside the group, and thus the
+ability of these parties to infer the group's properties, depends on several
+aspects of the DS design, such as:</t>
+        <ul spacing="normal">
+          <li>How KeyPackages are distributed</li>
+          <li>How the ratchet tree is distributed</li>
+          <li>How prospective external joiners get a GroupInfo object for the group</li>
+          <li>Whether Proposal and Commit messages are sent as PublicMessage or PrivateMessage</li>
+        </ul>
+        <t>In the remainder of this section, we note the ways that the above properties of
+the group are reflected in unprotected group messages, as a guide to
+understanding how they might be exposed or protected in a given application.</t>
+        <section anchor="groupid-epoch-and-message-frequency">
+          <name>GroupID, Epoch, and Message Frequency</name>
+          <t>MLS provides no mechanism to protect the group ID and epoch of a message from
+the DS, so the group ID and the frequency of messages and epoch changes are not
+protected against inspection by the DS. However, any modifications to these
+will cause decryption failure.</t>
+        </section>
+        <section anchor="group-extensions">
+          <name>Group Extensions</name>
+          <t>A group's extensions are first set by the group's creator and then updated by
+GroupContextExtensions proposals.  A GroupContextExtensions proposal sent as
+a PublicMessage leaks the group's extensions.</t>
+          <t>A new member learns the group's extensions via a GroupInfo object.  When the new
+member joins via a Welcome message, the Welcome message's encryption protects
+the GroupInfo message.  When the new member joins via an external join, they
+must be provided with a GroupInfo object.  Protection of this GroupInfo object
+is up to the application -- if it is transmitted over a channel that is not
+confidential to the group and the new joiner, then it will leak the group's
+extensions.</t>
+        </section>
+        <section anchor="group-membership">
+          <name>Group Membership</name>
+          <t>The group's membership is represented directly by its ratchet tree, since each
+member's LeafNode contains members' cryptographic keys, a credential that
+contains information about the member's identity, and possibly other
+identifiers.  Applications that expose the group's ratchet tree outside the
+group also leak the group's membership.</t>
+          <t>Changes to the group's membership are made by means of Add and Remove proposals.
+If these proposals are sent as PublicMessage, then information will be leaked
+about the corresponding changes to the group's membership.  A party that sees
+all of these changes can reconstruct the group membership.</t>
+          <t>Welcome messages contain a hash of each KeyPackage for which the Welcome message
+is encrypted.  If a party has access to a pool of KeyPackages and observes a
+Welcome message, then they can identify the KeyPackage representing the new
+member.  If the party can also associate the Welcome with a group, then the
+party can infer that the identified new member was added to that group.</t>
+          <t>Note that these information leaks reveal the group's membership only to the degree
+that membership is revealed by the contents of a member's LeafNode in the
+ratchet tree.  In some cases, this may be quite direct, e.g., due to credentials
+attesting to identifiers such as email addresses.  An application could
+construct a member's leaf node to be less identifying, e.g., by using a
+pseudonymous credential and frequently rotating encryption and signature keys.</t>
+        </section>
+      </section>
+      <section anchor="authentication">
+        <name>Authentication</name>
+        <t>The first form of authentication we provide is that group members can verify a
+message originated from one of the members of the group. For encrypted messages,
+this is guaranteed because messages are encrypted with an AEAD under a key
+derived from the group secrets. For plaintext messages, this is guaranteed by
+the use of a <tt>membership_tag</tt>, which constitutes a MAC over the message, under a
+key derived from the group secrets.</t>
+        <t>The second form of authentication is that group members can verify a message
+originated from a particular member of the group. This is guaranteed by a
+digital signature on each message from the sender's signature key.</t>
+        <t>The signature keys held by group members are critical to the security of MLS
+against active attacks.  If a member's signature key is compromised, then an
+attacker can create LeafNodes and KeyPackages impersonating the member; depending on the
+application, this can then allow the attacker to join the group with the
+compromised member's identity.  For example, if a group has enabled external
+parties to join via external commits, then an attacker that has compromised a
+member's signature key could use an external Commit to insert themselves into
+the group -- even using a "resync"-style external Commit to replace the
+compromised member in the group.</t>
+        <t>Applications can mitigate the risks of signature key compromise using pre-shared
+keys.  If a group requires joiners to know a PSK in addition to authenticating
+with a credential, then in order to mount an impersonation attack, the attacker
+would need to compromise the relevant PSK as well as the victim's signature key.
+The cost of this mitigation is that the application needs some external
+arrangement that ensures that the legitimate members of the group have the
+required PSKs.</t>
+      </section>
+      <section anchor="forward-secrecy-and-post-compromise-security">
+        <name>Forward Secrecy and Post-Compromise Security</name>
+        <t>Forward secrecy and post-compromise security are important security notions for
+long-lived MLS groups.  Forward secrecy means that messages sent at a certain
+point in time are secure in the face of later compromise of a group member.
+Post-compromise security means that messages are secure even if a group member
+was compromised at some point in the past.</t>
+        <figure>
+          <name>Forward Secrecy and Post-Compromise Security</name>
+          <artset>
+            <artwork type="svg">
+              <svg xmlns="http://www.w3.org/2000/svg" class="diagram" font-family="monospace" font-size="13px" height="176" text-anchor="middle" version="1.1" viewBox="0 0 448 176" width="448">
+                <path d="M 152,80 L 152,160" fill="none" stroke="black"/>
+                <path d="M 192,48 L 192,88" fill="none" stroke="black"/>
+                <path d="M 232,80 L 232,160" fill="none" stroke="black"/>
+                <path d="M 8,96 L 144,96" fill="none" stroke="black"/>
+                <path d="M 160,96 L 224,96" fill="none" stroke="black"/>
+                <path d="M 240,96 L 440,96" fill="none" stroke="black"/>
+                <path d="M 8,128 L 144,128" fill="none" stroke="black"/>
+                <path d="M 240,128 L 368,128" fill="none" stroke="black"/>
+                <polygon class="arrowhead" fill="black" points="448,96 436,90.4 436,101.6" transform="rotate(0,440,96)"/>
+                <polygon class="arrowhead" fill="black" points="376,128 364,122.4 364,133.6" transform="rotate(0,368,128)"/>
+                <polygon class="arrowhead" fill="black" points="200,88 188,82.4 188,93.6" transform="rotate(90,192,88)"/>
+                <polygon class="arrowhead" fill="black" points="16,128 4,122.4 4,133.6" transform="rotate(180,8,128)"/>
+                <g class="text">
+                  <text x="196" y="36">Compromise</text>
+                  <text x="420" y="116">Time</text>
+                  <text x="48" y="148">Forward</text>
+                  <text x="112" y="148">Secrecy</text>
+                  <text x="304" y="148">Post-Compromise</text>
+                  <text x="292" y="164">Security</text>
+                </g>
+              </svg>
+            </artwork>
+            <artwork type="ascii-art">
+                   Compromise
+                       |
+                       |
+                  |    V    |
+------------------|---------|-------------------------&gt;
+                  |         |                     Time
+&lt;-----------------|         |----------------&gt;
+  Forward Secrecy |         | Post-Compromise
+                  |         |   Security
+</artwork>
+          </artset>
+        </figure>
+        <t>Post-compromise security is provided between epochs by members
+regularly updating their leaf key in the ratchet tree. Updating their
+leaf key prevents group secrets from continuing to be encrypted to
+public keys whose private keys had previously been compromised. Note
+that sending an Update proposal does not achieve PCS until another
+member includes it in a Commit. Members can achieve immediate PCS by
+sending their own Commit and populating the <tt>path</tt> field, as described
+in <xref target="commit"/>. To be clear, in all these cases, the PCS guarantees
+come into effect when the members of the group process the relevant
+Commit, not when the sender creates it.</t>
+        <t>Forward secrecy between epochs is provided by deleting private keys from past
+versions of the ratchet tree, as this prevents old group secrets from being
+re-derived. Forward secrecy <em>within</em> an epoch is provided by deleting message
+encryption keys once they've been used to encrypt or decrypt a message.
+Note that group secrets and message encryption keys are shared by the
+group.  There is thus a risk to forward secrecy as long as any
+member has not deleted these keys. This is a particular risk if a member
+is offline for a long period of time. Applications <bcp14>SHOULD</bcp14> have mechanisms
+for evicting group members that are offline for too long (i.e., have
+not changed their key within some period).</t>
+        <t>New groups are also at risk of using previously compromised keys (as with
+post-compromise security) if a member is added to a new group via an old
+KeyPackage whose corresponding private key has been compromised.  This risk can
+be mitigated by having clients regularly generate new KeyPackages and upload
+them to the Delivery Service.  This way, the key material used to add a member
+to a new group is more likely to be fresh and less likely to be compromised.</t>
+      </section>
+      <section anchor="uniqueness-of-ratchet-tree-key-pairs">
+        <name>Uniqueness of Ratchet Tree Key Pairs</name>
+        <t>The encryption and signature keys stored in the <tt>encryption_key</tt> and
+<tt>signature_key</tt> fields of ratchet tree nodes <bcp14>MUST</bcp14> be distinct from one another.
+If two members' leaf nodes have the same signature key, for example, then the
+data origin authentication properties afforded by signatures within the group
+are degraded.</t>
+        <t>Uniqueness of keys in leaf nodes is assured by explicitly checking each leaf node
+as it is added to the tree, whether in an Add proposal, in an Update proposal, or in the <tt>path</tt> field of a
+Commit.  Details can be found in Sections <xref format="counter" target="leaf-node-validation"/>,
+<xref format="counter" target="proposal-list-validation"/>, and <xref format="counter" target="processing-a-commit"/>.  Uniqueness of
+encryption keys in parent nodes is assured by checking that the keys in an
+UpdatePath are not found elsewhere in the tree (see <xref target="processing-a-commit"/>).</t>
+      </section>
+      <section anchor="keypackage-reuse">
+        <name>KeyPackage Reuse</name>
+        <t>KeyPackages are intended to be used only once.  That is, once a KeyPackage
+has been used to introduce the corresponding client to a group, it <bcp14>SHOULD</bcp14> be
+deleted from the KeyPackage publication system.  Reuse of KeyPackages can lead
+to replay attacks.</t>
+        <t>An application <bcp14>MAY</bcp14> allow for reuse of a "last resort" KeyPackage in order to
+prevent denial-of-service attacks.  Since a KeyPackage is needed to add a
+client to a new group, an attacker could prevent a client from being added to new
+groups by exhausting all available KeyPackages. To prevent such a denial-of-service
+attack, the KeyPackage publication system <bcp14>SHOULD</bcp14> rate-limit KeyPackage
+requests, especially if not authenticated.</t>
+      </section>
+      <section anchor="delivery-service-compromise">
+        <name>Delivery Service Compromise</name>
+        <t>MLS is designed to protect the confidentiality and integrity of
+the group data even in the face of a compromised DS. However, a compromised
+DS can still mount some attacks. While it cannot forge messages,
+it can selectively delay or remove them. In some cases, this can be
+observed by detecting gaps in the per-sender generation counter,
+though it may not always be possible to distinguish an attack from message
+loss. In addition, the DS can permanently block messages to and from
+a group member. This will not always be detectable by other members.
+If an application uses the DS to resolve conflicts between
+simultaneous Commits (see <xref target="sequencing"/>), it is also possible for the
+DS to influence which Commit is applied, even to the point of
+preventing a member from ever having its Commits applied.</t>
+        <t>When put together, these abilities potentially allow a DS to collude
+with an attacker who has compromised a member's state to defeat PCS by
+suppressing the valid Update and Commit messages from the member that
+would lock out the attacker and update the member's leaf to a new,
+uncompromised state. Aside from the SenderData.generation value, MLS
+leaves loss detection up to the application.</t>
+      </section>
+      <section anchor="authentication-service-compromise">
+        <name>Authentication Service Compromise</name>
+        <t>Authentication Service compromise is much more serious than compromise
+of the Delivery Service. A compromised AS can assert a binding for a
+signature key and identity pair of its choice, thus allowing
+impersonation of a given user. This ability is sufficient to allow the
+AS to join new groups as if it were that user. Depending on the
+application architecture, it may also be sufficient to allow the
+compromised AS to join the group as an existing user, for instance, as
+if it were a new device associated with the same user. If
+the application uses a transparency mechanism such as CONIKS
+<xref target="CONIKS"/> or Key Transparency <xref target="KT"/>, then it may be possible for end
+users to detect this kind of misbehavior by the AS.  It is also possible to
+construct schemes in which the various clients owned by a user vouch
+for each other, e.g., by signing each others' keys.</t>
+      </section>
+      <section anchor="additional-policy-enforcement">
+        <name>Additional Policy Enforcement</name>
+        <t>The DS and AS may also apply additional policies to MLS operations to obtain
+additional security properties. For example, MLS enables any participant to add
+or remove members of a group; a DS could enforce a policy that only certain
+members are allowed to perform these operations. MLS authenticates all members
+of a group; a DS could help ensure that only clients with certain types of
+credentials are admitted. MLS provides no inherent protection against denial of
+service; a DS could also enforce rate limits in order to mitigate
+these risks.</t>
+      </section>
+      <section anchor="group-fragmentation-by-malicious-insiders">
+        <name>Group Fragmentation by Malicious Insiders</name>
+        <t>It is possible for a malicious member of a group to "fragment" the group by
+crafting an invalid UpdatePath.  Recall that an UpdatePath encrypts a sequence
+of path secrets to different subtrees of the group's ratchet trees.  These path
+secrets should be derived in a sequence as described in
+<xref target="ratchet-tree-evolution"/>, but the UpdatePath syntax allows the sender to
+encrypt arbitrary, unrelated secrets.  The syntax also does not guarantee that
+the encrypted path secret for a given node corresponds to the public
+key provided for that node.</t>
+        <t>Both of these types of corruption will cause processing of a Commit to fail for
+some members of the group.  If the public key for a node does not match the path
+secret, then the members that decrypt that path secret will reject the Commit
+based on this mismatch.  If the path secret sequence is incorrect at some point,
+then members that can decrypt nodes before that point will compute a different
+public key for the mismatched node than the one in the UpdatePath, which also
+causes the Commit to fail.  Applications <bcp14>SHOULD</bcp14> provide mechanisms for failed
+commits to be reported, so that group members who were not able to recognize the
+error themselves can reinitialize the group if necessary.</t>
+        <t>Even with such an error reporting mechanism in place, however, it is still
+possible for members to get locked out of the group by a malformed Commit.
+Since malformed Commits can only be recognized by certain members of the group,
+in an asynchronous application, it may be the case that all members that could
+detect a fault in a Commit are offline.  In such a case, the Commit will be
+accepted by the group, and the resulting state will possibly be used as the basis for
+further Commits.  When the affected members come back online, they will reject
+the first Commit, and thus be unable to catch up with the group. These members
+will need to either add themselves back with an external Commit or reinitialize
+the group from scratch.</t>
+        <t>Applications can address this risk by requiring certain members of the group to
+acknowledge successful processing of a Commit before the group regards the
+Commit as accepted.  The minimum set of acknowledgements necessary to verify
+that a Commit is well-formed comprises an acknowledgement from one member per
+node in the UpdatePath, that is, one member from each subtree rooted in the
+copath node corresponding to the node in the UpdatePath. MLS does not
+provide a built-in mechanism for such acknowledgements, but they can
+be added at the application layer.</t>
+      </section>
+    </section>
+    <section anchor="iana-considerations">
+      <name>IANA Considerations</name>
+      <t>IANA has created the following registries:</t>
+      <ul spacing="normal">
+        <li>MLS Cipher Suites (<xref target="mls-cipher-suites"/>)</li>
+        <li>MLS Wire Formats (<xref target="mls-wire-formats"/>)</li>
+        <li>MLS Extension Types (<xref target="mls-extension-types"/>)</li>
+        <li>MLS Proposal Types (<xref target="mls-proposal-types"/>)</li>
+        <li>MLS Credential Types (<xref target="mls-credential-types"/>)</li>
+        <li>MLS Signature Labels (<xref target="mls-signature-labels"/>)</li>
+        <li>MLS Public Key Encryption Labels (<xref target="mls-public-key-encryption-labels"/>)</li>
+        <li>MLS Exporter Labels (<xref target="mls-exporter-labels"/>)</li>
+      </ul>
+      <t>All of these registries are under the "Messaging Layer Security" group registry heading,
+and assignments are made via the Specification Required policy <xref target="RFC8126"/>. See
+<xref target="de"/> for additional information about the MLS Designated Experts (DEs).</t>
+      <section anchor="mls-cipher-suites">
+        <name>MLS Cipher Suites</name>
+        <t>A cipher suite is a combination of a protocol version and the set of
+cryptographic algorithms that should be used.</t>
+        <t>Cipher suite names follow the naming convention:</t>
+        <sourcecode type="pseudocode">
+CipherSuite MLS_LVL_KEM_AEAD_HASH_SIG = VALUE;
+</sourcecode>
+        <t>Where VALUE is represented as a 16-bit integer:</t>
+        <sourcecode type="tls-presentation">
+uint16 CipherSuite;
+</sourcecode>
+        <table>
+          <thead>
+            <tr>
+              <th align="left">Component</th>
+              <th align="left">Contents</th>
+            </tr>
+          </thead>
+          <tbody>
+            <tr>
+              <td align="left">LVL</td>
+              <td align="left">The security level (in bits)</td>
+            </tr>
+            <tr>
+              <td align="left">KEM</td>
+              <td align="left">The KEM algorithm used for HPKE in ratchet tree operations</td>
+            </tr>
+            <tr>
+              <td align="left">AEAD</td>
+              <td align="left">The AEAD algorithm used for HPKE and message protection</td>
+            </tr>
+            <tr>
+              <td align="left">HASH</td>
+              <td align="left">The hash algorithm used for HPKE and the MLS transcript hash</td>
+            </tr>
+            <tr>
+              <td align="left">SIG</td>
+              <td align="left">The signature algorithm used for message authentication</td>
+            </tr>
+          </tbody>
+        </table>
+        <t>The columns in the registry are as follows:</t>
+        <ul spacing="normal">
+          <li>Value: The numeric value of the cipher suite</li>
+          <li>Name: The name of the cipher suite</li>
+          <li>
+            <t>Recommended: Whether support for this cipher suite is recommended by the IETF.
+Valid values are "Y", "N", and "D", as described below.  The default
+value of the "Recommended" column is "N".  Setting the Recommended item to "Y"
+or "D", or changing an item whose current value is "Y" or "D", requires
+Standards Action <xref target="RFC8126"/>.  </t>
+            <ul spacing="normal">
+              <li>Y: Indicates that the IETF has consensus that the item is <bcp14>RECOMMENDED</bcp14>. This
+only means that the associated mechanism is fit for the purpose for which it
+was defined. Careful reading of the documentation for the mechanism is
+necessary to understand the applicability of that mechanism. The IETF could
+recommend mechanisms that have limited applicability, but it will provide
+applicability statements that describe any limitations of the mechanism or
+necessary constraints on its use.</li>
+              <li>N: Indicates that the item has not been evaluated by the IETF and that the
+IETF has made no statement about the suitability of the associated
+mechanism. This does not necessarily mean that the mechanism is flawed, only
+that no consensus exists. The IETF might have consensus to leave an item
+marked as "N" on the basis of it having limited applicability or usage
+constraints.</li>
+              <li>D: Indicates that the item is discouraged and <bcp14>SHOULD NOT</bcp14> or <bcp14>MUST NOT</bcp14> be
+used. This marking could be used to identify mechanisms that might result in
+problems if they are used, such as a weak cryptographic algorithm or a
+mechanism that might cause interoperability problems in deployment.</li>
+            </ul>
+          </li>
+          <li>Reference: The document where this cipher suite is defined</li>
+        </ul>
+        <t>Initial contents:</t>
+        <table>
+          <name>MLS Extension Types Registry</name>
+          <thead>
+            <tr>
+              <th align="left">Value</th>
+              <th align="left">Name</th>
+              <th align="left">R</th>
+              <th align="left">Ref</th>
+            </tr>
+          </thead>
+          <tbody>
+            <tr>
+              <td align="left">0x0000</td>
+              <td align="left">RESERVED</td>
+              <td align="left">-</td>
+              <td align="left">RFC 9420</td>
+            </tr>
+            <tr>
+              <td align="left">0x0001</td>
+              <td align="left">MLS_128_DHKEMX25519_AES128GCM_SHA256_Ed25519</td>
+              <td align="left">Y</td>
+              <td align="left">RFC 9420</td>
+            </tr>
+            <tr>
+              <td align="left">0x0002</td>
+              <td align="left">MLS_128_DHKEMP256_AES128GCM_SHA256_P256</td>
+              <td align="left">Y</td>
+              <td align="left">RFC 9420</td>
+            </tr>
+            <tr>
+              <td align="left">0x0003</td>
+              <td align="left">MLS_128_DHKEMX25519_CHACHA20POLY1305_SHA256_Ed25519</td>
+              <td align="left">Y</td>
+              <td align="left">RFC 9420</td>
+            </tr>
+            <tr>
+              <td align="left">0x0004</td>
+              <td align="left">MLS_256_DHKEMX448_AES256GCM_SHA512_Ed448</td>
+              <td align="left">Y</td>
+              <td align="left">RFC 9420</td>
+            </tr>
+            <tr>
+              <td align="left">0x0005</td>
+              <td align="left">MLS_256_DHKEMP521_AES256GCM_SHA512_P521</td>
+              <td align="left">Y</td>
+              <td align="left">RFC 9420</td>
+            </tr>
+            <tr>
+              <td align="left">0x0006</td>
+              <td align="left">MLS_256_DHKEMX448_CHACHA20POLY1305_SHA512_Ed448</td>
+              <td align="left">Y</td>
+              <td align="left">RFC 9420</td>
+            </tr>
+            <tr>
+              <td align="left">0x0007</td>
+              <td align="left">MLS_256_DHKEMP384_AES256GCM_SHA384_P384</td>
+              <td align="left">Y</td>
+              <td align="left">RFC 9420</td>
+            </tr>
+            <tr>
+              <td align="left">0x0A0A</td>
+              <td align="left">GREASE</td>
+              <td align="left">Y</td>
+              <td align="left">RFC 9420</td>
+            </tr>
+            <tr>
+              <td align="left">0x1A1A</td>
+              <td align="left">GREASE</td>
+              <td align="left">Y</td>
+              <td align="left">RFC 9420</td>
+            </tr>
+            <tr>
+              <td align="left">0x2A2A</td>
+              <td align="left">GREASE</td>
+              <td align="left">Y</td>
+              <td align="left">RFC 9420</td>
+            </tr>
+            <tr>
+              <td align="left">0x3A3A</td>
+              <td align="left">GREASE</td>
+              <td align="left">Y</td>
+              <td align="left">RFC 9420</td>
+            </tr>
+            <tr>
+              <td align="left">0x4A4A</td>
+              <td align="left">GREASE</td>
+              <td align="left">Y</td>
+              <td align="left">RFC 9420</td>
+            </tr>
+            <tr>
+              <td align="left">0x5A5A</td>
+              <td align="left">GREASE</td>
+              <td align="left">Y</td>
+              <td align="left">RFC 9420</td>
+            </tr>
+            <tr>
+              <td align="left">0x6A6A</td>
+              <td align="left">GREASE</td>
+              <td align="left">Y</td>
+              <td align="left">RFC 9420</td>
+            </tr>
+            <tr>
+              <td align="left">0x7A7A</td>
+              <td align="left">GREASE</td>
+              <td align="left">Y</td>
+              <td align="left">RFC 9420</td>
+            </tr>
+            <tr>
+              <td align="left">0x8A8A</td>
+              <td align="left">GREASE</td>
+              <td align="left">Y</td>
+              <td align="left">RFC 9420</td>
+            </tr>
+            <tr>
+              <td align="left">0x9A9A</td>
+              <td align="left">GREASE</td>
+              <td align="left">Y</td>
+              <td align="left">RFC 9420</td>
+            </tr>
+            <tr>
+              <td align="left">0xAAAA</td>
+              <td align="left">GREASE</td>
+              <td align="left">Y</td>
+              <td align="left">RFC 9420</td>
+            </tr>
+            <tr>
+              <td align="left">0xBABA</td>
+              <td align="left">GREASE</td>
+              <td align="left">Y</td>
+              <td align="left">RFC 9420</td>
+            </tr>
+            <tr>
+              <td align="left">0xCACA</td>
+              <td align="left">GREASE</td>
+              <td align="left">Y</td>
+              <td align="left">RFC 9420</td>
+            </tr>
+            <tr>
+              <td align="left">0xDADA</td>
+              <td align="left">GREASE</td>
+              <td align="left">Y</td>
+              <td align="left">RFC 9420</td>
+            </tr>
+            <tr>
+              <td align="left">0xEAEA</td>
+              <td align="left">GREASE</td>
+              <td align="left">Y</td>
+              <td align="left">RFC 9420</td>
+            </tr>
+            <tr>
+              <td align="left">0xF000 - 0xFFFF</td>
+              <td align="left">Reserved for Private Use</td>
+              <td align="left">-</td>
+              <td align="left">RFC 9420</td>
+            </tr>
+          </tbody>
+        </table>
+        <t>All of the non-GREASE cipher suites use HMAC <xref target="RFC2104"/> as their MAC function, with
+different hashes per cipher suite.  The mapping of cipher suites to HPKE
+primitives <xref target="RFC9180"/>, HMAC hash functions, and TLS signature schemes
+<xref target="RFC8446"/> is as follows:</t>
+        <table>
+          <thead>
+            <tr>
+              <th align="left">Value</th>
+              <th align="left">KEM</th>
+              <th align="left">KDF</th>
+              <th align="left">AEAD</th>
+              <th align="left">Hash</th>
+              <th align="left">Signature</th>
+            </tr>
+          </thead>
+          <tbody>
+            <tr>
+              <td align="left">0x0001</td>
+              <td align="left">0x0020</td>
+              <td align="left">0x0001</td>
+              <td align="left">0x0001</td>
+              <td align="left">SHA256</td>
+              <td align="left">ed25519</td>
+            </tr>
+            <tr>
+              <td align="left">0x0002</td>
+              <td align="left">0x0010</td>
+              <td align="left">0x0001</td>
+              <td align="left">0x0001</td>
+              <td align="left">SHA256</td>
+              <td align="left">ecdsa_secp256r1_sha256</td>
+            </tr>
+            <tr>
+              <td align="left">0x0003</td>
+              <td align="left">0x0020</td>
+              <td align="left">0x0001</td>
+              <td align="left">0x0003</td>
+              <td align="left">SHA256</td>
+              <td align="left">ed25519</td>
+            </tr>
+            <tr>
+              <td align="left">0x0004</td>
+              <td align="left">0x0021</td>
+              <td align="left">0x0003</td>
+              <td align="left">0x0002</td>
+              <td align="left">SHA512</td>
+              <td align="left">ed448</td>
+            </tr>
+            <tr>
+              <td align="left">0x0005</td>
+              <td align="left">0x0012</td>
+              <td align="left">0x0003</td>
+              <td align="left">0x0002</td>
+              <td align="left">SHA512</td>
+              <td align="left">ecdsa_secp521r1_sha512</td>
+            </tr>
+            <tr>
+              <td align="left">0x0006</td>
+              <td align="left">0x0021</td>
+              <td align="left">0x0003</td>
+              <td align="left">0x0003</td>
+              <td align="left">SHA512</td>
+              <td align="left">ed448</td>
+            </tr>
+            <tr>
+              <td align="left">0x0007</td>
+              <td align="left">0x0011</td>
+              <td align="left">0x0002</td>
+              <td align="left">0x0002</td>
+              <td align="left">SHA384</td>
+              <td align="left">ecdsa_secp384r1_sha384</td>
+            </tr>
+          </tbody>
+        </table>
+        <t>The hash used for the MLS transcript hash is the one referenced in the
+cipher suite name.  In the cipher suites defined above, "SHA256", "SHA384", and
+"SHA512" refer, respectively, to the SHA-256, SHA-384, and SHA-512 functions
+defined in <xref target="SHS"/>.</t>
+        <t>In addition to the general requirements of <xref target="additional-cipher-suites"/>, future
+cipher suites <bcp14>MUST</bcp14> meet the requirements of <xref target="confidentiality-of-sender-data"/>.</t>
+        <t>It is advisable to keep the number of cipher suites low to increase the likelihood
+that clients can interoperate in a federated environment. The cipher suites therefore
+include only modern, yet well-established algorithms.  Depending on their
+requirements, clients can choose between two security levels (roughly 128-bit
+and 256-bit). Within the security levels, clients can choose between faster
+X25519/X448 curves and curves compliant with FIPS 140-2 for Diffie-Hellman key
+negotiations. Clients may also choose ChaCha20Poly1305 or AES-GCM, e.g., for
+performance reasons. Since ChaCha20Poly1305 is not listed by FIPS 140-2, it is
+not paired with curves compliant with FIPS 140-2. The security level of symmetric
+encryption algorithms and hash functions is paired with the security level of
+the curves.</t>
+        <t>The mandatory-to-implement cipher suite for MLS 1.0 is
+<tt>MLS_128_DHKEMX25519_AES128GCM_SHA256_Ed25519</tt>, which uses
+Curve25519 for key exchange, AES-128-GCM for HPKE, HKDF over SHA2-256, and
+Ed25519 for signatures.  MLS clients <bcp14>MUST</bcp14> implement this cipher suite.</t>
+      </section>
+      <section anchor="mls-wire-formats">
+        <name>MLS Wire Formats</name>
+        <t>The "MLS Wire Formats" registry lists identifiers for the types of messages that can be sent in
+MLS.  The wire format field is two bytes wide, so the valid wire format values
+are in the range 0x0000 to 0xFFFF.</t>
+        <t>Template:</t>
+        <ul spacing="normal">
+          <li>Value: The numeric value of the wire format</li>
+          <li>Name: The name of the wire format</li>
+          <li>Recommended: Same as in <xref target="mls-cipher-suites"/></li>
+          <li>Reference: The document where this wire format is defined</li>
+        </ul>
+        <t>Initial contents:</t>
+        <table>
+          <name>MLS Wire Formats Registry</name>
+          <thead>
+            <tr>
+              <th align="left">Value</th>
+              <th align="left">Name</th>
+              <th align="left">R</th>
+              <th align="left">Ref</th>
+            </tr>
+          </thead>
+          <tbody>
+            <tr>
+              <td align="left">0x0000</td>
+              <td align="left">RESERVED</td>
+              <td align="left">-</td>
+              <td align="left">RFC 9420</td>
+            </tr>
+            <tr>
+              <td align="left">0x0001</td>
+              <td align="left">mls_public_message</td>
+              <td align="left">Y</td>
+              <td align="left">RFC 9420</td>
+            </tr>
+            <tr>
+              <td align="left">0x0002</td>
+              <td align="left">mls_private_message</td>
+              <td align="left">Y</td>
+              <td align="left">RFC 9420</td>
+            </tr>
+            <tr>
+              <td align="left">0x0003</td>
+              <td align="left">mls_welcome</td>
+              <td align="left">Y</td>
+              <td align="left">RFC 9420</td>
+            </tr>
+            <tr>
+              <td align="left">0x0004</td>
+              <td align="left">mls_group_info</td>
+              <td align="left">Y</td>
+              <td align="left">RFC 9420</td>
+            </tr>
+            <tr>
+              <td align="left">0x0005</td>
+              <td align="left">mls_key_package</td>
+              <td align="left">Y</td>
+              <td align="left">RFC 9420</td>
+            </tr>
+            <tr>
+              <td align="left">0xF000 - 0xFFFF</td>
+              <td align="left">Reserved for Private Use</td>
+              <td align="left">-</td>
+              <td align="left">RFC 9420</td>
+            </tr>
+          </tbody>
+        </table>
+      </section>
+      <section anchor="mls-extension-types">
+        <name>MLS Extension Types</name>
+        <t>The "MLS Extension Types" registry lists identifiers for extensions to the MLS protocol.  The
+extension type field is two bytes wide, so valid extension type values are in
+the range 0x0000 to 0xFFFF.</t>
+        <t>Template:</t>
+        <ul spacing="normal">
+          <li>Value: The numeric value of the extension type</li>
+          <li>Name: The name of the extension type</li>
+          <li>
+            <t>Message(s): The messages in which the extension may appear, drawn from the following
+list:  </t>
+            <ul spacing="normal">
+              <li>KP: KeyPackage objects</li>
+              <li>LN: LeafNode objects</li>
+              <li>GC: GroupContext objects</li>
+              <li>GI: GroupInfo objects</li>
+            </ul>
+          </li>
+          <li>Recommended: Same as in <xref target="mls-cipher-suites"/></li>
+          <li>Reference: The document where this extension is defined</li>
+        </ul>
+        <t>Initial contents:</t>
+        <table>
+          <name>MLS Extension Types Registry</name>
+          <thead>
+            <tr>
+              <th align="left">Value</th>
+              <th align="left">Name</th>
+              <th align="left">Message(s)</th>
+              <th align="left">R</th>
+              <th align="left">Ref</th>
+            </tr>
+          </thead>
+          <tbody>
+            <tr>
+              <td align="left">0x0000</td>
+              <td align="left">RESERVED</td>
+              <td align="left">N/A</td>
+              <td align="left">-</td>
+              <td align="left">RFC 9420</td>
+            </tr>
+            <tr>
+              <td align="left">0x0001</td>
+              <td align="left">application_id</td>
+              <td align="left">LN</td>
+              <td align="left">Y</td>
+              <td align="left">RFC 9420</td>
+            </tr>
+            <tr>
+              <td align="left">0x0002</td>
+              <td align="left">ratchet_tree</td>
+              <td align="left">GI</td>
+              <td align="left">Y</td>
+              <td align="left">RFC 9420</td>
+            </tr>
+            <tr>
+              <td align="left">0x0003</td>
+              <td align="left">required_capabilities</td>
+              <td align="left">GC</td>
+              <td align="left">Y</td>
+              <td align="left">RFC 9420</td>
+            </tr>
+            <tr>
+              <td align="left">0x0004</td>
+              <td align="left">external_pub</td>
+              <td align="left">GI</td>
+              <td align="left">Y</td>
+              <td align="left">RFC 9420</td>
+            </tr>
+            <tr>
+              <td align="left">0x0005</td>
+              <td align="left">external_senders</td>
+              <td align="left">GC</td>
+              <td align="left">Y</td>
+              <td align="left">RFC 9420</td>
+            </tr>
+            <tr>
+              <td align="left">0x0A0A</td>
+              <td align="left">GREASE</td>
+              <td align="left">KP, GI, LN</td>
+              <td align="left">Y</td>
+              <td align="left">RFC 9420</td>
+            </tr>
+            <tr>
+              <td align="left">0x1A1A</td>
+              <td align="left">GREASE</td>
+              <td align="left">KP, GI, LN</td>
+              <td align="left">Y</td>
+              <td align="left">RFC 9420</td>
+            </tr>
+            <tr>
+              <td align="left">0x2A2A</td>
+              <td align="left">GREASE</td>
+              <td align="left">KP, GI, LN</td>
+              <td align="left">Y</td>
+              <td align="left">RFC 9420</td>
+            </tr>
+            <tr>
+              <td align="left">0x3A3A</td>
+              <td align="left">GREASE</td>
+              <td align="left">KP, GI, LN</td>
+              <td align="left">Y</td>
+              <td align="left">RFC 9420</td>
+            </tr>
+            <tr>
+              <td align="left">0x4A4A</td>
+              <td align="left">GREASE</td>
+              <td align="left">KP, GI, LN</td>
+              <td align="left">Y</td>
+              <td align="left">RFC 9420</td>
+            </tr>
+            <tr>
+              <td align="left">0x5A5A</td>
+              <td align="left">GREASE</td>
+              <td align="left">KP, GI, LN</td>
+              <td align="left">Y</td>
+              <td align="left">RFC 9420</td>
+            </tr>
+            <tr>
+              <td align="left">0x6A6A</td>
+              <td align="left">GREASE</td>
+              <td align="left">KP, GI, LN</td>
+              <td align="left">Y</td>
+              <td align="left">RFC 9420</td>
+            </tr>
+            <tr>
+              <td align="left">0x7A7A</td>
+              <td align="left">GREASE</td>
+              <td align="left">KP, GI, LN</td>
+              <td align="left">Y</td>
+              <td align="left">RFC 9420</td>
+            </tr>
+            <tr>
+              <td align="left">0x8A8A</td>
+              <td align="left">GREASE</td>
+              <td align="left">KP, GI, LN</td>
+              <td align="left">Y</td>
+              <td align="left">RFC 9420</td>
+            </tr>
+            <tr>
+              <td align="left">0x9A9A</td>
+              <td align="left">GREASE</td>
+              <td align="left">KP, GI, LN</td>
+              <td align="left">Y</td>
+              <td align="left">RFC 9420</td>
+            </tr>
+            <tr>
+              <td align="left">0xAAAA</td>
+              <td align="left">GREASE</td>
+              <td align="left">KP, GI, LN</td>
+              <td align="left">Y</td>
+              <td align="left">RFC 9420</td>
+            </tr>
+            <tr>
+              <td align="left">0xBABA</td>
+              <td align="left">GREASE</td>
+              <td align="left">KP, GI, LN</td>
+              <td align="left">Y</td>
+              <td align="left">RFC 9420</td>
+            </tr>
+            <tr>
+              <td align="left">0xCACA</td>
+              <td align="left">GREASE</td>
+              <td align="left">KP, GI, LN</td>
+              <td align="left">Y</td>
+              <td align="left">RFC 9420</td>
+            </tr>
+            <tr>
+              <td align="left">0xDADA</td>
+              <td align="left">GREASE</td>
+              <td align="left">KP, GI, LN</td>
+              <td align="left">Y</td>
+              <td align="left">RFC 9420</td>
+            </tr>
+            <tr>
+              <td align="left">0xEAEA</td>
+              <td align="left">GREASE</td>
+              <td align="left">KP, GI, LN</td>
+              <td align="left">Y</td>
+              <td align="left">RFC 9420</td>
+            </tr>
+            <tr>
+              <td align="left">0xF000  - 0xFFFF</td>
+              <td align="left">Reserved for Private Use</td>
+              <td align="left">N/A</td>
+              <td align="left">-</td>
+              <td align="left">RFC 9420</td>
+            </tr>
+          </tbody>
+        </table>
+      </section>
+      <section anchor="mls-proposal-types">
+        <name>MLS Proposal Types</name>
+        <t>The "MLS Proposal Types" registry lists identifiers for types of proposals that can be made for
+changes to an MLS group.  The extension type field is two bytes wide, so valid
+extension type values are in the range 0x0000 to 0xFFFF.</t>
+        <t>Template:</t>
+        <ul spacing="normal">
+          <li>Value: The numeric value of the proposal type</li>
+          <li>Name: The name of the proposal type</li>
+          <li>Recommended: Same as in <xref target="mls-cipher-suites"/></li>
+          <li>External: Whether a proposal of this type may be sent by an <tt>external</tt> sender
+(see <xref target="external-proposals"/>)</li>
+          <li>Path Required: Whether a Commit covering a proposal of this type is required
+to have its <tt>path</tt> field populated (see <xref target="commit"/>)</li>
+          <li>Reference: The document where this extension is defined</li>
+        </ul>
+        <t>Initial contents:</t>
+        <table>
+          <name>MLS Proposal Types Registry</name>
+          <thead>
+            <tr>
+              <th align="left">Value</th>
+              <th align="left">Name</th>
+              <th align="left">R</th>
+              <th align="left">Ext</th>
+              <th align="left">Path</th>
+              <th align="left">Ref</th>
+            </tr>
+          </thead>
+          <tbody>
+            <tr>
+              <td align="left">0x0000</td>
+              <td align="left">RESERVED</td>
+              <td align="left">-</td>
+              <td align="left">-</td>
+              <td align="left">-</td>
+              <td align="left">RFC 9420</td>
+            </tr>
+            <tr>
+              <td align="left">0x0001</td>
+              <td align="left">add</td>
+              <td align="left">Y</td>
+              <td align="left">Y</td>
+              <td align="left">N</td>
+              <td align="left">RFC 9420</td>
+            </tr>
+            <tr>
+              <td align="left">0x0002</td>
+              <td align="left">update</td>
+              <td align="left">Y</td>
+              <td align="left">N</td>
+              <td align="left">Y</td>
+              <td align="left">RFC 9420</td>
+            </tr>
+            <tr>
+              <td align="left">0x0003</td>
+              <td align="left">remove</td>
+              <td align="left">Y</td>
+              <td align="left">Y</td>
+              <td align="left">Y</td>
+              <td align="left">RFC 9420</td>
+            </tr>
+            <tr>
+              <td align="left">0x0004</td>
+              <td align="left">psk</td>
+              <td align="left">Y</td>
+              <td align="left">Y</td>
+              <td align="left">N</td>
+              <td align="left">RFC 9420</td>
+            </tr>
+            <tr>
+              <td align="left">0x0005</td>
+              <td align="left">reinit</td>
+              <td align="left">Y</td>
+              <td align="left">Y</td>
+              <td align="left">N</td>
+              <td align="left">RFC 9420</td>
+            </tr>
+            <tr>
+              <td align="left">0x0006</td>
+              <td align="left">external_init</td>
+              <td align="left">Y</td>
+              <td align="left">N</td>
+              <td align="left">Y</td>
+              <td align="left">RFC 9420</td>
+            </tr>
+            <tr>
+              <td align="left">0x0007</td>
+              <td align="left">group_context_extensions</td>
+              <td align="left">Y</td>
+              <td align="left">Y</td>
+              <td align="left">Y</td>
+              <td align="left">RFC 9420</td>
+            </tr>
+            <tr>
+              <td align="left">0x0A0A</td>
+              <td align="left">GREASE</td>
+              <td align="left">Y</td>
+              <td align="left">-</td>
+              <td align="left">-</td>
+              <td align="left">RFC 9420</td>
+            </tr>
+            <tr>
+              <td align="left">0x1A1A</td>
+              <td align="left">GREASE</td>
+              <td align="left">Y</td>
+              <td align="left">-</td>
+              <td align="left">-</td>
+              <td align="left">RFC 9420</td>
+            </tr>
+            <tr>
+              <td align="left">0x2A2A</td>
+              <td align="left">GREASE</td>
+              <td align="left">Y</td>
+              <td align="left">-</td>
+              <td align="left">-</td>
+              <td align="left">RFC 9420</td>
+            </tr>
+            <tr>
+              <td align="left">0x3A3A</td>
+              <td align="left">GREASE</td>
+              <td align="left">Y</td>
+              <td align="left">-</td>
+              <td align="left">-</td>
+              <td align="left">RFC 9420</td>
+            </tr>
+            <tr>
+              <td align="left">0x4A4A</td>
+              <td align="left">GREASE</td>
+              <td align="left">Y</td>
+              <td align="left">-</td>
+              <td align="left">-</td>
+              <td align="left">RFC 9420</td>
+            </tr>
+            <tr>
+              <td align="left">0x5A5A</td>
+              <td align="left">GREASE</td>
+              <td align="left">Y</td>
+              <td align="left">-</td>
+              <td align="left">-</td>
+              <td align="left">RFC 9420</td>
+            </tr>
+            <tr>
+              <td align="left">0x6A6A</td>
+              <td align="left">GREASE</td>
+              <td align="left">Y</td>
+              <td align="left">-</td>
+              <td align="left">-</td>
+              <td align="left">RFC 9420</td>
+            </tr>
+            <tr>
+              <td align="left">0x7A7A</td>
+              <td align="left">GREASE</td>
+              <td align="left">Y</td>
+              <td align="left">-</td>
+              <td align="left">-</td>
+              <td align="left">RFC 9420</td>
+            </tr>
+            <tr>
+              <td align="left">0x8A8A</td>
+              <td align="left">GREASE</td>
+              <td align="left">Y</td>
+              <td align="left">-</td>
+              <td align="left">-</td>
+              <td align="left">RFC 9420</td>
+            </tr>
+            <tr>
+              <td align="left">0x9A9A</td>
+              <td align="left">GREASE</td>
+              <td align="left">Y</td>
+              <td align="left">-</td>
+              <td align="left">-</td>
+              <td align="left">RFC 9420</td>
+            </tr>
+            <tr>
+              <td align="left">0xAAAA</td>
+              <td align="left">GREASE</td>
+              <td align="left">Y</td>
+              <td align="left">-</td>
+              <td align="left">-</td>
+              <td align="left">RFC 9420</td>
+            </tr>
+            <tr>
+              <td align="left">0xBABA</td>
+              <td align="left">GREASE</td>
+              <td align="left">Y</td>
+              <td align="left">-</td>
+              <td align="left">-</td>
+              <td align="left">RFC 9420</td>
+            </tr>
+            <tr>
+              <td align="left">0xCACA</td>
+              <td align="left">GREASE</td>
+              <td align="left">Y</td>
+              <td align="left">-</td>
+              <td align="left">-</td>
+              <td align="left">RFC 9420</td>
+            </tr>
+            <tr>
+              <td align="left">0xDADA</td>
+              <td align="left">GREASE</td>
+              <td align="left">Y</td>
+              <td align="left">-</td>
+              <td align="left">-</td>
+              <td align="left">RFC 9420</td>
+            </tr>
+            <tr>
+              <td align="left">0xEAEA</td>
+              <td align="left">GREASE</td>
+              <td align="left">Y</td>
+              <td align="left">-</td>
+              <td align="left">-</td>
+              <td align="left">RFC 9420</td>
+            </tr>
+            <tr>
+              <td align="left">0xF000  - 0xFFFF</td>
+              <td align="left">Reserved for Private Use</td>
+              <td align="left">-</td>
+              <td align="left">-</td>
+              <td align="left">-</td>
+              <td align="left">RFC 9420</td>
+            </tr>
+          </tbody>
+        </table>
+      </section>
+      <section anchor="mls-credential-types">
+        <name>MLS Credential Types</name>
+        <t>The "MLS Credential Types" registry lists identifiers for types of credentials that can be used for
+authentication in the MLS protocol.  The credential type field is two bytes wide,
+so valid credential type values are in the range 0x0000 to 0xFFFF.</t>
+        <t>Template:</t>
+        <ul spacing="normal">
+          <li>Value: The numeric value of the credential type</li>
+          <li>Name: The name of the credential type</li>
+          <li>Recommended: Same as in <xref target="mls-cipher-suites"/></li>
+          <li>Reference: The document where this credential is defined</li>
+        </ul>
+        <t>Initial contents:</t>
+        <table>
+          <name>MLS Credential Types Registry</name>
+          <thead>
+            <tr>
+              <th align="left">Value</th>
+              <th align="left">Name</th>
+              <th align="left">R</th>
+              <th align="left">Ref</th>
+            </tr>
+          </thead>
+          <tbody>
+            <tr>
+              <td align="left">0x0000</td>
+              <td align="left">RESERVED</td>
+              <td align="left">-</td>
+              <td align="left">RFC 9420</td>
+            </tr>
+            <tr>
+              <td align="left">0x0001</td>
+              <td align="left">basic</td>
+              <td align="left">Y</td>
+              <td align="left">RFC 9420</td>
+            </tr>
+            <tr>
+              <td align="left">0x0002</td>
+              <td align="left">x509</td>
+              <td align="left">Y</td>
+              <td align="left">RFC 9420</td>
+            </tr>
+            <tr>
+              <td align="left">0x0A0A</td>
+              <td align="left">GREASE</td>
+              <td align="left">Y</td>
+              <td align="left">RFC 9420</td>
+            </tr>
+            <tr>
+              <td align="left">0x1A1A</td>
+              <td align="left">GREASE</td>
+              <td align="left">Y</td>
+              <td align="left">RFC 9420</td>
+            </tr>
+            <tr>
+              <td align="left">0x2A2A</td>
+              <td align="left">GREASE</td>
+              <td align="left">Y</td>
+              <td align="left">RFC 9420</td>
+            </tr>
+            <tr>
+              <td align="left">0x3A3A</td>
+              <td align="left">GREASE</td>
+              <td align="left">Y</td>
+              <td align="left">RFC 9420</td>
+            </tr>
+            <tr>
+              <td align="left">0x4A4A</td>
+              <td align="left">GREASE</td>
+              <td align="left">Y</td>
+              <td align="left">RFC 9420</td>
+            </tr>
+            <tr>
+              <td align="left">0x5A5A</td>
+              <td align="left">GREASE</td>
+              <td align="left">Y</td>
+              <td align="left">RFC 9420</td>
+            </tr>
+            <tr>
+              <td align="left">0x6A6A</td>
+              <td align="left">GREASE</td>
+              <td align="left">Y</td>
+              <td align="left">RFC 9420</td>
+            </tr>
+            <tr>
+              <td align="left">0x7A7A</td>
+              <td align="left">GREASE</td>
+              <td align="left">Y</td>
+              <td align="left">RFC 9420</td>
+            </tr>
+            <tr>
+              <td align="left">0x8A8A</td>
+              <td align="left">GREASE</td>
+              <td align="left">Y</td>
+              <td align="left">RFC 9420</td>
+            </tr>
+            <tr>
+              <td align="left">0x9A9A</td>
+              <td align="left">GREASE</td>
+              <td align="left">Y</td>
+              <td align="left">RFC 9420</td>
+            </tr>
+            <tr>
+              <td align="left">0xAAAA</td>
+              <td align="left">GREASE</td>
+              <td align="left">Y</td>
+              <td align="left">RFC 9420</td>
+            </tr>
+            <tr>
+              <td align="left">0xBABA</td>
+              <td align="left">GREASE</td>
+              <td align="left">Y</td>
+              <td align="left">RFC 9420</td>
+            </tr>
+            <tr>
+              <td align="left">0xCACA</td>
+              <td align="left">GREASE</td>
+              <td align="left">Y</td>
+              <td align="left">RFC 9420</td>
+            </tr>
+            <tr>
+              <td align="left">0xDADA</td>
+              <td align="left">GREASE</td>
+              <td align="left">Y</td>
+              <td align="left">RFC 9420</td>
+            </tr>
+            <tr>
+              <td align="left">0xEAEA</td>
+              <td align="left">GREASE</td>
+              <td align="left">Y</td>
+              <td align="left">RFC 9420</td>
+            </tr>
+            <tr>
+              <td align="left">0xF000  - 0xFFFF</td>
+              <td align="left">Reserved for Private Use</td>
+              <td align="left">-</td>
+              <td align="left">RFC 9420</td>
+            </tr>
+          </tbody>
+        </table>
+      </section>
+      <section anchor="mls-signature-labels">
+        <name>MLS Signature Labels</name>
+        <t>The <tt>SignWithLabel</tt> function defined in <xref target="signing"/> avoids the risk of
+confusion between signatures in different contexts.  Each context is assigned a
+distinct label that is incorporated into the signature.  The "MLS Signature Labels" registry records
+the labels defined in this document and allows additional labels to be
+registered in case extensions add other types of signatures using the same
+signature keys used elsewhere in MLS.</t>
+        <t>Template:</t>
+        <ul spacing="normal">
+          <li>Label: The string to be used as the <tt>Label</tt> parameter to <tt>SignWithLabel</tt></li>
+          <li>Recommended: Same as in <xref target="mls-cipher-suites"/></li>
+          <li>Reference: The document where this label is defined</li>
+        </ul>
+        <t>Initial contents:</t>
+        <table>
+          <name>MLS Signature Labels Registry</name>
+          <thead>
+            <tr>
+              <th align="left">Label</th>
+              <th align="left">R</th>
+              <th align="left">Ref</th>
+            </tr>
+          </thead>
+          <tbody>
+            <tr>
+              <td align="left">"FramedContentTBS"</td>
+              <td align="left">Y</td>
+              <td align="left">RFC 9420</td>
+            </tr>
+            <tr>
+              <td align="left">"LeafNodeTBS"</td>
+              <td align="left">Y</td>
+              <td align="left">RFC 9420</td>
+            </tr>
+            <tr>
+              <td align="left">"KeyPackageTBS"</td>
+              <td align="left">Y</td>
+              <td align="left">RFC 9420</td>
+            </tr>
+            <tr>
+              <td align="left">"GroupInfoTBS"</td>
+              <td align="left">Y</td>
+              <td align="left">RFC 9420</td>
+            </tr>
+          </tbody>
+        </table>
+      </section>
+      <section anchor="mls-public-key-encryption-labels">
+        <name>MLS Public Key Encryption Labels</name>
+        <t>The <tt>EncryptWithLabel</tt> function defined in <xref target="public-key-encryption"/> avoids the
+risk of confusion between ciphertexts produced for different purposes in
+different contexts.  Each context is assigned a distinct label that is
+incorporated into the signature.  The "MLS Public Key Encryption Labels" registry records the labels defined in
+this document and allows additional labels to be registered in case extensions
+add other types of public key encryption using the same HPKE keys used elsewhere
+in MLS.</t>
+        <t>Template:</t>
+        <ul spacing="normal">
+          <li>Label: The string to be used as the <tt>Label</tt> parameter to <tt>EncryptWithLabel</tt></li>
+          <li>Recommended: Same as in <xref target="mls-cipher-suites"/></li>
+          <li>Reference: The document where this label is defined</li>
+        </ul>
+        <t>Initial contents:</t>
+        <table>
+          <name>MLS Public Key Encryption Labels Registry</name>
+          <thead>
+            <tr>
+              <th align="left">Label</th>
+              <th align="left">R</th>
+              <th align="left">Ref</th>
+            </tr>
+          </thead>
+          <tbody>
+            <tr>
+              <td align="left">"UpdatePathNode"</td>
+              <td align="left">Y</td>
+              <td align="left">RFC 9420</td>
+            </tr>
+            <tr>
+              <td align="left">"Welcome"</td>
+              <td align="left">Y</td>
+              <td align="left">RFC 9420</td>
+            </tr>
+          </tbody>
+        </table>
+      </section>
+      <section anchor="mls-exporter-labels">
+        <name>MLS Exporter Labels</name>
+        <t>The exporter function defined in <xref target="exporters"/> allows applications to derive key
+material from the MLS key schedule.  Like the TLS exporter <xref target="RFC8446"/>, the MLS
+exporter uses a label to distinguish between different applications' use of the
+exporter.  The "MLS Exporter Labels" registry allows applications to register their usage to avoid
+collisions.</t>
+        <t>Template:</t>
+        <ul spacing="normal">
+          <li>Label: The string to be used as the <tt>Label</tt> parameter to <tt>MLS-Exporter</tt></li>
+          <li>Recommended: Same as in <xref target="mls-cipher-suites"/></li>
+          <li>Reference: The document where this label is defined</li>
+        </ul>
+        <t>The registry has no initial contents, since it is intended to be used by
+applications, not the core protocol.  The table below is intended only to show
+the column layout of the registry.</t>
+        <table>
+          <name>MLS Exporter Labels Registry</name>
+          <thead>
+            <tr>
+              <th align="left">Label</th>
+              <th align="left">Recommended</th>
+              <th align="left">Reference</th>
+            </tr>
+          </thead>
+          <tbody>
+            <tr>
+              <td align="left">(N/A)</td>
+              <td align="left">(N/A)</td>
+              <td align="left">(N/A)</td>
+            </tr>
+          </tbody>
+        </table>
+      </section>
+      <section anchor="de">
+        <name>MLS Designated Expert Pool</name>
+        <t>Specification Required <xref target="RFC8126"/> registry requests are registered
+after a three-week review period on the MLS Designated Expert (DE) mailing list
+<eref brackets="angle" target="mailto:mls-reg-review@ietf.org"/> on the advice of one or more of the MLS DEs. However,
+to allow for the allocation of values prior to publication, the MLS
+DEs may approve registration once they are satisfied that such a
+specification will be published.</t>
+        <t>Registration requests sent to the MLS DEs' mailing list for review
+<bcp14>SHOULD</bcp14> use an appropriate subject (e.g., "Request to register value
+in MLS Bar registry").</t>
+        <t>Within the review period, the MLS DEs will either approve or deny
+the registration request, communicating this decision to the MLS DEs'
+mailing list and IANA. Denials <bcp14>SHOULD</bcp14> include an explanation and, if
+applicable, suggestions as to how to make the request successful.
+Registration requests that are undetermined for a period longer than
+21 days can be brought to the IESG's attention for resolution using
+the <eref brackets="angle" target="mailto:iesg@ietf.org"/> mailing list.</t>
+        <t>Criteria that <bcp14>SHOULD</bcp14> be applied by the MLS DEs includes determining
+whether the proposed registration duplicates existing functionality,
+whether it is likely to be of general applicability or useful only
+for a single application, and whether the registration description
+is clear. For example, for cipher suite registrations, the MLS DEs will apply the
+advisory found in <xref target="mls-cipher-suites"/>.</t>
+        <t>IANA <bcp14>MUST</bcp14> only accept registry updates from the MLS DEs and <bcp14>SHOULD</bcp14>
+direct all requests for registration to the MLS DEs' mailing list.</t>
+        <t>It is suggested that multiple MLS DEs who are able to
+represent the perspectives of different applications using this
+specification be appointed, in order to enable a broadly informed review of
+registration decisions. In cases where a registration decision could
+be perceived as creating a conflict of interest for a particular
+MLS DE, that MLS DE <bcp14>SHOULD</bcp14> defer to the judgment of the other MLS DEs.</t>
+      </section>
+      <section anchor="the-messagemls-media-type">
+        <name>The "message/mls" Media Type</name>
+        <t>This document registers the "message/mls" media type in the "message" registry in order to allow other
+protocols (e.g., HTTP <xref target="RFC9113"/>) to convey MLS messages.</t>
+        <dl>
+          <dt>Type name:</dt>
+          <dd>message</dd>
+          <dt>Subtype name:</dt>
+          <dd>mls</dd>
+          <dt>Required parameters:</dt>
+          <dd>none</dd>
+          <dt>Optional parameters:</dt>
+          <dd>
+            <dl>
+              <dt>version</dt>
+              <dd/>
+              <dt>  version:</dt>
+              <dd>The MLS protocol version expressed as a string
+  <tt>&lt;major&gt;.&lt;minor&gt;</tt>.  If omitted, the version is "1.0", which
+  corresponds to MLS ProtocolVersion mls10. If for some reason
+  the version number in the media type parameter differs from the
+  ProtocolVersion embedded in the protocol, the protocol takes
+  precedence.</dd>
+            </dl>
+          </dd>
+          <dt>Encoding considerations:</dt>
+          <dd>MLS messages are represented using the TLS
+presentation language <xref target="RFC8446"/>. Therefore, MLS messages need to be
+treated as binary data.</dd>
+          <dt>Security considerations:</dt>
+          <dd>MLS is an encrypted messaging layer designed
+to be transmitted over arbitrary lower-layer protocols. The
+security considerations in this document (RFC 9420) also apply.</dd>
+          <dt>Interoperability considerations:</dt>
+          <dd>N/A</dd>
+          <dt>Published specification:</dt>
+          <dd>RFC 9420</dd>
+          <dt>Applications that use this media type:</dt>
+          <dd>MLS-based messaging applications</dd>
+          <dt>Fragment identifier considerations:</dt>
+          <dd>N/A</dd>
+          <dt>Additional information:</dt>
+          <dd>
+            <t>
+              <br/>
+            </t>
+            <dl indent="3" newline="false" spacing="compact">
+              <dt>Deprecated alias names for this type:</dt>
+              <dd>N/A</dd>
+              <dt>Magic number(s):</dt>
+              <dd>N/A</dd>
+              <dt>File extension(s):</dt>
+              <dd>N/A</dd>
+              <dt>Macintosh file type code(s):</dt>
+              <dd>N/A</dd>
+            </dl>
+          </dd>
+        </dl>
+        <dl>
+          <dt>Person &amp; email address to contact for further information:</dt>
+          <dd>
+            <t>IETF MLS Working Group <eref brackets="angle" target="mailto:mls@ietf.org"/></t>
+          </dd>
+          <dt>Intended usage:</dt>
+          <dd>
+            <t>COMMON</t>
+          </dd>
+          <dt>Restrictions on usage:</dt>
+          <dd>
+            <t>N/A</t>
+          </dd>
+          <dt>Author:</dt>
+          <dd>
+            <t>IETF MLS Working Group</t>
+          </dd>
+          <dt>Change controller:</dt>
+          <dd>
+            <t>IETF</t>
+          </dd>
+        </dl>
+      </section>
+    </section>
+  </middle>
+  <back>
+    <displayreference target="I-D.ietf-mls-architecture" to="MLS-ARCH"/>
+    <displayreference target="I-D.irtf-cfrg-aead-limits" to="CFRG-AEAD-LIMITS"/>
+    <references>
+      <name>References</name>
+      <references>
+        <name>Normative References</name>
+        <xi:include xmlns:xi="http://www.w3.org/2001/XInclude" href="https://bib.ietf.org/public/rfc/bibxml/reference.RFC.2119.xml"/>
+        <xi:include xmlns:xi="http://www.w3.org/2001/XInclude" href="https://bib.ietf.org/public/rfc/bibxml/reference.RFC.8174.xml"/>
+        <xi:include xmlns:xi="http://www.w3.org/2001/XInclude" href="https://bib.ietf.org/public/rfc/bibxml/reference.RFC.9180.xml"/>
+        <xi:include xmlns:xi="http://www.w3.org/2001/XInclude" href="https://bib.ietf.org/public/rfc/bibxml/reference.RFC.8446.xml"/>
+        <xi:include xmlns:xi="http://www.w3.org/2001/XInclude" href="https://bib.ietf.org/public/rfc/bibxml/reference.RFC.8126.xml"/>
+        <xi:include xmlns:xi="http://www.w3.org/2001/XInclude" href="https://bib.ietf.org/public/rfc/bibxml/reference.RFC.2104.xml"/>
+      </references>
+      <references>
+        <name>Informative References</name>
+        <reference anchor="ART" target="https://eprint.iacr.org/2017/666.pdf">
+          <front>
+            <title>On Ends-to-Ends Encryption: Asynchronous Group Messaging with Strong Security Guarantees</title>
+            <author fullname="Katriel Cohn-Gordon" initials="K." surname="Cohn-Gordon">
+              <organization/>
+            </author>
+            <author fullname="Cas Cremers" initials="C." surname="Cremers">
+              <organization/>
+            </author>
+            <author fullname="Luke Garratt" initials="L." surname="Garratt">
+              <organization/>
+            </author>
+            <author fullname="Jon Millican" initials="J." surname="Millican">
+              <organization/>
+            </author>
+            <author fullname="Kevin Milner" initials="K." surname="Milner">
+              <organization/>
+            </author>
+            <date month="March" year="2020"/>
+          </front>
+          <refcontent>Version 2.3</refcontent>
+          <seriesInfo name="DOI" value="10.1145/3243734.3243747"/>
+        </reference>
+        <reference anchor="DoubleRatchet">
+          <front>
+            <title>A Formal Security Analysis of the Signal Messaging Protocol</title>
+            <author fullname="Katriel Cohn-Gordon" initials="K." surname="Cohn-Gordon">
+              <organization/>
+            </author>
+            <author fullname="Cas Cremers" initials="C." surname="Cremers">
+              <organization/>
+            </author>
+            <author fullname="Benjamin Dowling" initials="B." surname="Dowling">
+              <organization/>
+            </author>
+            <author fullname="Luke Garratt" initials="L." surname="Garratt">
+              <organization/>
+            </author>
+            <author fullname="Douglas Stebila" initials="D." surname="Stebila">
+              <organization/>
+            </author>
+            <date month="April" year="2017"/>
+          </front>
+          <refcontent>2017 IEEE European Symposium on Security and Privacy (EuroS&amp;P)</refcontent>
+          <seriesInfo name="DOI" value="10.1109/eurosp.2017.27"/>
+        </reference>
+        <reference anchor="Signal" target="https://www.signal.org/docs/specifications/doubleratchet/">
+          <front>
+            <title>The Double Ratchet Algorithm</title>
+            <author fullname="Trevor Perrin(ed)" initials="T." surname="Perrin(ed)">
+              <organization/>
+            </author>
+            <author fullname="Moxie Marlinspike" initials="M." surname="Marlinspike">
+              <organization/>
+            </author>
+            <date month="November" year="2016"/>
+          </front>
+          <refcontent>Revision 1</refcontent>
+        </reference>
+        <reference anchor="SHS" target="https://doi.org/10.6028/NIST.FIPS.180-4">
+          <front>
+            <title>Secure Hash Standard (SHS)</title>
+            <author>
+              <organization>National Institute of Standards and Technology (NIST)</organization>
+            </author>
+            <date month="August" year="2015"/>
+          </front>
+          <seriesInfo name="FIPS PUB" value="180-4"/>
+          <seriesInfo name="DOI" value="10.6028/NIST.FIPS.180-4"/>
+        </reference>
+        <reference anchor="NAN">
+          <front>
+            <title>Nonces Are Noticed: AEAD Revisited</title>
+            <author fullname="Mihir Bellare" initials="M." surname="Bellare">
+              <organization/>
+            </author>
+            <author fullname="Ruth Ng" initials="R." surname="Ng">
+              <organization/>
+            </author>
+            <author fullname="Bj&#xF6;rn Tackmann" initials="B." surname="Tackmann">
+              <organization/>
+            </author>
+            <date month="August" year="2019"/>
+          </front>
+          <refcontent>Advances in Cryptology - CRYPTO 2019, pp. 235-265</refcontent>
+          <seriesInfo name="DOI" value="10.1007/978-3-030-26948-7_9"/>
+        </reference>
+        <reference anchor="CONIKS" target="https://www.usenix.org/system/files/conference/usenixsecurity15/sec15-paper-melara.pdf">
+          <front>
+            <title>CONIKS: Bringing Key Transparency to End Users</title>
+            <author fullname="Marcela S. Melara" initials="M. S." surname="Melara">
+              <organization/>
+            </author>
+            <author fullname="Aaron Blankstein" initials="A." surname="Blankstein">
+              <organization/>
+            </author>
+            <author fullname="Joseph Bonneau" initials="J." surname="Bonneau">
+              <organization/>
+            </author>
+            <author fullname="Edward W. Felten" initials="E. W." surname="Felten">
+              <organization/>
+            </author>
+            <author fullname="Michael J. Freedman" initials="M. J." surname="Freedman">
+              <organization/>
+            </author>
+            <date month="August" year="2015"/>
+          </front>
+          <refcontent>Proceedings of the 24th USENIX Security Symposium</refcontent>
+          <refcontent>ISBN 978-1-939133-11-3</refcontent>
+        </reference>
+        <reference anchor="KT" target="https://github.com/google/keytransparency/blob/master/docs/design.md">
+          <front>
+            <title>Key Transparency Design Doc</title>
+            <author>
+              <organization/>
+            </author>
+            <date month="June" year="2020"/>
+          </front>
+          <refcontent>commit fb0f87f</refcontent>
+        </reference>
+        <xi:include xmlns:xi="http://www.w3.org/2001/XInclude" href="https://bib.ietf.org/public/rfc/bibxml/reference.RFC.9000.xml"/>
+        <!--[I-D.ietf-mls-architecture] IESG state IESG Evaluation::Revised I-D Needed -->
+        <xi:include xmlns:xi="http://www.w3.org/2001/XInclude" href="https://datatracker.ietf.org/doc/bibxml3/reference.I-D.ietf-mls-architecture.xml"/>
+        <xi:include xmlns:xi="http://www.w3.org/2001/XInclude" href="https://bib.ietf.org/public/rfc/bibxml/reference.RFC.5116.xml"/>
+        <xi:include xmlns:xi="http://www.w3.org/2001/XInclude" href="https://bib.ietf.org/public/rfc/bibxml/reference.RFC.8032.xml"/>
+        <xi:include xmlns:xi="http://www.w3.org/2001/XInclude" href="https://bib.ietf.org/public/rfc/bibxml/reference.RFC.6125.xml"/>
+        <xi:include xmlns:xi="http://www.w3.org/2001/XInclude" href="https://bib.ietf.org/public/rfc/bibxml/reference.RFC.5905.xml"/>
+        <xi:include xmlns:xi="http://www.w3.org/2001/XInclude" href="https://bib.ietf.org/public/rfc/bibxml/reference.RFC.7696.xml"/>
+        <xi:include xmlns:xi="http://www.w3.org/2001/XInclude" href="https://bib.ietf.org/public/rfc/bibxml/reference.RFC.9113.xml"/>
+        <xi:include xmlns:xi="http://www.w3.org/2001/XInclude" href="https://bib.ietf.org/public/rfc/bibxml/reference.RFC.8701.xml"/>
+        <reference anchor="CLINIC">
+          <front>
+            <title>I Know Why You Went to the Clinic: Risks and Realization of HTTPS Traffic Analysis</title>
+            <author fullname="Brad Miller" initials="B." surname="Miller">
+              <organization/>
+            </author>
+            <author fullname="Ling Huang" initials="L." surname="Huang">
+              <organization/>
+            </author>
+            <author fullname="A. D. Joseph" initials="A." surname="Joseph">
+              <organization/>
+            </author>
+            <author fullname="J. D. Tygar" initials="J." surname="Tygar">
+              <organization/>
+            </author>
+            <date year="2014"/>
+          </front>
+          <refcontent>Privacy Enhancing Technologies, pp. 143-163</refcontent>
+          <seriesInfo name="DOI" value="10.1007/978-3-319-08506-7_8"/>
+        </reference>
+        <reference anchor="HCJ16">
+          <front>
+            <title>HTTPS traffic analysis and client identification using passive SSL/TLS fingerprinting</title>
+            <author fullname="Martin Hus&#xE1;k" initials="M." surname="Hus&#xE1;k">
+              <organization/>
+            </author>
+            <author fullname="Milan &#x10C;erm&#xE1;k" initials="M." surname="&#x10C;erm&#xE1;k">
+              <organization/>
+            </author>
+            <author fullname="Tom&#xE1;&#x161; Jirs&#xED;k" initials="T." surname="Jirs&#xED;k">
+              <organization/>
+            </author>
+            <author fullname="Pavel &#x10C;eleda" initials="P." surname="&#x10C;eleda">
+              <organization/>
+            </author>
+            <date month="February" year="2016"/>
+          </front>
+          <refcontent>EURASIP Journal on Information Security, Vol. 2016, Issue 1</refcontent>
+          <seriesInfo name="DOI" value="10.1186/s13635-016-0030-7"/>
+        </reference>
+        <!--[I-D.irtf-cfrg-aead-limits] IESG state I-D Exists -->
+        <xi:include xmlns:xi="http://www.w3.org/2001/XInclude" href="https://datatracker.ietf.org/doc/bibxml3/reference.I-D.irtf-cfrg-aead-limits.xml"/>
+        <xi:include xmlns:xi="http://www.w3.org/2001/XInclude" href="https://bib.ietf.org/public/rfc/bibxml/reference.RFC.9001.xml"/>
+      </references>
+    </references>
+    <section anchor="protocol-origins-of-example-trees">
+      <name>Protocol Origins of Example Trees</name>
+      <t>Protocol operations in MLS give rise to specific forms of ratchet tree,
+typically affecting a whole direct path at once.  In this section, we describe
+the protocol operations that could have given rise to the various example trees
+in this document.</t>
+      <t>To construct the tree in <xref target="full-tree"/>:</t>
+      <ul spacing="normal">
+        <li>A creates a group with B, ..., G</li>
+        <li>F sends an empty Commit, setting X, Y, and W</li>
+        <li>G removes C and D, blanking V, U, and setting Y and W</li>
+        <li>B sends an empty Commit, setting T and W</li>
+      </ul>
+      <t>To construct the tree in <xref target="resolution-tree"/>:</t>
+      <ul spacing="normal">
+        <li>A creates a group with B, ..., H, as well as some members outside this subtree</li>
+        <li>F sends an empty Commit, setting Y and its ancestors</li>
+        <li>
+          <t>D removes B and C, with the following effects:
+          </t>
+          <ul spacing="normal">
+            <li>Blank the direct paths of B and C</li>
+            <li>Set X, the top node, and any further nodes in the direct path of D</li>
+          </ul>
+        </li>
+        <li>Someone outside this subtree removes G, blanking the direct path of G</li>
+        <li>A adds a new member at B with a partial Commit, adding B as unmerged at X</li>
+      </ul>
+      <t>To construct the tree in <xref target="evolution-tree"/>:</t>
+      <ul spacing="normal">
+        <li>A creates a group with B, C, and D</li>
+        <li>B sends a full Commit, setting X and Y</li>
+        <li>D removes C, setting Z and Y</li>
+        <li>
+          <t>B adds a new member at C with a full Commit
+          </t>
+          <ul spacing="normal">
+            <li>The Add proposal adds C as unmerged at Z and Y</li>
+            <li>The path in the Commit resets X and Y, clearing Y's unmerged leaves</li>
+          </ul>
+        </li>
+      </ul>
+      <t>To construct the tree in <xref target="parent-hash-tree"/>:</t>
+      <ul spacing="normal">
+        <li>A creates a group with B, ..., G</li>
+        <li>A removes F in a full Commit, setting T, U, and W</li>
+        <li>E sends an empty Commit, setting Y and W</li>
+        <li>A adds a new member at F in a partial Commit, adding F as unmerged at Y and W</li>
+      </ul>
+    </section>
+    <section anchor="ph-evolution">
+      <name>Evolution of Parent Hashes</name>
+      <t>To better understand how parent hashes are maintained, let's look in detail at
+how they evolve in a small group.  Consider the following sequence of
+operations:</t>
+      <ol spacing="normal" type="1">
+        <li>A initializes a new group</li>
+        <li>A adds B to the group with a full Commit</li>
+        <li>B adds C and D to the group with a full Commit</li>
+        <li>C sends an empty Commit</li>
+      </ol>
+      <figure>
+        <name>Building a Four-Member Tree to Illustrate Parent Hashes</name>
+        <artset>
+          <artwork type="svg">
+            <svg xmlns="http://www.w3.org/2000/svg" class="diagram" font-family="monospace" font-size="13px" height="144" text-anchor="middle" version="1.1" viewBox="0 0 432 144" width="432">
+              <path d="M 216,48 L 216,64" fill="none" stroke="black"/>
+              <path d="M 376,48 L 376,64" fill="none" stroke="black"/>
+              <path d="M 200,64 L 232,64" fill="none" stroke="black"/>
+              <path d="M 360,64 L 392,64" fill="none" stroke="black"/>
+              <path d="M 32,78 L 48,78" fill="none" stroke="black"/>
+              <path d="M 32,82 L 48,82" fill="none" stroke="black"/>
+              <path d="M 128,78 L 144,78" fill="none" stroke="black"/>
+              <path d="M 128,82 L 144,82" fill="none" stroke="black"/>
+              <path d="M 288,78 L 304,78" fill="none" stroke="black"/>
+              <path d="M 288,82 L 304,82" fill="none" stroke="black"/>
+              <path d="M 232,64 L 240,80" fill="none" stroke="black"/>
+              <path d="M 252,104 L 256,112" fill="none" stroke="black"/>
+              <path d="M 392,64 L 400,80" fill="none" stroke="black"/>
+              <path d="M 192,80 L 200,64" fill="none" stroke="black"/>
+              <path d="M 240,112 L 244,104" fill="none" stroke="black"/>
+              <path d="M 352,80 L 360,64" fill="none" stroke="black"/>
+              <polygon class="arrowhead" fill="black" points="312,80 300,74.4 300,85.6" transform="rotate(0,304,80)"/>
+              <polygon class="arrowhead" fill="black" points="152,80 140,74.4 140,85.6" transform="rotate(0,144,80)"/>
+              <polygon class="arrowhead" fill="black" points="56,80 44,74.4 44,85.6" transform="rotate(0,48,80)"/>
+              <g class="text">
+                <text x="216" y="36">Y</text>
+                <text x="380" y="36">Y'</text>
+                <text x="88" y="100">X</text>
+                <text x="188" y="100">X'</text>
+                <text x="256" y="100">_=Z</text>
+                <text x="348" y="100">X'</text>
+                <text x="412" y="100">Z'</text>
+                <text x="80" y="116">/</text>
+                <text x="96" y="116">\</text>
+                <text x="176" y="116">/</text>
+                <text x="192" y="116">\</text>
+                <text x="336" y="116">/</text>
+                <text x="352" y="116">\</text>
+                <text x="400" y="116">/</text>
+                <text x="416" y="116">\</text>
+                <text x="8" y="132">A</text>
+                <text x="72" y="132">A</text>
+                <text x="104" y="132">B</text>
+                <text x="168" y="132">A</text>
+                <text x="200" y="132">B</text>
+                <text x="232" y="132">C</text>
+                <text x="264" y="132">D</text>
+                <text x="328" y="132">A</text>
+                <text x="360" y="132">B</text>
+                <text x="392" y="132">C</text>
+                <text x="424" y="132">D</text>
+              </g>
+            </svg>
+          </artwork>
+          <artwork type="ascii-art">
+                          Y                   Y'
+                          |                   |
+                        .-+-.               .-+-.
+   ==&gt;         ==&gt;     /     \     ==&gt;     /     \
+          X           X'      _=Z         X'      Z'
+         / \         / \     / \         / \     / \
+A       A   B       A   B   C   D       A   B   C   D
+</artwork>
+        </artset>
+      </figure>
+      <t>Then the parent hashes associated to the nodes will be updated as follows (where
+we use the shorthand <tt>ph</tt> for parent hash, <tt>th</tt> for tree hash, and <tt>osth</tt> for
+original sibling tree hash):</t>
+      <ol spacing="normal" type="1">
+        <li>
+          <t>A adds B: set X  </t>
+          <ul spacing="normal">
+            <li>
+              <tt>A.parent_hash = ph(X) = H(X, ph="", osth=th(B))</tt>
+            </li>
+          </ul>
+        </li>
+        <li>
+          <t>B adds C, D: set B', X', and Y  </t>
+          <ul spacing="normal">
+            <li><tt>X'.parent_hash = ph(Y)  = H(Y, ph="", osth=th(Z))</tt>,
+where <tt>th(Z)</tt> covers <tt>(C, _, D)</tt></li>
+            <li>
+              <tt>B'.parent_hash = ph(X') = H(X', ph=X'.parent_hash, osth=th(A))</tt>
+            </li>
+          </ul>
+        </li>
+        <li>
+          <t>C sends empty Commit: set C', Z', Y'  </t>
+          <ul spacing="normal">
+            <li><tt>Z'.parent_hash = ph(Y') = H(Y', ph="", osth=th(X'))</tt>, where
+<tt>th(X')</tt> covers <tt>(A, X', B')</tt></li>
+            <li>
+              <tt>C'.parent_hash = ph(Z') = H(Z', ph=Z'.parent_hash, osth=th(D))</tt>
+            </li>
+          </ul>
+        </li>
+      </ol>
+      <t>When a new member joins, they will receive a tree that has the following parent
+hash values and compute the indicated parent hash validity relationships:</t>
+      <table>
+        <thead>
+          <tr>
+            <th align="left">Node</th>
+            <th align="left">Parent Hash Value</th>
+            <th align="left">Valid?</th>
+          </tr>
+        </thead>
+        <tbody>
+          <tr>
+            <td align="left">A</td>
+            <td align="left">H(X, ph="", osth=th(B))</td>
+            <td align="left">No, B changed</td>
+          </tr>
+          <tr>
+            <td align="left">B'</td>
+            <td align="left">H(X', ph=X'.parent_hash, osth=th(A))</td>
+            <td align="left">Yes</td>
+          </tr>
+          <tr>
+            <td align="left">C'</td>
+            <td align="left">H(Z', ph=Z'.parent_hash, osth=th(D))</td>
+            <td align="left">Yes</td>
+          </tr>
+          <tr>
+            <td align="left">D</td>
+            <td align="left">(none, never sent an UpdatePath)</td>
+            <td align="left">N/A</td>
+          </tr>
+          <tr>
+            <td align="left">X'</td>
+            <td align="left">H(Y, ph="", osth=th(Z))</td>
+            <td align="left">No, Y and Z changed</td>
+          </tr>
+          <tr>
+            <td align="left">Z'</td>
+            <td align="left">H(Y', ph="", osth=th(X'))</td>
+            <td align="left">Yes</td>
+          </tr>
+        </tbody>
+      </table>
+      <t>In other words, the joiner will find the following path-hash links in the tree:</t>
+      <figure>
+        <name>Parent-hash links connect all non-empty parent nodes to leaves</name>
+        <artset>
+          <artwork type="svg">
+            <svg xmlns="http://www.w3.org/2000/svg" class="diagram" font-family="monospace" font-size="13px" height="160" text-anchor="middle" version="1.1" viewBox="0 0 112 160" width="112">
+              <path d="M 56,48 L 56,64" fill="none" stroke="black"/>
+              <path d="M 56,64 L 72,64" fill="none" stroke="black"/>
+              <path d="M 72,64 L 80,80" fill="none" stroke="black"/>
+              <g class="text">
+                <text x="60" y="36">Y'</text>
+                <text x="28" y="100">X'</text>
+                <text x="92" y="100">Z'</text>
+                <text x="32" y="116">\</text>
+                <text x="80" y="116">/</text>
+                <text x="8" y="132">A</text>
+                <text x="44" y="132">B'</text>
+                <text x="76" y="132">C'</text>
+                <text x="104" y="132">D</text>
+              </g>
+            </svg>
+          </artwork>
+          <artwork type="ascii-art">
+       Y'
+       |
+       +-.
+          \
+   X'      Z'
+    \     /
+ A   B'  C'  D
+</artwork>
+        </artset>
+      </figure>
+      <t>Since these chains collectively cover all non-blank parent nodes in the tree,
+the tree is parent-hash valid.</t>
+      <t>Note that this tree, though valid, contains invalid parent-hash links. If a
+client were checking parent hashes top-down from Y', for example, they would
+find that X' has an invalid parent hash relative to Y', but that Z' has a valid
+parent hash.  Likewise, if the client were checking bottom-up, they would find
+that the chain from B' ends in an invalid link from X' to Y'.  These invalid
+links are the natural result of multiple clients having committed.</t>
+      <t>Note also the way the tree hash and the parent hash interact.  The parent hash
+of node C' includes the tree hash of node D.  The parent hash of node Z'
+includes the tree hash of X', which covers nodes A and B' (including the parent
+hash of B').  Although the tree hash and the parent hash depend on each other,
+the dependency relationships are structured so that there is never a circular
+dependency.</t>
+      <t>In the particular case where a new member first receives the tree for a group
+(e.g., in a ratchet tree GroupInfo extension <xref target="ratchet-tree-extension"/>), the
+parent hashes will be expressed in the tree representation, but the tree hash
+need not be.  Instead, the new member will recompute the tree hashes for all the
+nodes in the tree, verifying that this matches the tree hash in the GroupInfo
+object.  If the tree is valid, then the subtree hashes computed in this
+way will align with the inputs needed for parent hash validation (except where
+recomputation is needed to account for unmerged leaves).</t>
+    </section>
+    <section anchor="array-based-trees">
+      <name>Array-Based Trees</name>
+      <t>One benefit of using complete balanced trees is that they admit a simple
+flat array representation.  In this representation, leaf nodes are
+even-numbered nodes, with the <tt>n</tt>-th leaf at <tt>2*n</tt>.  Intermediate nodes
+are held in odd-numbered nodes.  For example, the tree with 8 leaves has
+the following structure:</t>
+      <figure>
+        <name>An Eight-Member Tree Represented as an Array</name>
+        <artset>
+          <artwork type="svg">
+            <svg xmlns="http://www.w3.org/2000/svg" class="diagram" font-family="monospace" font-size="13px" height="288" text-anchor="middle" version="1.1" viewBox="0 0 400 288" width="400">
+              <path d="M 128,112 L 128,128" fill="none" stroke="black"/>
+              <path d="M 224,48 L 224,64" fill="none" stroke="black"/>
+              <path d="M 320,112 L 320,128" fill="none" stroke="black"/>
+              <path d="M 144,64 L 304,64" fill="none" stroke="black"/>
+              <path d="M 96,128 L 160,128" fill="none" stroke="black"/>
+              <path d="M 288,128 L 352,128" fill="none" stroke="black"/>
+              <path d="M 88,176 L 96,192" fill="none" stroke="black"/>
+              <path d="M 160,128 L 168,144" fill="none" stroke="black"/>
+              <path d="M 184,176 L 192,192" fill="none" stroke="black"/>
+              <path d="M 280,176 L 288,192" fill="none" stroke="black"/>
+              <path d="M 304,64 L 312,80" fill="none" stroke="black"/>
+              <path d="M 352,128 L 360,144" fill="none" stroke="black"/>
+              <path d="M 376,176 L 384,192" fill="none" stroke="black"/>
+              <path d="M 64,192 L 72,176" fill="none" stroke="black"/>
+              <path d="M 88,144 L 96,128" fill="none" stroke="black"/>
+              <path d="M 136,80 L 144,64" fill="none" stroke="black"/>
+              <path d="M 160,192 L 168,176" fill="none" stroke="black"/>
+              <path d="M 256,192 L 264,176" fill="none" stroke="black"/>
+              <path d="M 280,144 L 288,128" fill="none" stroke="black"/>
+              <path d="M 352,192 L 360,176" fill="none" stroke="black"/>
+              <g class="text">
+                <text x="224" y="36">X</text>
+                <text x="128" y="100">X</text>
+                <text x="320" y="100">X</text>
+                <text x="80" y="164">X</text>
+                <text x="176" y="164">X</text>
+                <text x="272" y="164">X</text>
+                <text x="368" y="164">X</text>
+                <text x="56" y="212">X</text>
+                <text x="104" y="212">X</text>
+                <text x="152" y="212">X</text>
+                <text x="200" y="212">X</text>
+                <text x="248" y="212">X</text>
+                <text x="296" y="212">X</text>
+                <text x="344" y="212">X</text>
+                <text x="392" y="212">X</text>
+                <text x="24" y="244">Node:</text>
+                <text x="56" y="244">0</text>
+                <text x="80" y="244">1</text>
+                <text x="104" y="244">2</text>
+                <text x="128" y="244">3</text>
+                <text x="152" y="244">4</text>
+                <text x="176" y="244">5</text>
+                <text x="200" y="244">6</text>
+                <text x="224" y="244">7</text>
+                <text x="248" y="244">8</text>
+                <text x="272" y="244">9</text>
+                <text x="292" y="244">10</text>
+                <text x="316" y="244">11</text>
+                <text x="340" y="244">12</text>
+                <text x="364" y="244">13</text>
+                <text x="388" y="244">14</text>
+                <text x="24" y="276">Leaf:</text>
+                <text x="56" y="276">0</text>
+                <text x="104" y="276">1</text>
+                <text x="152" y="276">2</text>
+                <text x="200" y="276">3</text>
+                <text x="248" y="276">4</text>
+                <text x="296" y="276">5</text>
+                <text x="344" y="276">6</text>
+                <text x="392" y="276">7</text>
+              </g>
+            </svg>
+          </artwork>
+          <artwork type="ascii-art">
+                           X
+                           |
+                 .---------+---------.
+                /                     \
+               X                       X
+               |                       |
+           .---+---.               .---+---.
+          /         \             /         \
+         X           X           X           X
+        / \         / \         / \         / \
+       /   \       /   \       /   \       /   \
+      X     X     X     X     X     X     X     X
+
+Node: 0  1  2  3  4  5  6  7  8  9 10 11 12 13 14
+
+Leaf: 0     1     2     3     4     5     6     7
+</artwork>
+        </artset>
+      </figure>
+      <t>This allows us to compute relationships between tree nodes simply by
+manipulating indices, rather than having to maintain complicated structures in
+memory. The basic rule is that the high-order bits of parent and child nodes
+indices have the following relation (where <tt>x</tt> is an arbitrary bit string):</t>
+      <sourcecode type="pseudocode">
+parent=01x =&gt; left=00x, right=10x
+</sourcecode>
+      <t>Since node relationships are implicit, the algorithms for adding and removing
+nodes at the right edge of the tree are quite simple.  If there are <tt>N</tt> nodes in
+the array:</t>
+      <ul spacing="normal">
+        <li>Add: Append <tt>N + 1</tt> blank values to the end of the array.</li>
+        <li>Remove: Truncate the array to its first <tt>(N-1) / 2</tt> entries.</li>
+      </ul>
+      <t>The following python code demonstrates the tree computations necessary to use an
+array-based tree for MLS.</t>
+      <sourcecode type="python">
+# The exponent of the largest power of 2 less than x. Equivalent to:
+#   int(math.floor(math.log(x, 2)))
+def log2(x):
+    if x == 0:
+        return 0
+
+    k = 0
+    while (x &gt;&gt; k) &gt; 0:
+        k += 1
+    return k-1
+
+# The level of a node in the tree. Leaves are level 0, their parents
+# are level 1, etc. If a node's children are at different levels,
+# then its level is the max level of its children plus one.
+def level(x):
+    if x &amp; 0x01 == 0:
+        return 0
+
+    k = 0
+    while ((x &gt;&gt; k) &amp; 0x01) == 1:
+        k += 1
+    return k
+
+# The number of nodes needed to represent a tree with n leaves.
+def node_width(n):
+    if n == 0:
+        return 0
+    else:
+        return 2*(n - 1) + 1
+
+# The index of the root node of a tree with n leaves.
+def root(n):
+    w = node_width(n)
+    return (1 &lt;&lt; log2(w)) - 1
+
+# The left child of an intermediate node.
+def left(x):
+    k = level(x)
+    if k == 0:
+        raise Exception('leaf node has no children')
+
+    return x ^ (0x01 &lt;&lt; (k - 1))
+
+# The right child of an intermediate node.
+def right(x):
+    k = level(x)
+    if k == 0:
+        raise Exception('leaf node has no children')
+
+    return x ^ (0x03 &lt;&lt; (k - 1))
+
+# The parent of a node.
+def parent(x, n):
+    if x == root(n):
+        raise Exception('root node has no parent')
+
+    k = level(x)
+    b = (x &gt;&gt; (k + 1)) &amp; 0x01
+    return (x | (1 &lt;&lt; k)) ^ (b &lt;&lt; (k + 1))
+
+# The other child of the node's parent.
+def sibling(x, n):
+    p = parent(x, n)
+    if x &lt; p:
+        return right(p)
+    else:
+        return left(p)
+
+# The direct path of a node, ordered from leaf to root.
+def direct_path(x, n):
+    r = root(n)
+    if x == r:
+        return []
+
+    d = []
+    while x != r:
+        x = parent(x, n)
+        d.append(x)
+    return d
+
+# The copath of a node, ordered from leaf to root.
+def copath(x, n):
+    if x == root(n):
+        return []
+
+    d = direct_path(x, n)
+    d.insert(0, x)
+    d.pop()
+    return [sibling(y, n) for y in d]
+
+# The common ancestor of two nodes is the lowest node that is in the
+# direct paths of both leaves.
+def common_ancestor_semantic(x, y, n):
+    dx = set([x]) | set(direct_path(x, n))
+    dy = set([y]) | set(direct_path(y, n))
+    dxy = dx &amp; dy
+    if len(dxy) == 0:
+        raise Exception('failed to find common ancestor')
+
+    return min(dxy, key=level)
+
+# The common ancestor of two nodes is the lowest node that is in the
+# direct paths of both leaves.
+def common_ancestor_direct(x, y, _):
+    # Handle cases where one is an ancestor of the other
+    lx, ly = level(x)+1, level(y)+1
+    if (lx &lt;= ly) and (x&gt;&gt;ly == y&gt;&gt;ly):
+      return y
+    elif (ly &lt;= lx) and (x&gt;&gt;lx == y&gt;&gt;lx):
+      return x
+
+    # Handle other cases
+    xn, yn = x, y
+    k = 0
+    while xn != yn:
+       xn, yn = xn &gt;&gt; 1, yn &gt;&gt; 1
+       k += 1
+    return (xn &lt;&lt; k) + (1 &lt;&lt; (k-1)) - 1
+</sourcecode>
+    </section>
+    <section anchor="link-based-trees">
+      <name>Link-Based Trees</name>
+      <t>An implementation may choose to store ratchet trees in a "link-based"
+representation, where each node stores references to its parents and/or
+children (as opposed to the array-based representation suggested above, where
+these relationships are computed from relationships between nodes' indices in
+the array). Such an implementation needs to update these links to maintain the
+balanced structure of the tree as the tree is extended to add new members
+or truncated when members are removed.</t>
+      <t>The following code snippet shows how these algorithms could be implemented in
+Python.</t>
+      <sourcecode type="python">
+class Node:
+    def __init__(self, value, left=None, right=None):
+        self.value = value    # Value of the node
+        self.left = left      # Left child node
+        self.right = right    # Right child node
+
+    @staticmethod
+    def blank_subtree(depth):
+        if depth == 1:
+            return Node(None)
+
+        L = Node.blank_subtree(depth-1)
+        R = Node.blank_subtree(depth-1)
+        return Node(None, left=L, right=R)
+
+    def empty(self):
+        L_empty = (self.left == None) or self.left.empty()
+        R_empty = (self.right == None) or self.right.empty()
+        return (self.value == None) and L_empty and R_empty
+
+class Tree:
+    def __init__(self):
+        self.depth = 0    # Depth of the tree
+        self.root = None  # Root node of the tree, initially empty
+
+    # Add a blank subtree to the right
+    def extend(self):
+        if self.depth == 0:
+            self.depth = 1
+            self.root = Node(None)
+
+        L = self.root
+        R = Node.blank_subtree(self.depth)
+        self.root = Node(None, left=L, right=R)
+        self.depth += 1
+
+    # Truncate the right subtree
+    def truncate(self):
+        if self.root == None:
+            return
+
+        if not self.root.right.empty():
+            raise Exception("Cannot truncate non-blank subtree")
+
+        self.depth -= 1
+        self.root = self.root.left
+</sourcecode>
+    </section>
+    <section anchor="contributors" numbered="false" removeInRFC="false" toc="include">
+      <name>Contributors</name>
+      <contact fullname="Joel Alwen">
+        <organization>Amazon</organization>
+        <address>
+          <email>alwenjo@amazon.com</email>
+        </address>
+      </contact>
+      <contact fullname="Karthikeyan Bhargavan">
+        <organization>Inria</organization>
+        <address>
+          <email>karthikeyan.bhargavan@inria.fr</email>
+        </address>
+      </contact>
+      <contact fullname="Cas Cremers">
+        <organization>CISPA</organization>
+        <address>
+          <email>cremers@cispa.de</email>
+        </address>
+      </contact>
+      <contact fullname="Alan Duric">
+        <organization>Wire</organization>
+        <address>
+          <email>alan@wire.com</email>
+        </address>
+      </contact>
+      <contact fullname="Britta Hale">
+        <organization>Naval Postgraduate School</organization>
+        <address>
+          <email>britta.hale@nps.edu</email>
+        </address>
+      </contact>
+      <contact fullname="Srinivas Inguva">
+        <organization/>
+        <address>
+          <email>singuva@yahoo.com</email>
+        </address>
+      </contact>
+      <contact fullname="Konrad Kohbrok">
+        <organization>Phoenix R&amp;D</organization>
+        <address>
+          <email>konrad.kohbrok@datashrine.de</email>
+        </address>
+      </contact>
+      <contact fullname="Albert Kwon">
+        <organization>MIT</organization>
+        <address>
+          <email>kwonal@mit.edu</email>
+        </address>
+      </contact>
+      <contact fullname="Tom Leavy">
+        <organization>Amazon</organization>
+        <address>
+          <email>tomleavy@amazon.com</email>
+        </address>
+      </contact>
+      <contact fullname="Brendan McMillion">
+        <organization/>
+        <address>
+          <email>brendanmcmillion@gmail.com</email>
+        </address>
+      </contact>
+      <contact fullname="Marta Mularczyk">
+        <organization>Amazon</organization>
+        <address>
+          <email>mulmarta@amazon.com</email>
+        </address>
+      </contact>
+      <contact fullname="Eric Rescorla">
+        <organization>Mozilla</organization>
+        <address>
+          <email>ekr@rtfm.com</email>
+        </address>
+      </contact>
+      <contact fullname="Michael Rosenberg">
+        <organization>Trail of Bits</organization>
+        <address>
+          <email>michael.rosenberg@trailofbits.com</email>
+        </address>
+      </contact>
+      <contact fullname="Th&#xE9;ophile Wallez">
+        <organization>Inria</organization>
+        <address>
+          <email>theophile.wallez@inria.fr</email>
+        </address>
+      </contact>
+      <contact fullname="Thyla van der Merwe">
+        <organization>Royal Holloway, University of London</organization>
+        <address>
+          <email>tjvdmerwe@gmail.com</email>
+        </address>
+      </contact>
+    </section>
+  </back>
+</rfc>
+
diff --git a/auth48/text-clean.py b/auth48/text-clean.py
new file mode 100644
index 00000000..1ed4be76
--- /dev/null
+++ b/auth48/text-clean.py
@@ -0,0 +1,18 @@
+#!/usr/bin/env python
+
+import sys
+import re
+import xml.etree.ElementTree as ET
+
+file = sys.stdin.read()
+
+# Tweak XML namespace expressions 
+file = re.sub(r'xmlns:ns0=', 'xmlns=', file)
+file = re.sub(r'<ns0:', '<', file)
+file = re.sub(r'</ns0:', '</', file)
+
+# Make sure there are spaces around <bcp14> tags
+file = re.sub(r'><bcp14>', '> <bcp14>', file)
+file = re.sub(r'></bcp14>', '> </bcp14>', file)
+
+print(file)
diff --git a/auth48/xml-clean.py b/auth48/xml-clean.py
new file mode 100644
index 00000000..ce5165c9
--- /dev/null
+++ b/auth48/xml-clean.py
@@ -0,0 +1,40 @@
+#!/usr/bin/env python
+
+import sys
+import re
+import xml.etree.ElementTree as ET
+
+file = sys.stdin.read()
+root = ET.fromstring(file)
+
+# Strip "removeInRFC" notes
+for parent in root.findall(".//note[@removeInRFC='true']/.."):
+    for child in parent.findall("./note[@removeInRFC='true']"):
+        parent.remove(child)
+        parent.tail = None
+
+# Remove <t> internal to <dd>
+for dd in root.findall(".//dd/t/.."):
+    t = dd.find("./t")
+    dd.remove(t)
+
+    dd.text = t.text
+    for sub in t:
+        t.remove(sub)
+        dd.append(sub)
+    dd.tail = t.tail
+
+# Convert all-whitespace text/tail to None
+for node in root.findall(".//*"):
+    if node.text and re.match(r'^\s+$', node.text):
+        node.text = None
+    if node.tail and re.match(r'^\s+$', node.tail):
+        node.tail = None
+
+# Remove trailing whitespace from SVG polygons 
+for node in root.findall(".//{http://www.w3.org/2000/svg}polygon"):
+    if node.attrib['points']:
+        node.attrib['points'] = re.sub(r'\s*$', '', node.attrib['points'])
+
+xml = ET.tostring(root, encoding='unicode', method='xml')
+print(xml)
diff --git a/draft-ietf-mls-protocol.md b/rfc9420.md
similarity index 82%
rename from draft-ietf-mls-protocol.md
rename to rfc9420.md
index a4c80c84..0d15a3ae 100644
--- a/draft-ietf-mls-protocol.md
+++ b/rfc9420.md
@@ -5,8 +5,12 @@ docname: draft-ietf-mls-protocol-latest
 category: std
 
 ipr: trust200902
-area: Security
-keyword: Internet-Draft
+area: sec
+keyword:
+  - security
+  - authenticated key exchange
+  - end-to-end encryption
+workgroup: mls
 
 stand_alone: yes
 pi: [toc, sortrefs, symrefs]
@@ -34,8 +38,7 @@ author:
  -
     ins: E. Omara
     name: Emad Omara
-    organization: Google
-    email: emadomara@google.com
+    email: emad.omara@gmail.com
  -
     ins: K. Cohn-Gordon
     name: Katriel Cohn-Gordon
@@ -141,22 +144,14 @@ challenging for group chat settings, in which more than two
 clients need to agree on a key but may not be online at the same
 time.  In this document, we specify a key establishment
 protocol that provides efficient asynchronous group key establishment
-with forward secrecy and post-compromise security for groups
+with forward secrecy (FS) and post-compromise security (PCS) for groups
 in size ranging from two to thousands.
 
 
 --- middle
 
-
 # Introduction
 
-RFC EDITOR: PLEASE REMOVE THE FOLLOWING PARAGRAPH The source for
-this draft is maintained in GitHub. Suggested changes should be
-submitted as pull requests at https://github.com/mlswg/mls-protocol.
-Instructions are on that page as well. Editorial changes can be
-managed in GitHub, but any substantive change should be discussed on
-the MLS mailing list.
-
 A group of users who want to send each other encrypted messages needs
 a way to derive shared symmetric encryption keys. For two parties,
 this problem has been studied thoroughly, with the Double Ratchet
@@ -182,418 +177,16 @@ computation and communications resources that scale linearly with
 the size of the group.
 
 In this document, we describe a protocol based on tree structures
-that enable asynchronous group keying with forward secrecy and
+that enables asynchronous group keying with forward secrecy and
 post-compromise security.  Based on earlier work on "asynchronous
 ratcheting trees" {{ART}}, the protocol presented here uses an
 asynchronous key-encapsulation mechanism for tree structures.
 This mechanism allows the members of the group to derive and update
 shared keys with costs that scale as the log of the group size.
 
-##  Change Log
-
-RFC EDITOR PLEASE DELETE THIS SECTION.
-
-draft-18
-
-- Make the document standards track
-- Make the ratchet tree non-malleable (\*)
-- Use ExpandWithLabel to derive welcome key (\*)
-- Change MLS-Exporter label from "exporter" to "exported" (\*)
-- Loosen chain requirements (\*)
-- Clarify transcript hash initialization
-- GREASE for MLS registries
-- Move pseudocode out of KDFLabel definition.
-- Rename PrivateContentTBE to PrivateMessageContent
-- Fix DecryptWithLabel argument order for Welcome
-- Responses to IESG reviews
-- Describe varint length check more clearly
-
-draft-17
-
-- Rename MLSCiphertext and MLSPlaintext to PrivateMessage and PublicMesssage respectively (\*)
-- Add label and context to public-key encryption (\*)
-- Include leaf index in LeafNodeTBS for better parent-hash guarantees (\*)
-- Make ProtocolVersion two bytes (\*)
-- Clarify group creation (\*)
-- Validate additional properties of unmerged leaves (\*)
-- Clarify that the AS needs to vet the signature key
-- Remove "MLS" prefix on structs
-- Credentials should be replaced before expiring
-- Add a section discussing the security of the sender data protection
-- Minor fixes in presentation language.
-- Allow multiple welcomes per commit
-- Remove reference to BasicCredential.
-- Client aware of its own removal in group
-- Create IANA registries for signature and export labels
-- Complete IANA media type registration
-- Make more vendor code points available
-- Update Recommended column definition to match 8447bis
-- Responses to early ARTART review
-- Responses to early OPSDIR review
-- Responses to early TSV-ART review
-
-draft-16
-
-- Fix GroupInfoTBS (\*)
-- Make reference to h2 informative
-
-draft-15
-
-- Include ciphersuite in group context (\*)
-
-- Add new new_proposal_member SenderType (\*)
-
-- Always use a full tree (\*)
-
-- Change KeyPackage identifier extension to be LeafNode identifier (\*)
-
-- Use new tree for context in path secret encryption (\*)
-
-- Use a hash function for hash identifiers (\*)
-
-- Add a marker byte to tree hash input structs (\*)
-
-- Recommend that group ids are generated randomly (\*)
-
-- Update external senders extension to have SignaturePublicKey and Credential (\*)
-
-- Replace LeafNodeRef with leaf index (\*)
-
-- Remove AppAck proposal (\*)
-
-- Make padding arbitrary-size and all-zero (\*)
-
-- Require that unmerged_leaves be ordered
-
-- Derive the commit secret from the end of the UpdatePath, not the root
-
-- Specify the precise points in the protocol where credential validation must be done
-
-- Make PSK provisions more uniform, e.g., always generating a fresh random nonce
-
-- Improve parent hash guarantees with stricter checks on tree correctness
-
-- Streamline some structs, e.g., folding GroupContext into GroupInfo
-
-- Provide clearer rules for validating and applying commits
-
-- Clarify tree hash and parent hash, and correct examples
-
-- Clean up struct names and references to outdated structs
-
-- Cite AEAD limits draft
-
-draft-14
-
-- Ensure that a signature public key is always intelligible (\*)
-
-- Clean up terminology of derived secrets/keys
-
-- Fix parent hash (\*)
-
-- Specify compatibility behavior around new credentials
-
-- Add Path Required to Proposal Type template
-
-- Sub-group branching requires fresh key packages for each member
-
-- Use `aasvg` and typed code blocks
-
-- Require init key and leaf key to be different
-
-- Preconfigured senders extension and removal of signature key indirection
-
-draft-13
-
-- TLS syntax updates (including variable-header-length vectors) (\*)
-
-- Stop generating redundant PKE key pairs. (\*)
-
-- Move validation of identity change to the AS
-
-- Add message/mls MIME type registration
-
-- Split LeafNode from KeyPackage (\*)
-
-- Remove endpoint_id (\*)
-
-- Reorganize to make section layout more sane
-
-- Forbid proposals by reference in external commits (\*)
-
-- Domain separation for KeyPackage and Proposal references (\*)
-
-- Downgrade MUST to SHOULD for commit senders including all valid commits
-
-- Stronger parent hashes for authenticated identities (\*)
-
-- Move wire_format to a separate tagged-union structure MLSMessage
-
-- Generalize tree extend/truncate algorithms
-
-- Add algorithms for link-based trees
-
-- Forbid self-Update entirely (\*)
-
-- Consolidate resumption PSK cases (\*)
-
-- 384 Ciphersuite Addition
-
-- Remove explicit version pin on HPKE (\*)
-
-- Remove the requirement for Add in external commit (\*)
-
-- Use smaller, fixed-size hash-based identifiers (\*)
-
-- Be explicit that Credentials can attest to multiple identities (\*)
-
-draft-12
-
-- Use the GroupContext to derive the joiner_secret (\*)
-
-- Make PreSharedKeys non optional in GroupSecrets (\*)
-
-- Update name for this particular key (\*)
-
-- Truncate tree size on removal (\*)
-
-- Use HPKE draft-08 (\*)
-
-- Clarify requirements around identity in MLS groups (\*)
-
-- Signal the intended wire format for MLS messages (\*)
-
-- Inject GroupContext as HPKE info instead of AAD (\*)
-
-- Clarify extension handling and make extension updatable (\*)
-
-- Improve extensibility of Proposals (\*)
-
-- Constrain proposal in External Commit (\*)
-
-- Remove the notion of a 'leaf index' (\*)
-
-- Add group_context_extensions proposal ID (\*)
-
-- Add RequiredCapabilities extension (\*)
-
-- Use cascaded KDF instead of concatenation to consolidate PSKs (\*)
-
-- Use key package hash to index clients in message structs (\*)
-
-- Don't require PublicGroupState for external init (\*)
-
-- Make ratchet tree section clearer.
-
-- Handle non-member sender cases in MLSPlaintextTBS
-
-- Clarify encoding of signatures with NIST curves
-
-- Remove OPEN ISSUEs and TODOs
-
-- Normalize the description of the zero vector
-
-draft-11
-
-- Include subtree keys in parent hash (\*)
-
-- Pin HPKE to draft-07 (\*)
-
-- Move joiner secret to the end of the first key schedule epoch (\*)
-
-- Add an AppAck proposal
-
-- Make initializations of transcript hashes consistent
-
-draft-10
-
-- Allow new members to join via an external Commit (\*)
-
-- Enable proposals to be sent inline in a Commit (\*)
-
-- Re-enable constant-time Add (\*)
-
-- Change expiration extension to lifetime extension (\*)
-
-- Make the tree in the Welcome optional (\*)
-
-- PSK injection, re-init, sub-group branching (\*)
-
-- Require the initial init_secret to be a random value (\*)
-
-- Remove explicit sender data nonce (\*)
-
-- Do not encrypt to joiners in UpdatePath generation (\*)
-
-- Move MLSPlaintext signature under the confirmation tag (\*)
-
-- Explicitly authenticate group membership with MLSPLaintext (\*)
-
-- Clarify X509Credential structure (\*)
-
-- Remove unneeded interim transcript hash from GroupInfo (\*)
-
-- IANA considerations
-
-- Derive an authentication secret
-
-- Use Extract/Expand from HPKE KDF
-
-- Clarify that application messages MUST be encrypted
-
-draft-09
-
-- Remove blanking of nodes on Add (\*)
-
-- Change epoch numbers to uint64 (\*)
-
-- Add PSK inputs (\*)
-
-- Add key schedule exporter (\*)
-
-- Sign the updated direct path on Commit, using "parent hashes" and one
-  signature per leaf (\*)
-
-- Use structured types for external senders (\*)
-
-- Redesign Welcome to include confirmation and use derived keys (\*)
-
-- Remove ignored proposals (\*)
-
-- Always include an Update with a Commit (\*)
-
-- Add per-message entropy to guard against nonce reuse (\*)
-
-- Use the same hash ratchet construct for both application and handshake keys (\*)
-
-- Add more ciphersuites
-
-- Use HKDF to derive key pairs (\*)
-
-- Mandate expiration of ClientInitKeys (\*)
-
-- Add extensions to GroupContext and flesh out the extensibility story (\*)
-
-- Rename ClientInitKey to KeyPackage
-
-draft-08
-
-- Change ClientInitKeys so that they only refer to one ciphersuite (\*)
-
-- Decompose group operations into Proposals and Commits (\*)
-
-- Enable Add and Remove proposals from outside the group (\*)
-
-- Replace Init messages with multi-recipient Welcome message (\*)
-
-- Add extensions to ClientInitKeys for expiration and downgrade resistance (\*)
-
-- Allow multiple Proposals and a single Commit in one MLSPlaintext (\*)
-
-draft-07
-
-- Initial version of the Tree based Application Key Schedule (\*)
-
-- Initial definition of the Init message for group creation (\*)
-
-- Fix issue with the transcript used for newcomers (\*)
-
-- Clarifications on message framing and HPKE contexts (\*)
-
-draft-06
-
-- Reorder blanking and update in the Remove operation (\*)
-
-- Rename the GroupState structure to GroupContext (\*)
-
-- Rename UserInitKey to ClientInitKey
-
-- Resolve the circular dependency that draft-05 introduced in the
-  confirmation MAC calculation (\*)
-
-- Cover the entire MLSPlaintext in the transcript hash (\*)
-
-draft-05
-
-- Common framing for handshake and application messages (\*)
-
-- Handshake message encryption (\*)
-
-- Convert from literal state to a commitment via the "tree hash" (\*)
-
-- Add credentials to the tree and remove the "roster" concept (\*)
-
-- Remove the secret field from tree node values
-
-draft-04
-
-- Updating the language to be similar to the Architecture document
-
-- ECIES is now renamed in favor of HPKE (\*)
-
-- Using a KDF instead of a Hash in TreeKEM (\*)
-
-draft-03
-
-- Added ciphersuites and signature schemes (\*)
-
-- Re-ordered fields in UserInitKey to make parsing easier (\*)
-
-- Fixed inconsistencies between Welcome and GroupState (\*)
-
-- Added encryption of the Welcome message (\*)
-
-draft-02
-
-- Removed ART (\*)
-
-- Allowed partial trees to avoid double-joins (\*)
-
-- Added explicit key confirmation (\*)
-
-draft-01
-
-- Initial description of the Message Protection mechanism. (\*)
-
-- Initial specification proposal for the Application Key Schedule
-  using the per-participant chaining of the Application Secret design. (\*)
-
-- Initial specification proposal for an encryption mechanism to protect
-  Application Messages using an AEAD scheme. (\*)
-
-- Initial specification proposal for an authentication mechanism
-  of Application Messages using signatures. (\*)
-
-- Initial specification proposal for a padding mechanism to improving
-  protection of Application Messages against traffic analysis. (\*)
-
-- Inversion of the Group Init Add and Application Secret derivations
-  in the Handshake Key Schedule to be ease chaining in case we switch
-  design. (\*)
-
-- Removal of the UserAdd construct and split of GroupAdd into Add
-  and Welcome messages (\*)
-
-- Initial proposal for authenticating handshake messages by signing
-  over group state and including group state in the key schedule (\*)
-
-- Added an appendix with example code for tree math
-
-- Changed the ECIES mechanism used by TreeKEM so that it uses nonces
-  generated from the shared secret
-
-draft-00
-
-- Initial adoption of draft-barnes-mls-protocol-01 as a WG item.
-
-
 # Terminology
 
-The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
-"SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and
-"OPTIONAL" in this document are to be interpreted as described in
-BCP 14 {{!RFC2119}} {{!RFC8174}} when, and only when, they appear in all
-capitals, as shown here.
+{::boilerplate bcp14-tagged}
 
 Client:
 : An agent that uses this protocol to establish shared cryptographic
@@ -610,14 +203,14 @@ Epoch:
   shared cryptographic state.
 
 Member:
-: A client that is included in the shared state of a group, hence
+: A client that is included in the shared state of a group and hence
   has access to the group's secrets.
 
 Key Package:
-: A signed object describing a client's identity and capabilities, and including
-  a hybrid public-key encryption (HPKE {{!RFC9180}}) public key that
-  can be used to encrypt to that client, and which other clients can use to
-  introduce the client to a new group.
+: A signed object describing a client's identity and capabilities, including
+  a hybrid public key encryption (HPKE) {{!RFC9180}} public key that
+  can be used to encrypt to that client. Other clients can use a client's
+  KeyPackage to introduce the client to a new group.
 
 Group Context:
 : An object that summarizes the shared, public state of the group. The group
@@ -640,7 +233,7 @@ PublicMessage:
 coming from a member of the group in a particular epoch, but not encrypted.
 
 PrivateMessage:
-: An MLS protocol message that is both signed by its sender, authenticated as
+: An MLS protocol message that is signed by its sender, authenticated as
 coming from a member of the group in a particular epoch, and encrypted so
 that it is confidential to the members of the group in that epoch.
 
@@ -656,9 +249,10 @@ Terminology specific to tree computations is described in
 
 In general, symmetric values are referred to as "keys" or "secrets"
 interchangeably.  Either term denotes a value that MUST be kept confidential to
-a Client.  When labeling individual values, we typically use "secret" to refer
-to a value that is used derive further secret values, and "key" to refer to a
-value that is used with an algorithm such as HMAC or an AEAD algorithm.
+a client.  When labeling individual values, we typically use "secret" to refer
+to a value that is used to derive further secret values and "key" to refer to a
+value that is used with an algorithm such as Hashed Message Authentication Code
+(HMAC) or an Authenticated Encryption with Associated Data (AEAD) algorithm.
 
 The PublicMessage and PrivateMessage formats are defined in {{message-framing}}.
 Security notions such as forward secrecy and post-compromise
@@ -674,7 +268,7 @@ registries.
 
 We use the TLS presentation language {{!RFC8446}} to describe the structure of
 protocol messages.  In addition to the base syntax, we add two additional
-features, the ability for fields to be optional and the ability for vectors to
+features: the ability for fields to be optional and the ability for vectors to
 have variable-size length headers.
 
 ### Optional Value
@@ -683,7 +277,7 @@ An optional value is encoded with a presence-signaling octet, followed by the
 value itself if present.  When decoding, a presence octet with a value other
 than 0 or 1 MUST be rejected as malformed.
 
-~~~ tls
+~~~ tls-presentation
 struct {
     uint8 present;
     select (present) {
@@ -693,7 +287,7 @@ struct {
 } optional<T>;
 ~~~
 
-### Variable-size Vector Length Headers
+### Variable-Size Vector Length Headers
 
 In the TLS presentation language, vectors are encoded as a sequence of encoded
 elements prefixed with a length.  The length field has a fixed size set by
@@ -701,19 +295,19 @@ specifying the minimum and maximum lengths of the encoded sequence of elements.
 
 In MLS, there are several vectors whose sizes vary over significant ranges.  So
 instead of using a fixed-size length field, we use a variable-size length using
-a variable-length integer encoding based on the one in Section 16 of
-{{?RFC9000}}. They differ only in that the one here requires a minimum-size
+a variable-length integer encoding based on the one described in
+{{Section 16 of ?RFC9000}}. They differ only in that the one here requires a minimum-size
 encoding. Instead of presenting min and max values, the vector description
 simply includes a `V`. For example:
 
-~~~ tls
+~~~ tls-presentation
 struct {
     uint32 fixed<0..255>;
     opaque variable<V>;
 } StructWithVectors;
 ~~~
 
-Such a vector can represent values with length from 0 bytes to 2^30 bytes.
+Such a vector can represent values with length from 0 bytes to 2<sup>30</sup> bytes.
 The variable-length integer encoding reserves the two most significant bits
 of the first byte to encode the base 2 logarithm of the integer encoding length
 in bytes.  The integer value is encoded on the remaining bits, so that the
@@ -722,8 +316,8 @@ The encoded value MUST use the smallest number of bits required to
 represent the value.  When decoding, values using more bits than necessary MUST
 be treated as malformed.
 
-This means that integers are encoded on 1, 2, or 4 bytes and can encode 6-,
-14-, or 30-bit values respectively.
+This means that integers are encoded in 1, 2, or 4 bytes and can encode 6-,
+14-, or 30-bit values, respectively.
 
 | Prefix | Length  | Usable Bits | Min        | Max                 |
 |:-------|:--------|:------------|:-----------|:--------------------|
@@ -733,13 +327,13 @@ This means that integers are encoded on 1, 2, or 4 bytes and can encode 6-,
 | 11     | invalid | -           | -          | -                   |
 {: #integer-summary title="Summary of Integer Encodings"}
 
-Vectors that start with "11" prefix are invalid and MUST be rejected.
+Vectors that start with the prefix "11" are invalid and MUST be rejected.
 
 For example:
 
-* The four byte length value 0x9d7f3e7d decodes to 494878333.
-* The two byte length value 0x7bbd decodes to 15293.
-* The single byte length value 0x25 decodes to 37.
+* The four-byte length value 0x9d7f3e7d decodes to 494878333.
+* The two-byte length value 0x7bbd decodes to 15293.
+* The single-byte length value 0x25 decodes to 37.
 
 The following figure adapts the pseudocode provided in {{RFC9000}} to add a
 check for minimum-length encoding:
@@ -769,7 +363,7 @@ ReadVarint(data):
 ~~~
 
 The use of variable-size integers for vector lengths allows vectors to grow
-very large, up to 2^30 bytes.  Implementations should take care not to allow
+very large, up to 2<sup>30</sup> bytes.  Implementations should take care not to allow
 vectors to overflow available storage.  To facilitate debugging of potential
 interoperability problems, implementations SHOULD provide a clear error when
 such an overflow condition occurs.
@@ -790,7 +384,7 @@ MLS assumes a trusted AS but a largely untrusted DS. {{authentication-service-co
 describes the impact of compromise or
 misbehavior of an AS. MLS is designed to protect the confidentiality and integrity of
 the group data even in the face of a compromised DS;
-in general, the DS is just expected to reliably deliver messages.
+in general, the DS is only expected to reliably deliver messages.
 {{delivery-service-compromise}} describes the impact of compromise or
 misbehavior of a DS.
 
@@ -822,20 +416,19 @@ shared cryptographic state from one epoch to another by exchanging MLS messages.
 * A _KeyPackage_ object describes a client's capabilities and provides keys that
   can be used to add the client to a group.
 * A _Proposal_ message proposes a change to be made in the next epoch, such as
-  adding or removing a member
+  adding or removing a member.
 * A _Commit_ message initiates a new epoch by instructing members of the group
-  to implement a collection of proposals
+  to implement a collection of proposals.
 * A _Welcome_ message provides a new member to the group with the information to
   initialize their state for the epoch in which they were added or in which they
-  want to add themselves to the group
+  want to add themselves to the group.
 
 KeyPackage and Welcome messages are used to initiate a group or introduce new
 members, so they are exchanged between group members and clients not yet in the
 group. A client publishes a KeyPackage via the DS, thus enabling other
-clients to add it to groups. When a group member wants to add a new member
-to a group it uses the new member's KeyPackage to add the new member to
-the group and construct a Welcome message with which the new member can
-initialize its local state.
+clients to add it to groups. When a group member wants to add a new member to a
+group, it uses the new member's KeyPackage to add them and constructs a Welcome
+message with which the new member can initialize their local state.
 
 Proposal and Commit messages are sent from one member of a group to the others.
 MLS provides a common framing layer for sending messages within a group:
@@ -872,7 +465,7 @@ The cryptographic state at the core of MLS is divided into three areas of respon
                          |               |
                           '-    ...    -'
 ~~~
-{: title="Overview of MLS group evolution"}
+{: title="Overview of MLS Group Evolution"}
 
 * A _ratchet tree_ that represents the membership of the group, providing group
   members a way to authenticate each other and efficiently encrypt messages to
@@ -880,7 +473,7 @@ The cryptographic state at the core of MLS is divided into three areas of respon
   _key schedule_.
 * A _key schedule_ that describes the chain of key derivations used to progress from
   epoch to epoch (mainly using the _init_secret_ and _epoch_secret_), as well as the derivation of
-  a variety of other secrets (see {{epoch-derived-secrets}}), for example:
+  a variety of other secrets (see {{epoch-derived-secrets}}). For example:
   * An _encryption secret_ that is used to initialize the secret tree for the
     epoch.
   * An _exporter secret_ that allows other protocols to leverage MLS as a
@@ -910,17 +503,17 @@ in this epoch.
 
 ## Example Protocol Execution
 
-There are three major operations in the lifecycle of a group:
+There are three major operations in the life of a group:
 
 * Adding a member, initiated by a current member;
-* Updating the keys that represent a member in the tree;
+* Updating the keys that represent a member in the tree; and
 * Removing a member.
 
 Each of these operations is "proposed" by sending a message of the corresponding
 type (Add / Update / Remove).  The state of the group is not changed, however,
 until a Commit message is sent to provide the group with fresh entropy.  In this
 section, we show each proposal being committed immediately, but in more advanced
-deployment cases an application might gather several proposals before
+deployment cases, an application might gather several proposals before
 committing them all at once.  In the illustrations below, we show the Proposal
 and Commit messages directly, while in reality they would be sent encapsulated in
 PublicMessage or PrivateMessage objects.
@@ -948,11 +541,11 @@ A                B                C            Directory       Channel
 ~~~
 {: #prepublish-flow title="Clients A, B, and C publish KeyPackages to the directory"}
 
-{{create-flow}} shows how these prepublished KeyPackages are used to create a group.
-When a client A wants to establish a group with B and C, it first initializes a
+{{create-flow}} shows how these pre-published KeyPackages are used to create a group.
+When client A wants to establish a group with clients B and C, it first initializes a
 group state containing only itself and downloads KeyPackages for B and C. For
-each member, A generates an Add and Commit message adding that member, and
-broadcasts them to the group. It also generates a Welcome message and sends this
+each member, A generates an Add proposal and a Commit message to add that member and then
+broadcasts the two messages to the group. Client A also generates a Welcome message and sends it
 directly to the new member (there's no need to send it to the group). Only after
 A has received its Commit message back from the Delivery Service does it update its
 state to reflect the new member's addition.
@@ -999,18 +592,20 @@ A              B              C          Directory            Channel
 {: #create-flow title="Client A creates a group with clients B and C"}
 
 Subsequent additions of group members proceed in the same way.  Any
-member of the group can download a KeyPackage for a new client
-and broadcast Add and Commit messages that the current group will use to update
-their state, and a Welcome message that the new client can use to
+member of the group can download a KeyPackage for a new client,
+broadcast Add and Commit messages that the current group will use to update
+their state, and send a Welcome message that the new client can use to
 initialize its state and join the group.
 
 To enforce the forward secrecy and post-compromise security of messages, each
 member periodically updates the keys that represent them to the group.  A member
-does this by sending a Commit (possibly with no proposals), or by sending an
+does this by sending a Commit (possibly with no proposals) or by sending an
 Update message that is committed by another member (see {{update-flow}}).
 Once the other members of
 the group have processed these messages, the group's secrets will be unknown to
 an attacker that had compromised the secrets corresponding to the sender's leaf in the tree.
+At the end of the scenario shown in {{update-flow}}, the group has
+post-compromise security with respect to both A and B.
 
 Update messages SHOULD be sent at regular intervals of time as long as the group
 is active, and members that don't update SHOULD eventually be removed from the
@@ -1018,8 +613,8 @@ group. It's left to the application to determine an appropriate amount of time
 between Updates. Since the purpose of sending an Update is to proactively
 constrain a compromise window, the right frequency is usually on the order of
 hours or days, not milliseconds. For example, an application might send an
-Update each time a member sends an application message after receiving from
-other members, or daily if no application messages are sent.
+Update each time a member sends an application message after receiving any
+message from another member, or daily if no application messages are sent.
 
 The MLS architecture recommends that MLS be operated over a secure transport
 (see {{Section 7.1 of I-D.ietf-mls-architecture}}).  Such transport protocols
@@ -1049,8 +644,7 @@ A              B     ...      Z          Directory        Channel
 |              |              |              |              |
 ~~~
 {: #update-flow title="Client B proposes to update its key, and client A commits the
-proposal.  As a result, the keys for both B and A updated, so the group has
-post-compromise security with respect to both of them."}
+proposal"}
 
 Members are removed from the group in a similar way, as shown in {{remove-flow}}.
 Any member of the group can send a Remove proposal followed by a
@@ -1088,8 +682,8 @@ message flows in other ways.  For example:
   group.
 
 * Proposal messages don't need to be immediately sent to all group members.  They need to
-  be available to the committer before generating a commit, and to other members before
-  processing the commit.
+  be available to the committer before generating a Commit, and to other members before
+  processing the Commit.
 
 * The sender of a Commit doesn't necessarily have to wait to receive its own
   Commit back before advancing its state. It only needs to know that its Commit
@@ -1131,11 +725,11 @@ A              B              Z          Directory        Channel
 |              |              |<----------------------------+
 |              |              |              |              |
 ~~~
-{: #groupinfo-flow title="Client A publishes a GroupInfo object and Client Z uses
+{: #groupinfo-flow title="Client A publishes a GroupInfo object, and Client Z uses
 it to join the group"}
 
 
-## Relationships Between Epochs
+## Relationships between Epochs
 
 A group has a single linear sequence of epochs. Groups and epochs are generally
 independent of one another. However, it can sometimes be useful to link epochs
@@ -1151,7 +745,7 @@ guarantees that members entering the new epoch agree on a key if and only if
 they were members of the group during the epoch from which the resumption key
 was extracted.
 
-MLS supports two ways to tie a new group to an existing group, illustrated in
+MLS supports two ways to tie a new group to an existing group, which are illustrated in
 {{psk-reinit}} and {{psk-branch}}. Reinitialization
 closes one group and creates a new group comprising the same members with
 different parameters. Branching starts a new group with a subset of the original
@@ -1173,7 +767,7 @@ epoch_A_[n]           epoch_B_[0]
                            V
                       epoch_B_[1]
 ~~~
-{: #psk-reinit title="Reinitializing a group" }
+{: #psk-reinit title="Reinitializing a Group" }
 
 
 ~~~ aasvg
@@ -1185,7 +779,7 @@ epoch_A_[n]           epoch_B_[0]
      V                     V
 epoch_A_[n+1]         epoch_B_[1]
 ~~~
-{: #psk-branch title="Branching a group" }
+{: #psk-branch title="Branching a Group" }
 
 Applications may also choose to use resumption PSKs to link epochs in other
 ways.  For example, {{psk-reinject}} shows a case where a resumption PSK
@@ -1212,7 +806,7 @@ epoch_A_[n+k-1]           .
      V
 epoch_A_[n+k]
 ~~~
-{: #psk-reinject title="Reinjecting entropy from an earlier epoch" }
+{: #psk-reinject title="Reinjecting Entropy from an Earlier Epoch" }
 
 # Ratchet Tree Concepts
 
@@ -1236,23 +830,23 @@ replaced with a freshly generated instance.
 ## Ratchet Tree Terminology
 
 Trees consist of _nodes_. A node is a
-_leaf_ if it has no children, and a _parent_ otherwise; note that all
-parents in our trees have precisely
+_leaf_ if it has no children; otherwise, it is a _parent_.
+All parents in our trees have precisely
 two children, a _left_ child and a _right_ child. A node is the _root_
-of a tree if it has no parents, and _intermediate_ if it has both
-children and parents. The _descendants_ of a node are that node's
-children, and the descendants of its children, and we say a tree
+of a tree if it has no parent, and _intermediate_ if it has both
+children and a parent. The _descendants_ of a node are that node's
+children, and the descendants of its children.  We say a tree
 _contains_ a node if that node is a descendant of the root of the tree,
 or if the node itself is the root of the tree. Nodes are _siblings_ if they share the same parent.
 
 A _subtree_ of a tree is the tree given by any node (the _head_ of the
 subtree) and its descendants. The _size_ of a tree or subtree is the
 number of leaf nodes it contains.  For a given parent node, its _left
-subtree_ is the subtree with its left child as head (respectively
-_right subtree_).
+subtree_ is the subtree with its left child as head and its
+_right subtree_ is the subtree with its right child as head.
 
 Every tree used in this protocol is a perfect binary tree, that is, a complete
-balanced binary tree with `2^d` leaves all at the same depth `d`.  This
+balanced binary tree with 2<sup>d</sup> leaves all at the same depth `d`.  This
 structure is unique for a given depth `d`.
 
 There are multiple ways that an implementation might represent a ratchet tree in
@@ -1269,14 +863,14 @@ algorithms, as long as they produce correct protocol messages.
 ### Ratchet Tree Nodes
 
 Each leaf node in a ratchet tree is given an _index_ (or _leaf index_), starting
-at `0` from the left to `2^d - 1` at the right (for a tree with `2^d` leaves). A tree
-with `2^d` leaves has `2^(d+1) - 1` nodes, including parent nodes.
+at 0 from the left to 2<sup>d</sup> - 1 at the right (for a tree with 2<sup>d</sup> leaves). A tree
+with 2<sup>d</sup> leaves has 2<sup>d+1</sup> - 1 nodes, including parent nodes.
 
 Each node in a ratchet tree is either _blank_ (containing no value) or it holds
 an HPKE public key with some associated data:
 
-* A public key (for the HPKE scheme in use, see {{ciphersuites}})
-* A credential (only for leaf nodes, see {{credentials}})
+* A public key (for the HPKE scheme in use; see {{cipher-suites}})
+* A credential (only for leaf nodes; see {{credentials}})
 * An ordered list of "unmerged" leaves (see {{views}})
 * A hash of certain information about the node's parent, as of the last time the
   node was changed (see {{parent-hashes}}).
@@ -1284,7 +878,7 @@ an HPKE public key with some associated data:
 As described in {{views}}, different members know different subsets of the set
 of private keys corresponding to the public keys in nodes in the tree.  The
 private key corresponding to a parent node is known only to members at leaf
-nodes that are descedants of that node.  The private key corresponding to a leaf
+nodes that are descendants of that node.  The private key corresponding to a leaf
 node is known only to the member at that leaf node.  A leaf node is _unmerged_
 relative to one of its ancestor nodes if the member at the leaf node does not
 know the private key corresponding to the ancestor node.
@@ -1296,17 +890,17 @@ in {{tree-hashes}}.
 
 The _resolution_ of a node is an ordered list of non-blank nodes
 that collectively cover all non-blank descendants of the node.
-The resolution of the root contains the set of keys which are collectively necessary to
+The resolution of the root contains the set of keys that are collectively necessary to
 encrypt to every node in the group. The resolution
 of a node is effectively a depth-first, left-first enumeration of the nearest
 non-blank nodes below the node:
 
 * The resolution of a non-blank node comprises the node itself,
-  followed by its list of unmerged leaves, if any
-* The resolution of a blank leaf node is the empty list
+  followed by its list of unmerged leaves, if any.
+* The resolution of a blank leaf node is the empty list.
 * The resolution of a blank intermediate node is the result of
   concatenating the resolution of its left child with the resolution
-  of its right child, in that order
+  of its right child, in that order.
 
 For example, consider the following subtree, where the `_` character
 represents a blank node and unmerged leaves are indicated in square
@@ -1327,17 +921,17 @@ A   B   _   D   E   F   _   H
 
 0   1   2   3   4   5   6   7
 ~~~
-{: #resolution-tree title="A tree with blanks and unmerged leaves" }
+{: #resolution-tree title="A Tree with Blanks and Unmerged Leaves" }
 
 In this tree, we can see all of the above rules in play:
 
-* The resolution of node X is the list \[X, B\]
-* The resolution of leaf 2 or leaf 6 is the empty list \[\]
-* The resolution of top node is the list \[X, B, Y, H\]
+* The resolution of node X is the list \[X, B\].
+* The resolution of leaf 2 or leaf 6 is the empty list \[\].
+* The resolution of top node is the list \[X, B, Y, H\].
 
 ### Paths through a Ratchet Tree
 
-The _direct path_ of a root is the empty list, and of any other node
+The _direct path_ of a root is the empty list. The direct path of any other node
 is the concatenation of that node's parent along with the parent's direct path.
 
 The _copath_ of a node is the node's sibling concatenated with the list of
@@ -1367,8 +961,8 @@ A   B   _   _   E   F   G   _=H
 
 0   1   2   3   4   5   6   7
 ~~~
-{: #full-tree title="A complete tree with five members, with labels for blank
-parent nodes" }
+{: #full-tree title="A Complete Tree with Five Members, with Labels for Blank
+Parent Nodes" }
 
 In this tree, the direct paths, copaths, and filtered direct paths for the leaf
 nodes are as follows:
@@ -1396,8 +990,8 @@ credential stored at that leaf is one provided by the member.
 In particular, MLS maintains the members' views of the tree in such
 a way as to maintain the _tree invariant_:
 
-    The private key for a node in the tree is known to a member of
-    the group only if the node's subtree contains that member's leaf.
+> The private key for a node in the tree is known to a member of
+> the group only if the node's subtree contains that member's leaf.
 
 In other words, if a node is not blank, then it holds a public key.
 The corresponding private key is known only to members occupying
@@ -1448,9 +1042,9 @@ a given node, but not for that node, as in the case with D.
 
 # Cryptographic Objects
 
-## Ciphersuites
+## Cipher Suites
 
-Each MLS session uses a single ciphersuite that specifies the
+Each MLS session uses a single cipher suite that specifies the
 following primitives to be used in group key computations:
 
 * HPKE parameters:
@@ -1458,50 +1052,53 @@ following primitives to be used in group key computations:
   * A Key Derivation Function (KDF)
   * An Authenticated Encryption with Associated Data (AEAD) encryption algorithm
 * A hash algorithm
-* A MAC algorithm
+* A Message Authentication Code (MAC) algorithm
 * A signature algorithm
 
-MLS uses HPKE for public-key encryption {{!RFC9180}}.  The
-`DeriveKeyPair` function associated to the KEM for the ciphersuite maps octet
+MLS uses HPKE for public key encryption {{!RFC9180}}.  The
+`DeriveKeyPair` function associated to the KEM for the cipher suite maps octet
 strings to HPKE key pairs.  As in HPKE, MLS assumes that an AEAD algorithm
 produces a single ciphertext output from AEAD encryption (aligning with
 {{?RFC5116}}), as opposed to a separate ciphertext and tag.
 
-Ciphersuites are represented with the CipherSuite type. The ciphersuites are
-defined in {{mls-ciphersuites}}.
+Cipher suites are represented with the CipherSuite type. The cipher suites are
+defined in {{mls-cipher-suites}}.
 
 ### Public Keys
 
 HPKE public keys are opaque values in a format defined by the underlying
-protocol (see Section 4 of {{RFC9180}} for more information).
+protocol (see {{Section 4 of RFC9180}} for more information).
 
-~~~ tls
+~~~ tls-presentation
 opaque HPKEPublicKey<V>;
 ~~~
 
 Signature public keys are likewise represented as opaque values in a format
-defined by the ciphersuite's signature scheme.
+defined by the cipher suite's signature scheme.
 
-~~~ tls
+~~~ tls-presentation
 opaque SignaturePublicKey<V>;
 ~~~
 
-For ciphersuites using Ed25519 or Ed448 signature schemes, the public key is in
-the format specified in {{?RFC8032}}.  For ciphersuites using ECDSA with the
-NIST curves (P-256, P-384, or P-521), the public key is represented as an
-encoded UncompressedPointRepresentation struct, as defined in {{RFC8446}}.
+For cipher suites using the Edwards-curve Digital Signature Algorithm (EdDSA)
+signature schemes (Ed25519 or Ed448), the public key is in the format specified
+in {{?RFC8032}}.
+
+For cipher suites using the Elliptic Curve Digital Signature Algorithm (ECDSA)
+with the NIST curves (P-256, P-384, or P-521), the public key is represented as
+an encoded UncompressedPointRepresentation struct, as defined in {{RFC8446}}.
 
 ### Signing
 
-The signature algorithm specified in a group's ciphersuite is the mandatory algorithm
+The signature algorithm specified in a group's cipher suite is the mandatory algorithm
 to be used for signing messages within the group.  It
 MUST be the same as the signature algorithm specified in the credentials in the
 leaves of the tree (including the leaf node information in KeyPackages used to
 add new members).
 
 The signatures used in this document are encoded as specified in {{!RFC8446}}.
-In particular, ECDSA signatures are DER-encoded and EdDSA signatures are defined
-as the concatenation of `r` and `s` as specified in {{?RFC8032}}.
+In particular, ECDSA signatures are DER encoded, and EdDSA signatures are defined
+as the concatenation of `R` and `S`, as specified in {{?RFC8032}}.
 
 To disambiguate different signatures used in MLS, each signed value is prefixed
 by a label as shown below:
@@ -1516,27 +1113,27 @@ VerifyWithLabel(VerificationKey, Label, Content, SignatureValue) =
 
 Where SignContent is specified as:
 
-~~~ tls
+~~~ tls-presentation
 struct {
     opaque label<V>;
     opaque content<V>;
 } SignContent;
 ~~~
 
-And its fields set to:
+And its fields are set to:
 
 ~~~ pseudocode
 label = "MLS 1.0 " + Label;
 content = Content;
 ~~~
 
-Here, the functions `Signature.Sign` and `Signature.Verify` are defined by the
+The functions `Signature.Sign` and `Signature.Verify` are defined by the
 signature algorithm.  If MLS extensions require signatures by group members,
-they should re-use the SignWithLabel construction, using a distinct label.  To
+they should reuse the SignWithLabel construction, using a distinct label.  To
 avoid collisions in these labels, an IANA registry is defined in
 {{mls-signature-labels}}.
 
-### Public-Key Encryption
+### Public Key Encryption
 
 As with signing, MLS includes a label and context in encryption operations to
 avoid confusion between ciphertexts produced for different purposes.  Encryption
@@ -1552,25 +1149,26 @@ DecryptWithLabel(PrivateKey, Label, Context, KEMOutput, Ciphertext) =
 
 Where EncryptContext is specified as:
 
-~~~ tls
+~~~ tls-presentation
 struct {
   opaque label<V>;
   opaque context<V>;
 } EncryptContext;
 ~~~
 
-And its fields set to:
+And its fields are set to:
 
 ~~~
 label = "MLS 1.0 " + Label;
 context = Context;
 ~~~
 
-Here, the functions `SealBase` and `OpenBase` are defined {{RFC9180}}, using the
-HPKE algorithms specified by the group's ciphersuite.  If MLS extensions
-require HPKE encryption operations, they should re-use the EncryptWithLabel
-construction, using a distinct label.  To avoid collisions in these labels, an
-IANA registry is defined in {{mls-public-key-encryption-labels}}.
+The functions `SealBase` and `OpenBase` are defined in {{Section 6.1 of
+RFC9180}} (with "Base" as the MODE), using the HPKE algorithms specified by the
+group's cipher suite.  If MLS extensions require HPKE encryption operations, they
+should reuse the EncryptWithLabel construction, using a distinct label.  To
+avoid collisions in these labels, an IANA registry is defined in
+{{mls-public-key-encryption-labels}}.
 
 ## Hash-Based Identifiers
 
@@ -1578,7 +1176,7 @@ Some MLS messages refer to other MLS objects by hash.  For example, Welcome
 messages refer to KeyPackages for the members being welcomed, and Commits refer
 to Proposals they cover.  These identifiers are computed as follows:
 
-~~~ tls
+~~~ tls-presentation
 opaque HashReference<V>;
 
 HashReference KeyPackageRef;
@@ -1586,22 +1184,25 @@ HashReference ProposalRef;
 ~~~
 
 ~~~ pseudocode
-MakeKeyPackageRef(value) = RefHash("MLS 1.0 KeyPackage Reference", value)
-MakeProposalRef(value)   = RefHash("MLS 1.0 Proposal Reference", value)
+MakeKeyPackageRef(value)
+  = RefHash("MLS 1.0 KeyPackage Reference", value)
+
+MakeProposalRef(value)
+  = RefHash("MLS 1.0 Proposal Reference", value)
 
 RefHash(label, value) = Hash(RefHashInput)
 ~~~
 
 Where RefHashInput is defined as:
 
-~~~ tls
+~~~ tls-presentation
 struct {
   opaque label<V>;
   opaque value<V>;
 } RefHashInput;
 ~~~
 
-And its fields set to:
+And its fields are set to:
 
 ~~~ pseudocode
 label = label;
@@ -1609,30 +1210,30 @@ value = value;
 ~~~
 
 For a KeyPackageRef, the `value` input is the encoded KeyPackage, and the
-ciphersuite specified in the KeyPackage determines the KDF used.  For a
+cipher suite specified in the KeyPackage determines the KDF used.  For a
 ProposalRef, the `value` input is the AuthenticatedContent carrying the
-proposal.  In the latter two cases, the KDF is determined by the group's
-ciphersuite.
+Proposal.  In the latter two cases, the KDF is determined by the group's
+cipher suite.
 
 ## Credentials
 
 Each member of a group presents a credential that provides one or more
-identities for the member, and associates them with the member's signing key.
+identities for the member and associates them with the member's signing key.
 The identities and signing key are verified by the Authentication Service in use
 for a group.
 
-It is up to the application to decide which identifier or identifiers to use at
+It is up to the application to decide which identifiers to use at
 the application level.  For example,
 a certificate in an X509Credential may attest to several domain names or email
 addresses in its subjectAltName extension.  An application may decide to
 present all of these to a user, or if it knows a "desired" domain name or email
 address, it can check that the desired identifier is among those attested.
-Using the terminology from {{?RFC6125}}, a Credential provides "presented
+Using the terminology from {{?RFC6125}}, a credential provides "presented
 identifiers", and it is up to the application to supply a "reference identifier"
 for the authenticated client, if any.
 
-~~~ tls
-// See IANA registry for registered values
+~~~ tls-presentation
+// See the "MLS Credential Types" IANA registry for values
 uint16 CredentialType;
 
 struct {
@@ -1676,25 +1277,25 @@ The parts of the system that perform these functions are collectively referred
 to as the Authentication Service (AS) {{?I-D.ietf-mls-architecture}}.  A
 member's credential is said to be _validated with the AS_ when the AS verifies
 that the credential's presented identifiers are correctly associated with the
-`signature_key` field in the member's LeafNode, and verifies that those
+`signature_key` field in the member's LeafNode, and that those
 identifiers match the reference identifiers for the member.
 
 Whenever a new credential is introduced in the group, it MUST be validated with
 the AS.  In particular, at the following events in the protocol:
 
 * When a member receives a KeyPackage that it will use in an Add proposal to add
-  a new member to the group.
+  a new member to the group
 * When a member receives a GroupInfo object that it will use to join a group,
-  either via a Welcome or via an External Commit
-* When a member receives an Add proposal adding a member to the group.
+  either via a Welcome or via an external Commit
+* When a member receives an Add proposal adding a member to the group
 * When a member receives an Update proposal whose LeafNode has a new credential
-  for the member.
+  for the member
 * When a member receives a Commit with an UpdatePath whose LeafNode has a new
-  credential for the committer.
-* When an `external_senders` extension is added to the group, or an existing
-  `external_senders` extension is updated.
+  credential for the committer
+* When an `external_senders` extension is added to the group
+* When an existing `external_senders` extension is updated
 
-In cases where a member's credential is being replaced, such as Update and
+In cases where a member's credential is being replaced, such as the Update and
 Commit cases above, the AS MUST also verify that the set of presented
 identifiers in the new credential is valid as a successor to the set of
 presented identifiers in the old credential, according to the application's
@@ -1702,7 +1303,7 @@ policy.
 
 ### Credential Expiry and Revocation
 
-In some credential schemes, a valid credential can "expire", or become invalid
+In some credential schemes, a valid credential can "expire" or become invalid
 after a certain point in time. For example, each X.509 certificate has a
 `notAfter` field, expressing a time after which the certificate is not valid.
 
@@ -1734,10 +1335,10 @@ the group tree is known to the DS, the DS could also monitor the tree for
 expired credentials and issue external Remove proposals.
 
 Some credential schemes also allow credentials to be revoked.  Revocation is
-similar to expiry, in that a previously valid credential becomes invalid.
+similar to expiry in that a previously valid credential becomes invalid.
 As such, most of the considerations above also apply to revoked credentials.
 However, applications may want to treat revoked credentials differently, e.g.,
-removing members with revoked credentials while allowing members with expired
+by removing members with revoked credentials while allowing members with expired
 credentials time to update.
 
 ### Uniquely Identifying Clients
@@ -1761,7 +1362,7 @@ user's application-layer identifiers.
 If needed, applications may add application-specific identifiers to the
 `extensions` field of a LeafNode object with the `application_id` extension.
 
-~~~ tls
+~~~ tls-presentation
 opaque application_id<V>;
 ~~~
 
@@ -1780,10 +1381,10 @@ AuthenticatedContent objects.  These structures contain the content of the
 message itself as well as information to authenticate the sender (see
 {{content-authentication}}).  The additional protections required to transmit
 these messages over an untrusted channel (group membership authentication or
-AEAD encryption) are added by encoding the AuthenticatedContent as an
+AEAD encryption) are added by encoding the AuthenticatedContent as a
 PublicMessage or PrivateMessage message, which can then be sent as an MLSMessage.
 Likewise, these protections are enforced (via membership verification or AEAD
-decryption) when decoding an PublicMessage or PrivateMessage into an
+decryption) when decoding a PublicMessage or PrivateMessage into an
 AuthenticatedContent object.
 
 PrivateMessage represents a signed and encrypted message, with
@@ -1791,11 +1392,11 @@ protections for both the content of the message and related
 metadata.  PublicMessage represents a message that is only signed,
 and not encrypted.  Applications MUST use PrivateMessage to encrypt
 application messages and SHOULD use PrivateMessage to encode
-handshake messages, but MAY transmit handshake messages encoded
+handshake messages, but they MAY transmit handshake messages encoded
 as PublicMessage objects in cases where it is necessary for the
 Delivery Service to examine such messages.
 
-~~~ tls
+~~~ tls-presentation
 enum {
     reserved(0),
     mls10(1),
@@ -1832,7 +1433,7 @@ struct {
     };
 } Sender;
 
-// See IANA registry for registered values
+// See the "MLS Wire Formats" IANA registry for values
 uint16 WireFormat;
 
 struct {
@@ -1876,7 +1477,7 @@ Messages from senders that aren't in the group are sent as PublicMessage. See
 The following structure is used to fully describe the data transmitted in
 plaintexts or ciphertexts.
 
-~~~ tls
+~~~ tls-presentation
 struct {
     WireFormat wire_format;
     FramedContent content;
@@ -1889,44 +1490,42 @@ section relate to each other, and the high-level operations used to produce and
 consume them:
 
 ~~~ aasvg
-                              Proposal        Commit     Application Data
-                                 |              |              |
-                                 +--------------+--------------+
-                                                |
-                                                V
-                                         FramedContent
-                                             |  |                -.
-                                             |  |                  |
-                                    +--------+  |                  |
-                                    |           |                  |
-                                    V           |                  +-- Asymmetric
-                          FramedContentAuthData |                  |   Sign / Verify
-                                    |           |                  |
-                                    +--------+  |                  |
-                                             |  |                  |
-                                             V  V                -'
-                                       AuthenticatedContent
-                                                |                -.
-                                                |                  |
-                                                |                  |
-                                       +--------+--------+         +-- Symmetric
-                                       |                 |         |   Protect / Unprotect
-                                       V                 V         |
-Welcome  KeyPackage  GroupInfo   PublicMessage    PrivateMessage -'
-   |          |          |             |                 |
-   |          |          |             |                 |
-   +----------+----------+----+--------+-----------------+
-                              |
-                              V
-                          MLSMessage
+    Proposal        Commit     Application Data
+       |              |              |
+       +--------------+--------------+
+                      |
+                      V
+               FramedContent
+                   |  |                -.
+          +--------+  |                  |
+          |           |                  |
+          V           |                  +-- Asymmetric
+FramedContentAuthData |                  |   Sign / Verify
+          |           |                  |
+          +--------+  |                  |
+                   |  |                  |
+                   V  V                -'
+             AuthenticatedContent
+                      |                -.
+             +--------+--------+         |
+             |                 |         +-- Symmetric
+             V                 V         |   Protect / Unprotect
+       PublicMessage    PrivateMessage -'
+             |                 |
+             |                 |  Welcome  KeyPackage  GroupInfo
+             |                 |     |          |          |
+             +-----------------+-----+----------+----------+
+                               |
+                               V
+                           MLSMessage
 ~~~
-{: title="Relationships among MLS objects" }
+{: title="Relationships among MLS Objects" }
 
 ## Content Authentication
 
 FramedContent is authenticated using the FramedContentAuthData structure.
 
-~~~ tls
+~~~ tls-presentation
 struct {
     ProtocolVersion version = mls10;
     WireFormat wire_format;
@@ -1980,7 +1579,7 @@ depending on the sender's `sender_type`:
     the Commit's path (see {{joining-via-external-commits}}). The
     `content_type` of the message MUST be `commit`.
 * `new_member_proposal`: The signature key in the LeafNode in
-    the KeyPackage embedded in an External Add Proposal. The
+    the KeyPackage embedded in an external Add proposal. The
     `content_type` of the message MUST be `proposal` and the
     `proposal_type` of the Proposal MUST be `add`.
 
@@ -1995,7 +1594,7 @@ the `signature` and `confirmation_tag` fields are valid.
 
 Messages that are authenticated but not encrypted are encoded using the PublicMessage structure.
 
-~~~ tls
+~~~ tls-presentation
 struct {
     FramedContent content;
     FramedContentAuthData auth;
@@ -2014,7 +1613,7 @@ The `membership_tag` field in the PublicMessage object authenticates the sender'
 membership in the group. For messages sent by members, it MUST be set to the
 following value:
 
-~~~ tls
+~~~ tls-presentation
 struct {
   FramedContentTBS content_tbs;
   FramedContentAuthData auth;
@@ -2025,7 +1624,7 @@ struct {
 membership_tag = MAC(membership_key, AuthenticatedContentTBM)
 ~~~
 
-When decoding an PublicMessage into an AuthenticatedContent,
+When decoding a PublicMessage into an AuthenticatedContent,
 the application MUST check `membership_tag` and MUST check that the
 FramedContentAuthData is valid.
 
@@ -2033,7 +1632,7 @@ FramedContentAuthData is valid.
 
 Authenticated and encrypted messages are encoded using the PrivateMessage structure.
 
-~~~ tls
+~~~ tls-presentation
 struct {
     opaque group_id<V>;
     uint64 epoch;
@@ -2045,14 +1644,14 @@ struct {
 ~~~
 
 `encrypted_sender_data` and `ciphertext` are encrypted using the AEAD function
-specified by the ciphersuite in use, using as input the structures SenderData
-and PrivateMessageContent.
+specified by the cipher suite in use, using the SenderData
+and PrivateMessageContent structures as input.
 
 ### Content Encryption
 
 Content to be encrypted is encoded in a PrivateMessageContent structure.
 
-~~~ tls
+~~~ tls-presentation
 struct {
     select (PrivateMessage.content_type) {
         case application:
@@ -2071,7 +1670,7 @@ struct {
 ~~~
 
 The `padding` field is set by the sender, by first encoding the content (via the
-`select`) and the `auth` field, then appending the chosen number of zero bytes.
+`select`) and the `auth` field, and then appending the chosen number of zero bytes.
 A receiver identifies the padding field in a plaintext decoded from
 `PrivateMessage.ciphertext` by first decoding the content and the `auth` field;
 then the `padding` field comprises any remaining octets of plaintext.  The
@@ -2084,7 +1683,7 @@ used as a covert channel.
 In the MLS key schedule, the sender creates two distinct key ratchets for
 handshake and application messages for each member of the group. When encrypting
 a message, the sender looks at the ratchets it derived for its own member and
-chooses an unused generation from either the handshake or application ratchet
+chooses an unused generation from either the handshake ratchet or the application ratchet,
 depending on the content type of the message. This generation of the ratchet is
 used to derive a provisional nonce and key.
 
@@ -2119,7 +1718,7 @@ The Additional Authenticated Data (AAD) input to the encryption
 contains an object of the following form, with the values used to
 identify the key and nonce:
 
-~~~ tls
+~~~ tls-presentation
 struct {
     opaque group_id<V>;
     uint64 epoch;
@@ -2134,16 +1733,16 @@ FramedContentAuthData is valid.
 It is up to the application to decide what `authenticated_data` to provide and
 how much padding to add to a given message (if any).  The overall size of the
 AAD and ciphertext MUST fit within the limits established for the group's AEAD
-algorithm in [!I-D.irtf-cfrg-aead-limits].
+algorithm in {{!I-D.irtf-cfrg-aead-limits}}.
 
 ### Sender Data Encryption
 
 The "sender data" used to look up the key for content encryption is
-encrypted with the ciphersuite's AEAD with a key and nonce derived from both the
+encrypted with the cipher suite's AEAD with a key and nonce derived from both the
 `sender_data_secret` and a sample of the encrypted content. Before being
 encrypted, the sender data is encoded as an object of the following form:
 
-~~~ tls
+~~~ tls-presentation
 struct {
     uint32 leaf_index;
     uint32 generation;
@@ -2172,10 +1771,10 @@ sender_data_nonce = ExpandWithLabel(sender_data_secret, "nonce",
                       ciphertext_sample, AEAD.Nn)
 ~~~
 
-The Additional Authenticated Data (AAD) for the SenderData ciphertext is the
+The AAD for the SenderData ciphertext is the
 first three fields of PrivateMessage:
 
-~~~ tls
+~~~ tls-presentation
 struct {
     opaque group_id<V>;
     uint64 epoch;
@@ -2190,7 +1789,7 @@ non-blank node.
 # Ratchet Tree Operations
 
 The ratchet tree for an epoch describes the membership of a group in that epoch,
-providing public-key encryption (HPKE) keys that can be used to encrypt to subsets of
+providing public key encryption (HPKE) keys that can be used to encrypt to subsets of
 the group as well as information to authenticate the members.  In order to
 reflect changes to the membership of the group from one epoch to the next,
 corresponding changes are made to the ratchet tree.  In this section, we
@@ -2202,7 +1801,7 @@ As discussed in {{ratchet-tree-nodes}}, the nodes of a ratchet tree contain
 several types of data describing individual members (for leaf nodes) or
 subgroups of the group (for parent nodes).  Parent nodes are simpler:
 
-~~~ tls
+~~~ tls-presentation
 struct {
     HPKEPublicKey encryption_key;
     opaque parent_hash<V>;
@@ -2224,7 +1823,7 @@ appearance in the group, signed by that client. It is also used in client
 KeyPackage objects to store the information that will be needed to add a
 client to a group.
 
-~~~ tls
+~~~ tls-presentation
 enum {
     reserved(0),
     key_package(1),
@@ -2235,7 +1834,7 @@ enum {
 
 struct {
     ProtocolVersion versions<V>;
-    CipherSuite ciphersuites<V>;
+    CipherSuite cipher_suites<V>;
     ExtensionType extensions<V>;
     ProposalType proposals<V>;
     CredentialType credentials<V>;
@@ -2246,7 +1845,7 @@ struct {
     uint64 not_after;
 } Lifetime;
 
-// See IANA registry for registered values
+// See the "MLS Extension Types" IANA registry for values
 uint16 ExtensionType;
 
 struct {
@@ -2320,7 +1919,7 @@ authenticating both the member's identity and the provided signing key, as
 described in {{credentials}}.
 
 The `capabilities` field indicates the protocol features that the client
-supports, including protocol versions, ciphersuites, credential types,
+supports, including protocol versions, cipher suites, credential types,
 non-default proposal types, and non-default extension types.  The following
 proposal and extension types are considered "default" and MUST NOT be
 listed:
@@ -2356,8 +1955,9 @@ ignore unknown values.
 The `leaf_node_source` field indicates how this LeafNode came to be added to the
 tree.  This signal tells other members of the group whether the leaf node is
 required to have a `lifetime` or `parent_hash`, and whether the `group_id` is
-added as context to the signature.  Whether these fields can be computed by the
-client represented by the LeafNode depends on when the LeafNode was created.
+added as context to the signature.
+These fields are included selectively because the client creating a LeafNode is
+not always able to compute all of them.
 For example, a KeyPackage is created before the client knows which group it will
 be used with, so its signature can't bind to a `group_id`.
 
@@ -2377,7 +1977,7 @@ the `parent_hash` field contains the parent hash for this leaf node (see
 
 The LeafNodeTBS structure covers the fields above the signature in the LeafNode.
 In addition, when the leaf node was created in the context of a group (the
-update and commit cases), the group ID of the group is added as context to the
+`update` and `commit` cases), the group ID of the group is added as context to the
 signature.
 
 LeafNode objects stored in the group's ratchet tree
@@ -2388,18 +1988,18 @@ case of a newcomer joining the group.
 
 ## Leaf Node Validation
 
-The validity of a LeafNode needs to be verified at a few stages:
+The validity of a LeafNode needs to be verified at the following stages:
 
 * When a LeafNode is downloaded in a KeyPackage, before it is used
   to add the client to the group
 * When a LeafNode is received by a group member in an Add, Update, or Commit
   message
 * When a client validates a ratchet tree, e.g., when joining a group or after
-  processing a commit
+  processing a Commit
 
 The client verifies the validity of a LeafNode using the following steps:
 
-* Verify that the credential in the LeafNode is valid as described in
+* Verify that the credential in the LeafNode is valid, as described in
   {{credential-validation}}.
 
 * Verify that the signature on the LeafNode is valid using `signature_key`.
@@ -2416,10 +2016,10 @@ The client verifies the validity of a LeafNode using the following steps:
 
 * Verify the `lifetime` field:
   * If the LeafNode appears in a message being sent by the client, e.g., a
-    proposal or a commit, then the client MUST verify that the current time is within
+    Proposal or a Commit, then the client MUST verify that the current time is within
     the range of the `lifetime` field.
   * If instead the LeafNode appears in a message being received by the client, e.g.,
-    a proposal, a commit, or a ratchet tree of the group the client is joining, it is
+    a Proposal, a Commit, or a ratchet tree of the group the client is joining, it is
     RECOMMENDED that the client verifies that the current time is within the range
     of the `lifetime` field.  (This check is not mandatory because the LeafNode
     might have expired in the time between when the message was sent and when it
@@ -2478,7 +2078,7 @@ node_priv[n], node_pub[n] = KEM.DeriveKeyPair(node_secret[n])
 
 The node secret is derived as a temporary intermediate secret so that each
 secret is only used with one algorithm: The path secret is used as an input to
-DeriveSecret and the node secret is used as an input to DeriveKeyPair.
+DeriveSecret, and the node secret is used as an input to DeriveKeyPair.
 
 For example, suppose there is a group with four members, with C an unmerged leaf
 at Z:
@@ -2494,7 +2094,7 @@ A   B   C   D
 
 0   1   2   3
 ~~~
-{: #evolution-tree title="A full tree with one unmerged leaf" }
+{: #evolution-tree title="A Full Tree with One Unmerged Leaf" }
 
 If member B subsequently generates an UpdatePath based on a secret
 "leaf_secret", then it would generate the following sequence
@@ -2517,7 +2117,7 @@ leaf_secret ------> leaf_node_secret --+--> leaf_priv, leaf_pub
                                                      |
                                                  leaf_node
 ~~~
-{: title="Derivation of ratchet tree keys along a direct path" }
+{: title="Derivation of Ratchet Tree Keys along a Direct Path" }
 
 After applying the UpdatePath, the tree will have the following structure:
 
@@ -2533,12 +2133,12 @@ node_priv[0] ----> X'      Z[C]
 leaf_priv -----------+
                  0   1   2   3
 ~~~
-{: title="Placement of keys in a ratchet tree" }
+{: title="Placement of Keys in a Ratchet Tree" }
 
 ## Synchronizing Views of the Tree
 
-After generating fresh key material and applying it to ratchet forward their
-local tree state as described in the {{ratchet-tree-evolution}}, the
+After generating fresh key material and applying it to update their
+local tree state as described in {{ratchet-tree-evolution}}, the
 generator broadcasts
 this update to other members of the group in a Commit message, who
 apply it to keep their local views of the tree in
@@ -2565,13 +2165,13 @@ There is one encryption of the path secret to each public key in the resolution
 of the non-updated child.
 
 A member of the group _updates their direct path_ by computing new values for
-their leaf node and the nodes along their filtered direct path:
+their leaf node and the nodes along their filtered direct path as follows:
 
 1. Blank all nodes along the direct path of the sender's leaf.
 2. Compute updated path secrets and public keys for the nodes on the sender's
    filtered direct path.
     * Generate a sequence of path secrets of the same length as the filtered
-      direct path, as defined in {{ratchet-tree-evolution}}
+      direct path, as defined in {{ratchet-tree-evolution}}.
     * For each node in the filtered direct path, replace the node's public key
       with the `node_pub[n]` value derived from the corresponding path secret
       `path_secret[n]`.
@@ -2579,12 +2179,12 @@ their leaf node and the nodes along their filtered direct path:
    and the sender's leaf node.
 4. Update the leaf node for the sender.
     * Set the `leaf_node_source` to `commit`.
-    * Set the `encryption_key` to the public key of a freshly sampled key pair
+    * Set the `encryption_key` to the public key of a freshly sampled key pair.
     * Set the parent hash to the parent hash for the leaf.
-    * Re-sign the leaf node with its new contents
+    * Re-sign the leaf node with its new contents.
 
 Since the new leaf node effectively updates an existing leaf node in the group,
-it MUST adhere to the same restrictions as LeafNodes used in `Update` proposals
+it MUST adhere to the same restrictions as LeafNodes used in Update proposals
 (aside from `leaf_node_source`). The application MAY specify other changes to
 the leaf node, e.g., providing a new signature key, updated capabilities, or
 different extensions.
@@ -2598,9 +2198,9 @@ member's filtered direct path, the member takes the following steps:
    this resolution.
 2. For each node in the resolution, encrypt the path secret for the direct
    path node using the public key of the resolution node, as defined in
-   {{update-paths}}
+   {{update-paths}}.
 
-The recipient of an UpdatePath performs the corresponding steps.  First, the
+The recipient of an UpdatePath performs the corresponding steps. First, the
 recipient _merges UpdatePath into the tree_:
 
 1. Blank all nodes on the direct path of the sender's leaf.
@@ -2641,7 +2241,7 @@ values:
 In this table, the value node_pub\[i\] represents the public key
 derived from node_secret\[i\], pk(X) represents the current public key
 of node X, and E(K, S) represents
-the public-key encryption of the path secret S to the
+the public key encryption of the path secret S to the
 public key K (using HPKE).
 
 A recipient at node A would decrypt `E(pk(A), path_secret\[0\])` to obtain
@@ -2668,7 +2268,7 @@ filtered direct path. For each parent node, the UpdatePath contains a new
 public key and encrypted path secret. The parent nodes are kept in the same
 order as the filtered direct path.
 
-~~~ tls
+~~~ tls-presentation
 struct {
     opaque kem_output<V>;
     opaque ciphertext<V>;
@@ -2685,7 +2285,7 @@ struct {
 } UpdatePath;
 ~~~
 
-For each `UpdatePathNode`, the resolution of the corresponding copath node MUST
+For each UpdatePathNode, the resolution of the corresponding copath node MUST
 exclude all new leaf nodes added as part of the current Commit. The length of
 the `encrypted_path_secret` vector MUST be equal to the length of the resolution
 of the copath node (excluding new leaf nodes), with each ciphertext being the
@@ -2732,9 +2332,9 @@ This operation is typically done when adding a member to the group.
 A   B       A   B   _   _                           A   B   C   _
                                                             ^
                                                             |
-                                                            +-- new member
+                                               new member --+
 ~~~
-{: title="Extending the tree to make room for a third member"}
+{: title="Extending the Tree to Make Room for a Third Member"}
 
 When the right subtree of the tree no longer has any non-blank nodes, it can be
 safely removed.  The root of the tree and the right subtree are discarded
@@ -2753,7 +2353,7 @@ typically done after removing a member from the group.
                  |
 removed member --+
 ~~~
-{: title="Cleaning up after removing member C"}
+{: title="Cleaning Up after Removing Member C"}
 
 Concrete algorithms for these operations on array-based and link-based trees are
 provided in {{array-based-trees}} and {{link-based-trees}}.  The concrete
@@ -2778,17 +2378,17 @@ P --> th(P)
     /     \
 th(L)     th(R)
 ~~~
-{: title="Composition of the tree hash" }
+{: title="Composition of the Tree Hash" }
 
-The tree hash of an individual node is the hash of the node's `TreeHashInput`
-object, which may contain either a `LeafNodeHashInput` or a
-`ParentNodeHashInput` depending on the type of node. `LeafNodeHashInput` objects
-contain the `leaf_index` and the `LeafNode` (if any). `ParentNodeHashInput`
-objects contain the `ParentNode` (if any) and the tree hash of the node's left
+The tree hash of an individual node is the hash of the node's TreeHashInput
+object, which may contain either a LeafNodeHashInput or a
+ParentNodeHashInput depending on the type of node. LeafNodeHashInput objects
+contain the `leaf_index` and the LeafNode (if any). ParentNodeHashInput
+objects contain the ParentNode (if any) and the tree hash of the node's left
 and right children.  For both parent and leaf nodes, the optional node value
 MUST be absent if the node is blank and present if the node contains a value.
 
-~~~ tls
+~~~ tls-presentation
 enum {
     reserved(0),
     leaf(1),
@@ -2824,7 +2424,7 @@ which is computed recursively by starting at the leaf nodes and building up.
 While tree hashes summarize the state of a tree at point in time, parent hashes
 capture information about how keys in the tree were populated.
 
-When a client sends a commit to change a group, it can include an UpdatePath to
+When a client sends a Commit to change a group, it can include an UpdatePath to
 assign new keys to the nodes along its filtered direct path.  When a client
 computes an UpdatePath (as defined in {{synchronizing-views-of-the-tree}}), it
 computes and signs a parent hash that summarizes the state of the tree after the
@@ -2844,7 +2444,7 @@ P.public_key --> ph(P)
                V     \
    N.parent_hash     th(S)
 ~~~
-{: title="Inputs to a parent hash" }
+{: title="Inputs to a Parent Hash" }
 
 As a result, the signature over the parent hash in each member's leaf
 effectively signs the subtree of the tree that hasn't been changed since that
@@ -2869,14 +2469,14 @@ leaf node L (for "leaf"):
  /
 L
 ~~~
-{: #parent-hash-nodes title="Nodes involved in a parent hash computation" }
+{: #parent-hash-nodes title="Nodes Involved in a Parent Hash Computation" }
 
-The parent hash of P changes whenever an `UpdatePath` object is applied to
+The parent hash of P changes whenever an UpdatePath object is applied to
 the ratchet tree along a path from a leaf L traversing node D (and hence also
 P). The new "Parent hash of P (with copath child S)" is obtained by hashing P's
-`ParentHashInput` struct.
+ParentHashInput struct.
 
-~~~ tls
+~~~ tls-presentation
 struct {
     HPKEPublicKey encryption_key;
     opaque parent_hash<V>;
@@ -2886,12 +2486,12 @@ struct {
 
 The field `encryption_key` contains the HPKE public key of P. If P is the root,
 then the `parent_hash` field is set to a zero-length octet string. Otherwise,
-`parent_hash` is the Parent Hash of the next node after P on the filtered
-direct path of the leaf L. This way, P's Parent Hash fixes
+`parent_hash` is the parent hash of the next node after P on the filtered
+direct path of the leaf L. This way, P's parent hash fixes
 the new HPKE public key of each non-blank node on the path from P to the root. Note
 that the path from P to the root may contain some blank nodes that are not
-fixed by P's Parent Hash. However, for each node that has an HPKE key, this key
-is fixed by P's Parent Hash.
+fixed by P's parent hash. However, for each node that has an HPKE key, this key
+is fixed by P's parent hash.
 
 Finally, `original_sibling_tree_hash` is the tree hash of S in the ratchet tree
 modified as follows: For each leaf L in `P.unmerged_leaves`, blank L and remove
@@ -2919,7 +2519,7 @@ For example, in the following tree:
  / \     / \     / \     / \
 A   B   C   D   E   F   G   _
 ~~~
-{: #parent-hash-tree title="A tree illustrating parent hash computations." }
+{: #parent-hash-tree title="A Tree Illustrating Parent Hash Computations" }
 
 With P = W and S = Y, `original_sibling_tree_hash` is the tree hash of the
 following tree:
@@ -2941,7 +2541,7 @@ the last time P was updated. This is the case for computing or processing a
 Commit whose UpdatePath traverses P, since the Commit itself resets P. (In
 other words, it is only necessary to recompute the original sibling tree hash
 when validating a group's tree on joining.) More generally, if none of the entries
-in `P.unmerged_leaves` is in the subtree under S (and thus no leaves were blanked),
+in `P.unmerged_leaves` are in the subtree under S (and thus no leaves were blanked),
 then the original tree hash at S is the tree hash of S in the current tree.
 
 If it is necessary to recompute the original tree hash of a node, the efficiency
@@ -2961,10 +2561,11 @@ direct path).  Using the node labels in {{parent-hash-nodes}}, the
 This is the case even when the node D is a leaf node.
 
 The `parent_hash` field of a LeafNode is signed by the member.  The signature of
-such a LeafNode thus also attests to which keys the group member introduced into
-the ratchet tree and to whom the corresponding secret keys were sent. This
+such a LeafNode thus attests to which keys the group member introduced into
+the ratchet tree and to whom the corresponding secret keys were sent, in
+addition to the other contents of the LeafNode. This
 prevents malicious insiders from constructing artificial ratchet trees with a
-node D whose HPKE secret key is known to the insider yet where the insider isn't
+node D whose HPKE secret key is known to the insider, yet where the insider isn't
 assigned a leaf in the subtree rooted at D. Indeed, such a ratchet tree would
 violate the tree invariant.
 
@@ -2976,7 +2577,7 @@ and when processing a Commit.
 The parent hash in a node D is valid with respect to a parent node P if the
 following criteria hold.  Here C and S are the children of P (for "child" and
 "sibling"), with C being the child that is on the direct path of D (possibly D
-itself) and S the other child:
+itself) and S being the other child:
 
 * D is a descendant of P in the tree.
 
@@ -2993,7 +2594,7 @@ between them would have omitted from the filtered direct path.
 A parent node P is "parent-hash valid" if it can be chained back to a leaf node
 in this way.  That is, if there is leaf node L and a sequence of parent nodes
 P\_1, ..., P\_N such that P\_N = P and each step in the chain is authenticated
-by a parent hash: L's parent hash is valid with respect to P\_1, P\_1's parent
+by a parent hash, then L's parent hash is valid with respect to P\_1, P\_1's parent
 hash is valid with respect to P\_2, and so on.
 
 When joining a group, the new member MUST authenticate that each non-blank
@@ -3012,7 +2613,7 @@ chain from the committer's leaf to the root.
 # Key Schedule
 
 Group keys are derived using the `Extract` and `Expand` functions from the KDF
-for the group's ciphersuite, as well as the functions defined below:
+for the group's cipher suite, as well as the functions defined below:
 
 ~~~ pseudocode
 ExpandWithLabel(Secret, Label, Context, Length) =
@@ -3024,7 +2625,7 @@ DeriveSecret(Secret, Label) =
 
 Where KDFLabel is specified as:
 
-~~~ tls
+~~~ tls-presentation
 struct {
     uint16 length;
     opaque label<V>;
@@ -3032,7 +2633,7 @@ struct {
 } KDFLabel;
 ~~~
 
-And its fields set to:
+And its fields are set to:
 
 ~~~ pseudocode
 length = Length;
@@ -3044,8 +2645,8 @@ The value `KDF.Nh` is the size of an output from `KDF.Extract`, in bytes.  In
 the below diagram:
 
 * KDF.Extract takes its salt argument from the top and its Input
-  Key Material (IKM) argument from the left
-* DeriveSecret takes its Secret argument from the incoming arrow
+  Keying Material (IKM) argument from the left.
+* DeriveSecret takes its Secret argument from the incoming arrow.
 * `0` represents an all-zero byte string of length `KDF.Nh`.
 
 When processing a handshake message, a client combines the
@@ -3099,7 +2700,7 @@ psk_secret (or 0) --> KDF.Extract
                           V
                     init_secret_[n]
 ~~~
-{: title="The MLS key schedule" }
+{: title="The MLS Key Schedule" }
 
 A number of values are derived from the epoch secret for different purposes:
 
@@ -3110,10 +2711,10 @@ A number of values are derived from the epoch secret for different purposes:
 | "exporter"       | `exporter_secret`     | Deriving exported secrets                                   |
 | "external"       | `external_secret`     | Deriving the external init key                              |
 | "confirm"        | `confirmation_key`    | Computing the confirmation MAC for an epoch                 |
-| "membership"     | `membership_key`      | Computing the membership MAC for an PublicMessage            |
+| "membership"     | `membership_key`      | Computing the membership MAC for a PublicMessage            |
 | "resumption"     | `resumption_psk`      | Proving membership in this epoch (via a PSK injected later) |
 | "authentication" | `epoch_authenticator` | Confirming that two clients have the same view of the group |
-{: title="Epoch-derived secrets" #epoch-derived-secrets}
+{: title="Epoch-Derived Secrets" #epoch-derived-secrets}
 
 The `external_secret` is used to derive an HPKE key pair whose private key is
 held by the entire group:
@@ -3123,14 +2724,14 @@ external_priv, external_pub = KEM.DeriveKeyPair(external_secret)
 ~~~
 
 The public key `external_pub` can be published as part of the GroupInfo struct
-in order to allow non-members to join the group using an external commit.
+in order to allow non-members to join the group using an external Commit.
 
 ## Group Context
 
 Each member of the group maintains a GroupContext object that
 summarizes the state of the group:
 
-~~~ tls
+~~~ tls-presentation
 struct {
     ProtocolVersion version = mls10;
     CipherSuite cipher_suite;
@@ -3171,7 +2772,7 @@ The following general rules apply:
 * The `tree_hash` is updated to represent the current tree and
   credentials.
 * The `confirmed_transcript_hash` field is updated with the data for an
-  AuthenticatedContent encoding a Commit message as described below.
+  AuthenticatedContent encoding a Commit message, as described below.
 * The `extensions` field changes when a GroupContextExtensions proposal is
   committed.
 
@@ -3180,7 +2781,7 @@ The following general rules apply:
 The transcript hashes computed in MLS represent a running hash over all Proposal
 and Commit messages that have ever been sent in a group.  Commit messages are
 included directly. Proposal messages are indirectly included via the Commit that
-applied them. Both types of message are included by hashing the AuthenticatedContent
+applied them. Messages of both types are included by hashing the AuthenticatedContent
 object in which they were sent.
 
 The transcript hash comprises two individual hashes:
@@ -3197,10 +2798,10 @@ field of the GroupInfo struct, while existing members can compute it directly.
 Each Commit message updates these hashes by way of its enclosing
 AuthenticatedContent.  The AuthenticatedContent struct is split into
 ConfirmedTranscriptHashInput and InterimTranscriptHashInput. The former is used to
-update the confirmed transcript hash and the latter to update the interim
+update the confirmed transcript hash and the latter is used to update the interim
 transcript hash.
 
-~~~ tls
+~~~ tls-presentation
 struct {
     WireFormat wire_format;
     FramedContent content; /* with content_type == commit */
@@ -3279,7 +2880,7 @@ point, except for the latest Commit's confirmation tag.
 
                                                              ...
 ~~~
-{: title="Evolution of the transcript hashes through two epoch changes"}
+{: title="Evolution of the Transcript Hashes through Two Epoch Changes"}
 
 ## External Initialization
 
@@ -3309,8 +2910,8 @@ init_secret = context.export("MLS 1.0 external init secret", KDF.Nh)
 ## Pre-Shared Keys
 
 Groups that already have an out-of-band mechanism to generate
-shared group secrets can inject those into the MLS key schedule to seed
-the MLS group secrets computations by this external entropy.
+shared group secrets can inject them into the MLS key schedule to
+incorporate this external entropy in the computation of MLS group secrets.
 
 Injecting an external PSK can improve security in the case
 where having a full run of Updates across members is too expensive, or if
@@ -3318,13 +2919,13 @@ the external group key establishment mechanism provides
 stronger security against classical or quantum adversaries.
 
 Note that, as a PSK may have a different lifetime than an Update, it does not
-necessarily provide the same Forward Secrecy (FS) or Post-Compromise Security
-(PCS) guarantees as a Commit message.  Unlike the key pairs populated in the
+necessarily provide the same forward secrecy or post-compromise security
+guarantees as a Commit message.  Unlike the key pairs populated in the
 tree by an Update or Commit, which are always freshly generated, PSKs may be
 pre-distributed and stored. This creates the risk that a PSK may be compromised
 in the process of distribution and storage. The security that the group gets
 from injecting a PSK thus depends on both the entropy of the PSK and the risk of
-compromise.  These factors are outside of the scope of this document, but should
+compromise.  These factors are outside of the scope of this document, but they should
 be considered by application designers relying on PSKs.
 
 Each PSK in MLS has a type that designates how it was provisioned.
@@ -3339,7 +2940,7 @@ Welcome message corresponding to the Commit.  To ensure that existing and new
 members compute the same PSK input to the key schedule, the Commit and
 GroupSecrets objects MUST indicate the same set of PSKs, in the same order.
 
-~~~ tls
+~~~ tls-presentation
 enum {
   reserved(0),
   external(1),
@@ -3372,18 +2973,18 @@ struct {
 
 Each time a client injects a PSK into a group, the `psk_nonce` of its
 PreSharedKeyID MUST be set to a fresh random value of length `KDF.Nh`, where
-`KDF` is the KDF for the ciphersuite of the group into which the PSK is being
+`KDF` is the KDF for the cipher suite of the group into which the PSK is being
 injected. This ensures that even when a PSK is used multiple times, the value
 used as an input into the key schedule is different each time.
 
-Upon receiving a Commit with a `PreSharedKey` proposal or a GroupSecrets object
-with the `psks` field set, the receiving Client includes them in the key
-schedule in the order listed in the Commit, or in the `psks` field respectively.
+Upon receiving a Commit with a PreSharedKey proposal or a GroupSecrets object
+with the `psks` field set, the receiving client includes them in the key
+schedule in the order listed in the Commit, or in the `psks` field, respectively.
 For resumption PSKs, the PSK is defined as the `resumption_psk` of the group and
-epoch specified in the `PreSharedKeyID` object. Specifically, `psk_secret` is
+epoch specified in the PreSharedKeyID object. Specifically, `psk_secret` is
 computed as follows:
 
-~~~ tls
+~~~ tls-presentation
 struct {
     PreSharedKeyID id;
     uint16 index;
@@ -3402,7 +3003,7 @@ psk_secret     = psk_secret_[n]
 ~~~
 
 Here `0` represents the all-zero vector of length `KDF.Nh`. The `index` field in
-`PSKLabel` corresponds to the index of the PSK in the `psk` array, while the
+PSKLabel corresponds to the index of the PSK in the `psk` array, while the
 `count` field contains the total number of PSKs.  In other words, the PSKs are
 chained together with KDF.Extract invocations (labeled "Extract" for brevity
 in the diagram), as follows:
@@ -3423,14 +3024,14 @@ psk_[1]   --> Extract --> ExpandWithLabel --> Extract = psk_secret_[2]
                  V                               V
 psk_[n-1] --> Extract --> ExpandWithLabel --> Extract = psk_secret_[n]
 ~~~
-{: title="Computatation of a PSK secret from a set of PSKs" }
+{: title="Computation of a PSK Secret from a Set of PSKs" }
 
 In particular, if there are no PreSharedKey proposals in a given Commit, then
 the resulting `psk_secret` is `psk_secret_[0]`, the all-zero vector.
 
 ## Exporters
 
-The main MLS key schedule provides an `exporter_secret` which can
+The main MLS key schedule provides an `exporter_secret` that can
 be used by an application to derive new secrets for use outside of MLS.
 
 ~~~ pseudocode
@@ -3457,7 +3058,7 @@ to refresh those values after a Commit is processed.
 The main MLS key schedule provides a `resumption_psk` that is used as a PSK
 to inject entropy from one epoch into another.  This functionality is used in the
 reinitialization and branching processes described in {{reinitialization}} and
-{{subgroup-branching}}, but may be used by applications for other purposes.
+{{subgroup-branching}}, but it may be used by applications for other purposes.
 
 Some uses of resumption PSKs might call for the use of PSKs from historical
 epochs. The application SHOULD specify an upper limit on the number of past
@@ -3476,7 +3077,7 @@ example, if the users of the clients in an MLS group were to meet in person and
 reliably confirm that their epoch authenticator values were equal (using some
 suitable user interface), then each user would be assured that the others were
 not being impersonated in the current epoch. As soon as the epoch changed,
-though, they would need to re-do this confirmation. The state of the group would
+though, they would need to redo this confirmation. The state of the group would
 have changed, possibly introducing an attacker.
 
 More generally, in order for the members of an MLS group to obtain concrete
@@ -3489,11 +3090,11 @@ protections provided to the MLS group.
 
 For the generation of encryption keys and nonces, the key schedule begins with
 the `encryption_secret` at the root and derives a tree of secrets with the same
-structure as the group's ratchet tree. Each leaf in the Secret Tree is
+structure as the group's ratchet tree. Each leaf in the secret tree is
 associated with the same group member as the corresponding leaf in the ratchet
 tree.
 
-If N is a parent node in the Secret Tree then the secrets of the children of N
+If N is a parent node in the secret tree, then the secrets of the children of N
 are defined as follows (where left(N) and right(N) denote the children of N):
 
 ~~~ aasvg
@@ -3506,9 +3107,9 @@ tree_node_[N]_secret
         +--> ExpandWithLabel(., "tree", "right", KDF.Nh)
              = tree_node_[right(N)]_secret
 ~~~
-{: title="Derivation of secrets from parent to children within a secret tree" }
+{: title="Derivation of Secrets from Parent to Children within a Secret Tree" }
 
-The secret in the leaf of the Secret Tree is used to initiate two symmetric hash
+The secret in the leaf of the secret tree is used to initiate two symmetric hash
 ratchets, from which a sequence of single-use keys and nonces are derived, as
 described in {{encryption-keys}}. The root of each ratchet is computed as:
 
@@ -3522,7 +3123,7 @@ tree_node_[N]_secret
         +--> ExpandWithLabel(., "application", "", KDF.Nh)
              = application_ratchet_secret_[N]_[0]
 ~~~
-{: title="Initialization of the hash ratchets from the leaves of a secret tree" }
+{: title="Initialization of the Hash Ratchets from the Leaves of a Secret Tree" }
 
 ## Encryption Keys
 
@@ -3564,11 +3165,11 @@ first two application keys and nonces.
                 |
                AR2
 ~~~
-{: title="Secret tree for a four-member group" #secret-tree-example}
+{: title="Secret Tree for a Four-Member Group" #secret-tree-example}
 
 A sender ratchet starts from a per-sender base secret derived from a Secret
 Tree, as described in {{secret-tree}}. The base secret initiates a symmetric
-hash ratchet which generates a sequence of keys and nonces. The sender uses the
+hash ratchet, which generates a sequence of keys and nonces. The sender uses the
 j-th key/nonce pair in the sequence to encrypt (using the AEAD) the j-th message
 they send during that epoch. Each key/nonce pair MUST NOT be used to encrypt
 more than one message.
@@ -3598,24 +3199,24 @@ DeriveTreeSecret(., "secret", j, KDF.Nh)
 = ratchet_secret_[N]_[j+1]
 ~~~
 
-Here, `AEAD.Nn` and `AEAD.Nk` denote the lengths
+Here `AEAD.Nn` and `AEAD.Nk` denote the lengths
 in bytes of the nonce and key for the AEAD scheme defined by
-the ciphersuite.
+the cipher suite.
 
 ## Deletion Schedule
 
 It is important to delete all security-sensitive values as soon as they are
-_consumed_. A sensitive value S is said to be _consumed_ if
+_consumed_. A sensitive value S is said to be _consumed_ if:
 
-* S was used to encrypt or (successfully) decrypt a message, or if
+* S was used to encrypt or (successfully) decrypt a message, or
 * a key, nonce, or secret derived from S has been consumed. (This goes for
   values derived via DeriveSecret as well as ExpandWithLabel.)
 
-Here, S may be the `init_secret`, `commit_secret`, `epoch_secret`,
-`encryption_secret` as well as any secret in a Secret Tree or one of the
+Here S may be the `init_secret`, `commit_secret`, `epoch_secret`, or
+`encryption_secret` as well as any secret in a secret tree or one of the
 ratchets.
 
-As soon as a group member consumes a value they MUST immediately delete
+As soon as a group member consumes a value, they MUST immediately delete
 (all representations of) that value. This is crucial to ensuring
 forward secrecy for past messages. Members MAY keep unconsumed values around
 for some reasonable amount of time to handle out-of-order message delivery.
@@ -3625,14 +3226,14 @@ application message using the j-th key and nonce in the ratchet of leaf node
 L in some epoch n. Then, for that member, at least the following
 values have been consumed and MUST be deleted:
 
-* the `commit_secret`, `joiner_secret`, `epoch_secret`, `encryption_secret` of
+* the `commit_secret`, `joiner_secret`, `epoch_secret`, and `encryption_secret` of
   that epoch n as well as the `init_secret` of the previous epoch n-1,
-* all node secrets in the Secret Tree on the path from the root to the leaf with
+* all node secrets in the secret tree on the path from the root to the leaf with
   node L,
-* the first j secrets in the application data ratchet of node L and
+* the first j secrets in the application data ratchet of node L, and
 * `application_ratchet_nonce_[L]_[j]` and `application_ratchet_key_[L]_[j]`.
 
-Concretely, consider the Secret Tree shown in {{secret-tree-example}}.  Client
+Concretely, consider the secret tree shown in {{secret-tree-example}}.  Client
 A, B, or C would generate the illustrated values on receiving a message from D
 with generation equal to 1, having not received a message with generation 0
 (e.g., due to out-of-order delivery).  In such a case, the following values
@@ -3640,7 +3241,7 @@ would be consumed:
 
 * The key K1 and nonce N1 used to decrypt the message
 * The application ratchet secrets AR1 and AR0
-* The tree secrets D, F, G (recall that G is the `encryption_secret` for the
+* The tree secrets D, F, and G (recall that G is the `encryption_secret` for the
   epoch)
 * The `epoch_secret`, `commit_secret`, `psk_secret`, and `joiner_secret` for the
   current epoch
@@ -3655,33 +3256,33 @@ Other values may be retained (not consumed):
 # Key Packages
 
 In order to facilitate the asynchronous addition of clients to a
-group, key packages are pre-published that
+group, clients can pre-publish KeyPackage objects that
 provide some public information about a user. A KeyPackage object specifies:
 
-1. A protocol version and ciphersuite that the client supports,
+1. a protocol version and cipher suite that the client supports,
 2. a public key that others can use to encrypt a Welcome message to this client
    (an "init key"), and
 3. the content of the leaf node that should be added to the tree to represent
    this client.
 
 KeyPackages are intended to be used only once and SHOULD NOT
-be reused except in the case of last resort (see {{keypackage-reuse}}).
+be reused except in the case of a "last resort" KeyPackage (see {{keypackage-reuse}}).
 Clients MAY generate and publish multiple KeyPackages to
-support multiple ciphersuites.
+support multiple cipher suites.
 
 The value for `init_key` MUST be a public key for the asymmetric encryption
 scheme defined by `cipher_suite`, and it MUST be unique among the set of
 KeyPackages created by this client.  Likewise, the `leaf_node` field MUST be
-valid for the ciphersuite, including both the `encryption_key` and
+valid for the cipher suite, including both the `encryption_key` and
 `signature_key` fields.  The whole structure is signed using the client's
 signature key. A KeyPackage object with an invalid signature field MUST be
 considered malformed.
 
 The signature is computed by the function `SignWithLabel` with a label
-`KeyPackageTBS` and a `Content` input comprising all of the fields except for the
+`"KeyPackageTBS"` and a `Content` input comprising all of the fields except for the
 signature field.
 
-~~~ tls
+~~~ tls-presentation
 struct {
     ProtocolVersion version;
     CipherSuite cipher_suite;
@@ -3708,12 +3309,12 @@ provides an explicit signal of the intended version to the other members of
 group when they receive the KeyPackage in an Add proposal.
 
 The field `leaf_node.capabilities` indicates what protocol versions,
-ciphersuites, credential types, and non-default proposal/extension types are supported
-by the client.  (Proposal and extension types defined in this document are considered
-"default" and not listed.)  This information allows MLS session
+cipher suites, credential types, and non-default proposal/extension types are supported
+by the client.  (As discussed in {{leaf-node-contents}}, some proposal and extension types defined in this document are considered
+"default" and thus are not listed.)  This information allows MLS session
 establishment to be safe from downgrade attacks on the parameters described (as
-discussed in {{group-creation}}), while still only advertising one version /
-ciphersuite per KeyPackage.
+discussed in {{group-creation}}), while still only advertising one version and
+one cipher suite per KeyPackage.
 
 The field `leaf_node.leaf_node_source` of the LeafNode in a KeyPackage MUST be
 set to `key_package`.
@@ -3721,7 +3322,7 @@ set to `key_package`.
 Extensions included in the `extensions` or `leaf_node.extensions` fields MUST
 be included in the `leaf_node.capabilities` field.  As discussed in
 {{extensibility}}, unknown extensions in `KeyPackage.extensions` MUST be
-ignored, and the creator of a `KeyPackage` object SHOULD include some random GREASE
+ignored, and the creator of a KeyPackage object SHOULD include some random GREASE
 extensions to help ensure that other clients correctly ignore unknown
 extensions.
 
@@ -3735,8 +3336,8 @@ The validity of a KeyPackage needs to be verified at a few stages:
 
 The client verifies the validity of a KeyPackage using the following steps:
 
-* Verify that the ciphersuite and protocol version of the KeyPackage match
-  those in the `GroupContext`.
+* Verify that the cipher suite and protocol version of the KeyPackage match
+  those in the GroupContext.
 
 * Verify that the `leaf_node` of the KeyPackage is valid for a KeyPackage
   according to {{leaf-node-validation}}.
@@ -3752,25 +3353,26 @@ The client verifies the validity of a KeyPackage using the following steps:
 A group is always created with a single member, the "creator".  Other members
 are then added to the group using the usual Add/Commit mechanism.
 
-The creator of a group is responsible for setting the group ID, ciphersuite, and
+The creator of a group is responsible for setting the group ID, cipher suite, and
 initial extensions for the group.  If the creator intends to add other members
-at the time of creation, then it SHOULD Fetch KeyPackages for the members to be
-added, and select a ciphersuite and extensions according to the capabilities of
+at the time of creation, then it SHOULD fetch KeyPackages for the members to be
+added, and select a cipher suite and extensions according to the capabilities of
 the members.  To protect against downgrade attacks, the creator MUST use the
 `capabilities` information in these KeyPackages to verify that the chosen
-version and ciphersuite is the best option supported by all members.
+version and cipher suite is the best option supported by all members.
 
-Group IDs SHOULD be constructed in such a way that there's an overwhelmingly low
+Group IDs SHOULD be constructed in such a way that there is an overwhelmingly low
 probability of honest group creators generating the same group ID, even without
-assistance from the Delivery Service. For example, by making the group ID a
+assistance from the Delivery Service. This can be done, for example, by making the group ID a
 freshly generated random value of size `KDF.Nh`. The Delivery Service MAY
 attempt to ensure that group IDs are globally unique by rejecting the creation
 of new groups with a previously used ID.
 
-The creator of a group MUST take the following steps to initialize the group:
+To initialize a group, the creator of the group MUST take the
+following steps:
 
 * Initialize a one-member group with the following initial values:
-  * Ratchet tree: A tree with a single node, a leaf containing an HPKE public
+  * Ratchet tree: A tree with a single node, a leaf node containing an HPKE public
     key and credential for the creator
   * Group ID: A value set by the creator
   * Epoch: 0
@@ -3786,7 +3388,7 @@ The creator of a group MUST take the following steps to initialize the group:
     using the `confirmation_key` as described in {{content-authentication}}.
   * Compute the updated `interim_transcript_hash` from the
     `confirmed_transcript_hash` and the `confirmation_tag` as described in
-    {{transcript-hashes}}
+    {{transcript-hashes}}.
 
 At this point, the creator's state represents a one-member group with a fully
 initialized key schedule, transcript hashes, etc.  Proposals and Commits can be
@@ -3808,11 +3410,11 @@ KeyPackage and the leaf secret from which the Commit is built.
 ## Required Capabilities
 
 The configuration of a group imposes certain requirements on clients in the
-group.  At a minimum, all members of the group need to support the ciphersuite
+group.  At a minimum, all members of the group need to support the cipher suite
 and protocol version in use.  Additional requirements can be imposed by
 including a `required_capabilities` extension in the GroupContext.
 
-~~~ tls
+~~~ tls-presentation
 struct {
     ExtensionType extension_types<V>;
     ProposalType proposal_types<V>;
@@ -3839,14 +3441,14 @@ A group may be reinitialized by creating a new group with the same membership
 and different parameters, and linking it to the old group via a resumption PSK.
 The members of a group reinitialize it using the following steps:
 
-1. A member of the old group sends a ReInit proposal (see {{reinit}})
-2. A member of the old group sends a Commit covering the ReInit proposal
-3. A member of the old group creates an initial Commit setting up a new group
-   that matches the ReInit and sends a Welcome message
+1. A member of the old group sends a ReInit proposal (see {{reinit}}).
+2. A member of the old group sends a Commit covering the ReInit proposal.
+3. A member of the old group creates an initial Commit that sets up a new group
+   that matches the ReInit and sends a Welcome message:
     * The `version`, `cipher_suite`, `group_id`, and `extensions` fields of the GroupContext object in the Welcome
       message MUST be the same as the corresponding fields in the ReInit
       proposal. The `epoch` in the Welcome message MUST be 1.
-    * The Welcome MUST specify a PreSharedKeyID of type `resumption` with usage
+    * The Welcome message MUST specify a PreSharedKeyID of type `resumption` with usage
       `reinit`, where the `group_id` field matches the old group and the `epoch`
       field indicates the epoch after the Commit covering the ReInit.
 
@@ -3871,15 +3473,15 @@ A member can create a subgroup by performing the following steps:
    subgroup.
 2. Create an initial Commit message that sets up the new group and contains a
    PreSharedKey proposal of type `resumption` with usage `branch`. To avoid key
-   re-use, the `psk_nonce` included in the `PreSharedKeyID` object MUST be a
+   reuse, the `psk_nonce` included in the PreSharedKeyID object MUST be a
    randomly sampled nonce of length `KDF.Nh`.
 3. Send the corresponding Welcome message to the subgroup members.
 
-A client receiving a Welcome including a PreSharedKey of type `resumption` with
+A client receiving a Welcome message including a PreSharedKey of type `resumption` with
 usage `branch` MUST verify that the new group reflects a subgroup branched from
-the referenced group by checking:
+the referenced group by checking that:
 
-* The `version` and `ciphersuite` values in the Welcome are the same as
+* The `version` and `cipher_suite` values in the Welcome message are the same as
   those used by the old group.
 * The `epoch` in the Welcome message MUST be 1.
 * Each LeafNode in a new subgroup MUST match some LeafNode in the original
@@ -3897,9 +3499,10 @@ Over the lifetime of a group, its membership can change, and existing members
 might want to change their keys in order to achieve post-compromise security.
 In MLS, each such change is accomplished by a two-step process:
 
-1. A proposal to make the change is broadcast to the group in a Proposal message
+1. A proposal to make the change is broadcast to the group in a Proposal
+   message.
 2. A member of the group or a new member broadcasts a Commit message that causes
-   one or more proposed changes to enter into effect
+   one or more proposed changes to enter into effect.
 
 In cases where the Proposal and Commit are sent by the same member, these two steps
 can be combined by sending the proposals in the commit.
@@ -3915,8 +3518,8 @@ a state transition occurs, the epoch number is incremented by one.
 Proposals are included in a FramedContent by way of a Proposal structure
 that indicates their type:
 
-~~~ tls
-// See IANA registry for registered values
+~~~ tls-presentation
+// See the "MLS Proposal Types" IANA registry for values
 uint16 ProposalType;
 
 struct {
@@ -3944,7 +3547,7 @@ that it can be retrieved by hash (as a ProposalOrRef object) in a later Commit m
 An Add proposal requests that a client with a specified KeyPackage be added
 to the group.
 
-~~~ tls
+~~~ tls-presentation
 struct {
     KeyPackage key_package;
 } Add;
@@ -3962,7 +3565,7 @@ the right.
 
 * Identify the leaf L for the new member: if there are empty leaves in the tree,
   L is the leftmost empty leaf. Otherwise, the tree is extended to the right as
-  described in {{adding-and-removing-leaves}} and L is assigned the leftmost new
+  described in {{adding-and-removing-leaves}}, and L is assigned the leftmost new
   blank leaf.
 
 * For each non-blank intermediate node along the path from the leaf L
@@ -3977,7 +3580,7 @@ An Update proposal is a similar mechanism to Add with the distinction
 that it replaces the sender's LeafNode in the tree instead of adding a new leaf
 to the tree.
 
-~~~ tls
+~~~ tls-presentation
 struct {
     LeafNode leaf_node;
 } Update;
@@ -3989,16 +3592,17 @@ proposal according to {{leaf-node-validation}}.
 
 A member of the group applies an Update message by taking the following steps:
 
-* Replace the sender's LeafNode with the one contained in the Update proposal
+* Replace the sender's LeafNode with the one contained in the Update proposal.
 
-* Blank the intermediate nodes along the path from the sender's leaf to the root
+* Blank the intermediate nodes along the path from the sender's leaf to the
+  root.
 
 ### Remove
 
 A Remove proposal requests that the member with the leaf index `removed` be removed
 from the group.
 
-~~~ tls
+~~~ tls-presentation
 struct {
     uint32 removed;
 } Remove;
@@ -4011,21 +3615,21 @@ A member of the group applies a Remove message by taking the following steps:
 
 * Identify the leaf node matching `removed`.  Let L be this leaf node.
 
-* Replace the leaf node L with a blank node
+* Replace the leaf node L with a blank node.
 
-* Blank the intermediate nodes along the path from L to the root
+* Blank the intermediate nodes along the path from L to the root.
 
 * Truncate the tree by removing the right subtree until there is at least one
   non-blank leaf node in the right subtree.  If the rightmost non-blank leaf has
-  index L, then this will result in the tree having `2^d` leaves, where `d` is
-  the smallest value such that `2^d > L`.
+  index L, then this will result in the tree having 2<sup>d</sup> leaves, where `d` is
+  the smallest value such that 2<sup>d</sup> > `L`.
 
 ### PreSharedKey
 
 A PreSharedKey proposal can be used to request that a pre-shared key be
 injected into the key schedule in the process of advancing the epoch.
 
-~~~ tls
+~~~ tls-presentation
 struct {
     PreSharedKeyID psk;
 } PreSharedKey;
@@ -4046,17 +3650,17 @@ A PreSharedKey proposal is invalid if any of the following is true:
 The `psk_nonce` MUST be randomly sampled. When processing
 a Commit message that includes one or more PreSharedKey proposals, group
 members derive `psk_secret` as described in {{pre-shared-keys}}, where the
-order of the PSKs corresponds to the order of the `PreSharedKey` proposals
+order of the PSKs corresponds to the order of the PreSharedKey proposals
 in the Commit.
 
 ### ReInit
 
 A ReInit proposal represents a request to reinitialize the group with different
 parameters, for example, to increase the version number or to change the
-ciphersuite. The reinitialization is done by creating a completely new group
+cipher suite. The reinitialization is done by creating a completely new group
 and shutting down the old one.
 
-~~~ tls
+~~~ tls-presentation
 struct {
     opaque group_id<V>;
     ProtocolVersion version;
@@ -4078,7 +3682,7 @@ send the Welcome message that matches the ReInit, according to the criteria in
 An ExternalInit proposal is used by new members that want to join a group by
 using an external commit. This proposal can only be used in that context.
 
-~~~ tls
+~~~ tls-presentation
 struct {
   opaque kem_output<V>;
 } ExternalInit;
@@ -4093,7 +3697,7 @@ The `kem_output` field contains the required KEM output.
 A GroupContextExtensions proposal is used to update the list of extensions in
 the GroupContext for the group.
 
-~~~ tls
+~~~ tls-presentation
 struct {
   Extension extensions<V>;
 } GroupContextExtensions;
@@ -4101,14 +3705,14 @@ struct {
 
 A GroupContextExtensions proposal is invalid if it includes a
 `required_capabilities` extension and some members of the group do not support
-some of the required capabilities (including those added in the same commit,
+some of the required capabilities (including those added in the same Commit,
 and excluding those removed).
 
 A member of the group applies a GroupContextExtensions proposal with the
 following steps:
 
 * Remove all of the existing extensions from the GroupContext object for the
-  group and replacing them with the list of extensions in the proposal.  (This
+  group and replace them with the list of extensions in the proposal.  (This
   is a wholesale replacement, not a merge. An extension is only carried over if
   the sender of the proposal includes it in the new list.)
 
@@ -4119,26 +3723,27 @@ group agree on the extensions in use.
 ### External Proposals
 
 Proposals can be constructed and sent to the group by a party
-that is outside the group in two cases. One case, indicated by an `external` SenderType
-is useful in cases where, for example, an automated service might propose to
-remove a member of a group who has been inactive for a long time, or propose adding
-a newly-hired staff member to a group representing a real-world team.
-An `external` sender might send a ReInit proposal, to enforce a changed policy
-regarding MLS version or ciphersuite.
+that is outside the group in two cases. One case, indicated by the `external` SenderType,
+allows an entity outside the group to submit proposals to the group.
+For example, an automated service might propose
+removing a member of a group who has been inactive for a long time, or propose adding
+a newly hired staff member to a group representing a real-world team.
+An `external` sender might send a ReInit proposal to enforce a changed policy
+regarding MLS versions or cipher suites.
 
 The `external` SenderType requires that signers are pre-provisioned
 to the clients within a group and can only be used if the
 `external_senders` extension is present in the group's GroupContext.
 
-The other case, indicated by a `new_member_proposal` SenderType is useful when
-existing members of the group can independently authorize the addition of an
-MLS client proposing it be added to the group. External proposals which are not
-authorized are considered invalid.
+The other case, indicated by the `new_member_proposal` SenderType, is useful
+when existing members of the group can independenly verify that an Add proposal
+sent by the new joiner itself (not an existing member) is authorized. External
+proposals that are not authorized are considered invalid.
 
 An external proposal MUST be sent as a PublicMessage object, since the sender
 will not have the keys necessary to construct a PrivateMessage object.
 
-Some types of proposal cannot be sent by an `external` sender.  Among the
+Proposals of some types cannot be sent by an `external` sender.  Among the
 proposal types defined in this document, only the following types may be sent by
 an `external` sender:
 
@@ -4150,8 +3755,8 @@ an `external` sender:
 
 Messages from `external` senders containing proposal types other than the above
 MUST be rejected as malformed.  New proposal types defined in the future MUST
-define whether they may be sent by `external` senders.  A column is defined in
-the relevant IANA registry ({{mls-proposal-types}}) to reflect this property.
+define whether they may be sent by `external` senders.  The "Ext" column in
+the "MLS Proposal Types" registry ({{mls-proposal-types}}) reflects this property.
 
 #### External Senders Extension
 
@@ -4159,7 +3764,7 @@ The `external_senders` extension is a group context extension that contains
 the credentials and signature keys of senders that are permitted to send
 external proposals to the group.
 
-~~~ tls
+~~~ tls-presentation
 struct {
   SignaturePublicKey signature_key;
   Credential credential;
@@ -4170,13 +3775,13 @@ ExternalSender external_senders<V>;
 
 ## Proposal List Validation
 
-A group member creating a commit and a group member processing a Commit
+A group member creating a Commit and a group member processing a Commit
 MUST verify that the list of committed proposals is valid using one of the following
-procedures, depending on whether the commit is external or not.  If the list of
+procedures, depending on whether the Commit is external or not.  If the list of
 proposals is invalid, then the Commit message MUST be rejected as invalid.
 
-For a regular, i.e. not external, commit the list is invalid if any of the following
-occurs:
+For a regular, i.e., not external, Commit, the list is invalid if any of the
+following occurs:
 
 * It contains an individual proposal that is invalid as specified in {{proposals}}.
 
@@ -4206,32 +3811,32 @@ epoch.
 
 * It contains an ExternalInit proposal.
 
-* It contains a proposal with a non-default proposal type that is not supported by some
+* It contains a Proposal with a non-default proposal type that is not supported by some
 members of the group that will process the Commit (i.e., members being added
 or removed by the Commit do not need to support the proposal type).
 
-* After processing the commit the ratchet tree is invalid, in particular, if it
+* After processing the Commit the ratchet tree is invalid, in particular, if it
 contains any leaf node that is invalid according to {{leaf-node-validation}}.
 
 An application may extend the above procedure by additional rules, for example,
 requiring application-level permissions to add members, or rules concerning
 non-default proposal types.
 
-For an external commit, the list is valid if it contains only the following proposals
+For an external Commit, the list is valid if it contains only the following proposals
 (not necessarily in this order):
 
 * Exactly one ExternalInit
 
 * At most one Remove proposal, with which the joiner removes an
 old version of themselves. If a Remove proposal is present, then the LeafNode in the
-`path` field of the external commit MUST meet the same criteria as would the LeafNode
+`path` field of the external Commit MUST meet the same criteria as would the LeafNode
 in an Update for the removed leaf (see {{update}}). In particular, the `credential`
 in the LeafNode MUST present a set of identifiers that is acceptable to the
 application for the removed participant.
 
-* Zero or more PreSharedKey proposals.
+* Zero or more PreSharedKey proposals
 
-* No other proposals.
+* No other proposals
 
 Proposal types defined in the future may make updates to the above validation
 logic to incorporate considerations related to proposals of the new type.
@@ -4245,7 +3850,7 @@ the proposals in the list in the following order:
 
 * If there is a GroupContextExtensions proposal, replace the `extensions` field
   of the GroupContext for the group with the contents of the proposal.  The
-  new `extensions` MUST be used for evaluating other proposals in this list. For
+  new `extensions` MUST be used when evaluating other proposals in this list. For
   example, if a GroupContextExtensions proposal adds a `required_capabilities`
   extension, then any Add proposals need to indicate support for those
   capabilities.
@@ -4282,7 +3887,7 @@ for previously sent proposals from anyone (including the committer) can be sent
 by reference.  Proposals sent by reference are specified by including the hash of
 the AuthenticatedContent object in which the proposal was sent (see {{hash-based-identifiers}}).
 
-~~~ tls
+~~~ tls-presentation
 enum {
   reserved(0),
   proposal(1),
@@ -4315,7 +3920,7 @@ it includes an Update and a Remove for the same member, or an Add when the sende
 the application-level permission to add new users.
 
 The sender of a Commit SHOULD include all proposals that it has received
-during the current epoch, that are valid according to the rules for their
+during the current epoch that are valid according to the rules for their
 proposal types and according to application policy, as long as this results in
 a valid proposal list.
 
@@ -4346,7 +3951,7 @@ The only proposal types defined in this document that do not require a path are:
 New proposal types MUST state whether they require a path. If any instance of a
 proposal type requires a path, then the proposal type requires a path. This
 attribute of a proposal type is reflected in the "Path Required" field of the
-proposal type registry defined in {{mls-proposal-types}}.
+"MLS Proposal Types" registry defined in {{mls-proposal-types}}.
 
 Update and Remove proposals are the clearest examples of proposals that require
 a path.  An UpdatePath is required to evict the removed member or the old
@@ -4380,7 +3985,7 @@ uses:
    contribution to the group and provides PCS with regard to the committer.
 
 2. A "partial" Commit that references proposals that do not require a path, and
-   where the path is empty. Such a commit doesn't provide PCS with regard to the
+   where the path is empty. Such a Commit doesn't provide PCS with regard to the
    committer.
 
 3. A "full" Commit that references proposals of any type, which provides FS with
@@ -4402,21 +4007,20 @@ message at the same time, by taking the following steps:
 * Verify that the list of proposals to be committed is valid as specified in
   {{proposal-list-validation}}.
 
-* Construct an initial Commit object with the `proposals`
-  field populated from Proposals received during the current epoch, and an empty
-  `path` field.
+* Construct an initial Commit object with the `proposals` field populated from
+  Proposals received during the current epoch, and with the `path` field empty.
 
 * Create the new ratchet tree and GroupContext by applying the list of proposals
   to the old ratchet tree and GroupContext, as defined in
-  {{applying-a-proposal-list}}
+  {{applying-a-proposal-list}}.
 
 * Decide whether to populate the `path` field: If the `path` field is required
-  based on the proposals that are in the commit (see above), then it MUST be
+  based on the proposals that are in the Commit (see above), then it MUST be
   populated.  Otherwise, the sender MAY omit the `path` field at its discretion.
 
 * If populating the `path` field:
 
-  * If this is an external commit, assign the sender the leftmost blank leaf
+  * If this is an external Commit, assign the sender the leftmost blank leaf
     node in the new ratchet tree.  If there are no blank leaf nodes in the new
     ratchet tree, expand the tree to the right as defined in
     {{adding-and-removing-leaves}} and assign the leftmost new blank leaf to the
@@ -4455,8 +4059,8 @@ message at the same time, by taking the following steps:
   FramedContent using the old GroupContext as context.
   * Use the FramedContent to update the confirmed transcript hash and update
     the new GroupContext.
-  * Use the `init_secret` from the previous epoch, the `commit_secret` and the
-    `psk_secret` as defined in the previous steps, and the new GroupContext to
+  * Use the `init_secret` from the previous epoch, the `commit_secret` and
+    `psk_secret` defined in the previous steps, and the new GroupContext to
     compute the new `joiner_secret`, `welcome_secret`, `epoch_secret`, and
     derived secrets for the new epoch.
   * Use the `confirmation_key` for the new epoch to compute the
@@ -4472,28 +4076,30 @@ message at the same time, by taking the following steps:
     handshake ratchet.
 
 * Construct a GroupInfo reflecting the new state:
-  * Group ID, epoch, tree, confirmed transcript hash, interim transcript
-    hash, and group context extensions from the new state
-  * The confirmation_tag from the FramedContentAuthData object
-  * Other extensions as defined by the application
-  * Optionally derive an external keypair as described in {{key-schedule}}
-    (required for External Commits, see {{joining-via-external-commits}})
-  * Sign the GroupInfo using the member's private signing key
-  * Encrypt the GroupInfo using the key and nonce derived from the `joiner_secret`
-    for the new epoch (see {{joining-via-welcome-message}})
+  * Set the `group_id`, `epoch`, `tree`, `confirmed_transcript_hash`,
+    `interim_transcript_hash`, and `group_context_extensions` fields to reflect
+    the new state.
+  * Set the `confirmation_tag` field to the value of the corresponding field in
+    the FramedContentAuthData object.
+  * Add any other extensions as defined by the application.
+  * Optionally derive an external key pair as described in {{key-schedule}}.
+    (required for external Commits, see {{joining-via-external-commits}}).
+  * Sign the GroupInfo using the member's private signing key.
+  * Encrypt the GroupInfo using the key and nonce derived from the `joiner_secret`.
+    for the new epoch (see {{joining-via-welcome-message}}).
 
 * For each new member in the group:
   * Identify the lowest common ancestor in the tree of the new member's
-    leaf node and the member sending the Commit
+    leaf node and the member sending the Commit.
   * If the `path` field was populated above: Compute the path secret
-    corresponding to the common ancestor node
+    corresponding to the common ancestor node.
   * Compute an EncryptedGroupSecrets object that encapsulates the `init_secret`
     for the current epoch and the path secret (if present).
 
 * Construct one or more Welcome messages from the encrypted GroupInfo object,
   the encrypted key packages, and any PSKs for which a proposal was included in
   the Commit. The order of the `psks` MUST be the same as the order of
-  PreSharedKey proposals in the `proposals` vector.  As discussed on
+  PreSharedKey proposals in the `proposals` vector.  As discussed in
   {{joining-via-welcome-message}}, the committer is free to choose how many
   Welcome messages to construct.  However, the set of Welcome messages produced
   in this step MUST cover every new member added in the Commit.
@@ -4501,7 +4107,7 @@ message at the same time, by taking the following steps:
 * If a ReInit proposal was part of the Commit, the committer MUST create a new
   group with the parameters specified in the ReInit proposal,
   and with the same members as the original group.
-  The Welcome message MUST include a `PreSharedKeyID` with the following
+  The Welcome message MUST include a PreSharedKeyID with the following
   parameters:
   * `psktype`: `resumption`
   * `usage`: `reinit`
@@ -4513,25 +4119,26 @@ message at the same time, by taking the following steps:
 A member of the group applies a Commit message by taking the following steps:
 
 * Verify that the `epoch` field of the enclosing FramedContent is equal
-  to the `epoch` field of the current GroupContext object
+  to the `epoch` field of the current GroupContext object.
 
 * Unprotect the Commit using the keys from the current epoch:
   * If the message is encoded as PublicMessage, verify the membership MAC using
-    the `membership_key`
+    the `membership_key`.
   * If the message is encoded as PrivateMessage, decrypt the message using the
     `sender_data_secret` and the (key, nonce) pair from the step on the sender's
     hash ratchet indicated by the `generation` field.
 
-* Verify that the signature on the FramedContent message as described in
+* Verify the signature on the FramedContent message as described in
   {{content-authentication}}.
 
-* Verify that the `proposals` vector is valid as specified in {{proposal-list-validation}}.
+* Verify that the `proposals` vector is valid according to the rules in
+  {{proposal-list-validation}}.
 
 * Verify that all PreSharedKey proposals in the `proposals` vector are available.
 
 * Create the new ratchet tree and GroupContext by applying the list of proposals
   to the old ratchet tree and GroupContext, as defined in
-  {{applying-a-proposal-list}}
+  {{applying-a-proposal-list}}.
 
 * Verify that the `path` value is populated if the `proposals` vector contains
   any Update or Remove proposals, or if it's empty. Otherwise, the `path` value
@@ -4539,7 +4146,7 @@ A member of the group applies a Commit message by taking the following steps:
 
 * If the `path` value is populated, validate it and apply it to the tree:
 
-  * If this is an external commit, assign the sender the leftmost blank leaf
+  * If this is an external Commit, assign the sender the leftmost blank leaf
     node in the new ratchet tree.  If there are no blank leaf nodes in the new
     ratchet tree, add a blank leaf to the right side of the new ratchet tree and
     assign it to the sender.
@@ -4553,7 +4160,7 @@ A member of the group applies a Commit message by taking the following steps:
   * Verify that none of the public keys in the UpdatePath appear in any node of
     the new ratchet tree.
 
-  * Merge the UpdatePath into the new ratchet tree as described in
+  * Merge the UpdatePath into the new ratchet tree, as described in
     {{synchronizing-views-of-the-tree}}.
 
   * Construct a provisional GroupContext object containing the following values:
@@ -4571,7 +4178,7 @@ A member of the group applies a Commit message by taking the following steps:
   * Define `commit_secret` as the value `path_secret[n+1]` derived from the
     last path secret value (`path_secret[n]`) derived for the UpdatePath.
 
-* If the `path` value is not populated: Define `commit_secret` as the all-zero
+* If the `path` value is not populated, define `commit_secret` as the all-zero
   vector of length `KDF.Nh` (the same length as a `path_secret` value would be).
 
 * Update the confirmed and interim transcript hashes using the new Commit, and
@@ -4581,8 +4188,8 @@ A member of the group applies a Commit message by taking the following steps:
   of PSKs in the derivation corresponds to the order of PreSharedKey proposals
   in the `proposals` vector.
 
-* Use the `init_secret` from the previous epoch, the `commit_secret` and the
-  `psk_secret` as defined in the previous steps, and the new GroupContext to
+* Use the `init_secret` from the previous epoch, the `commit_secret` and
+  `psk_secret` defined in the previous steps, and the new GroupContext to
   compute the new `joiner_secret`, `welcome_secret`, `epoch_secret`, and
   derived secrets for the new epoch.
 
@@ -4597,18 +4204,18 @@ A member of the group applies a Commit message by taking the following steps:
   send messages anymore. Instead, it MUST wait for a Welcome message from the committer
   meeting the requirements of {{reinitialization}}.
 
-Note that clients need to be prepared to receive a valid Commit message which removes
+Note that clients need to be prepared to receive a valid Commit message that removes
 them from the group. In this case, the client cannot send any more messages in the
 group and SHOULD promptly delete its group state and secret tree. (A client might keep
 the secret tree for a short time to decrypt late messages in the previous epoch.)
 
 ### Adding Members to the Group
 
-New members can join the group in two ways. Either by being added by a group
-member, or by adding themselves through an external Commit. In both cases, the
+New members can join the group in two ways: by being added by a group
+member or by adding themselves through an external Commit. In both cases, the
 new members need information to bootstrap their local group state.
 
-~~~ tls
+~~~ tls-presentation
 struct {
     GroupContext group_context;
     Extension extensions<V>;
@@ -4628,12 +4235,12 @@ confirmation tag computed in the creation of the group (see {{group-creation}}).
 as `MAC(confirmation_key, confirmed_transcript_hash)`.)
 
 As discussed in {{extensibility}}, unknown extensions in `GroupInfo.extensions`
-MUST be ignored, and the creator of a `GroupInfo` object SHOULD include some
+MUST be ignored, and the creator of a GroupInfo object SHOULD include some
 random GREASE extensions to help ensure that other clients correctly ignore unknown
 extensions.  Extensions in `GroupInfo.group_context.extensions`, however, MUST
 be supported by the new joiner.
 
-New members MUST verify that `group_id` is unique among the groups they're
+New members MUST verify that `group_id` is unique among the groups they are
 currently participating in.
 
 New members also MUST verify the `signature` using the public key taken from the
@@ -4641,7 +4248,7 @@ leaf node of the ratchet tree with leaf index `signer`. The
 signature covers the following structure, comprising all the fields in the
 GroupInfo above `signature`:
 
-~~~ tls
+~~~ tls-presentation
 struct {
     GroupContext group_context;
     Extension extensions<V>;
@@ -4666,7 +4273,7 @@ encrypted.
 The Welcome message provides the new
 members with the current state of the group after the application of the Commit
 message.  The new members will not be able to decrypt or verify the Commit
-message, but will have the secrets they need to participate in the epoch
+message, but they will have the secrets they need to participate in the epoch
 initiated by the Commit message.
 
 In order to allow the same Welcome message to be sent to multiple new members,
@@ -4674,7 +4281,7 @@ information describing the group is encrypted with a symmetric key and nonce
 derived from the `joiner_secret` for the new epoch.  The `joiner_secret` is
 then encrypted to each new member using HPKE.  In the same encrypted package,
 the committer transmits the path secret for the lowest (closest to the leaf) node
-which is contained in the direct paths of both the committer and the new member.
+that is contained in the direct paths of both the committer and the new member.
 This allows the new
 member to compute private keys for nodes in its direct path that are being
 reset by the corresponding Commit.
@@ -4683,7 +4290,7 @@ If the sender of the Welcome message wants the receiving member to include a PSK
 in the derivation of the `epoch_secret`, they can populate the `psks` field
 indicating which PSK to use.
 
-~~~ tls
+~~~ tls-presentation
 struct {
   opaque path_secret<V>;
 } PathSecret;
@@ -4717,28 +4324,30 @@ On receiving a Welcome message, a client processes it using the following steps:
 * Identify an entry in the `secrets` array where the `new_member`
   value corresponds to one of this client's KeyPackages, using the hash
   indicated by the `cipher_suite` field. If no such field exists, or if the
-  ciphersuite indicated in the KeyPackage does not match the one in the
+  cipher suite indicated in the KeyPackage does not match the one in the
   Welcome message, return an error.
 
 * Decrypt the `encrypted_group_secrets` value with the algorithms indicated by
-  the ciphersuite and the private key `init_key_priv` corresponding to
+  the cipher suite and the private key `init_key_priv` corresponding to
   `init_key` in the referenced KeyPackage.
 
 ~~~ pseudocode
-encrypted_group_secrets = EncryptWithLabel(init_key, "Welcome",
-                                           encrypted_group_info, group_secrets)
+encrypted_group_secrets =
+  EncryptWithLabel(init_key, "Welcome",
+                   encrypted_group_info, group_secrets)
 
-group_secrets = DecryptWithLabel(init_key_priv, "Welcome",
-                                 encrypted_group_info, kem_output, ciphertext)
+group_secrets =
+  DecryptWithLabel(init_key_priv, "Welcome",
+                   encrypted_group_info, kem_output, ciphertext)
 ~~~
 
-* If a `PreSharedKeyID` is part of the GroupSecrets and the client is not in
+* If a PreSharedKeyID is part of the GroupSecrets and the client is not in
   possession of the corresponding PSK, return an error. Additionally, if a
-  `PreSharedKeyID` has type `resumption` with usage `reinit` or `branch`, verify
+  PreSharedKeyID has type `resumption` with usage `reinit` or `branch`, verify
   that it is the only such PSK.
 
 * From the `joiner_secret` in the decrypted GroupSecrets object and the PSKs
-  specified in the `GroupSecrets`, derive the `welcome_secret` and using that
+  specified in the GroupSecrets, derive the `welcome_secret` and then
   the `welcome_key` and `welcome_nonce`. Use the key and nonce to decrypt the
   `encrypted_group_info` field.
 
@@ -4776,7 +4385,7 @@ welcome_key = ExpandWithLabel(welcome_secret, "key", "", AEAD.Nk)
     * Verify that the entry represents a non-blank leaf node that is a
       descendant of the parent node.
 
-    * Verify that every non-blank intermediate node beween the leaf node and the
+    * Verify that every non-blank intermediate node between the leaf node and the
       parent node also has an entry for the leaf node in its `unmerged_leaves`.
 
     * Verify that the encryption key in the parent node does not appear in any
@@ -4787,13 +4396,13 @@ welcome_key = ExpandWithLabel(welcome_secret, "key", "", AEAD.Nk)
   error.  Let `my_leaf` represent this leaf in the tree.
 
 * Construct a new group state using the information in the GroupInfo object.
-    * The GroupContext is the `group_context` field from the GroupInfo object.
 
-    * The new member's position in the tree is at the leaf `my_leaf`, as defined
-      above.
+    * Initialize the GroupContext for the group from the `group_context` field
+      from the GroupInfo object.
 
     * Update the leaf `my_leaf` with the private key corresponding to the
-      public key in the node.
+      public key in the node, where `my_leaf` is the new member's leaf node in
+      the ratchet tree, as defined above.
 
     * If the `path_secret` value is set in the GroupSecrets object: Identify the
       lowest common ancestor of the leaf node `my_leaf` and of the node of
@@ -4801,7 +4410,7 @@ welcome_key = ExpandWithLabel(welcome_secret, "key", "", AEAD.Nk)
       this node to the private key derived from the `path_secret`.
 
     * For each parent of the common ancestor, up to the root of the tree, derive
-      a new path secret and set the private key for the node to the private key
+      a new path secret, and set the private key for the node to the private key
       derived from the path secret.  The private key MUST be the private key
       that corresponds to the public key in the node.
 
@@ -4817,7 +4426,7 @@ welcome_key = ExpandWithLabel(welcome_secret, "key", "", AEAD.Nk)
 * Use the confirmed transcript hash and confirmation tag to compute the interim
   transcript hash in the new state.
 
-* If a `PreSharedKeyID` was used that has type `resumption` with usage `reinit`
+* If a PreSharedKeyID was used that has type `resumption` with usage `reinit`
   or `branch`, verify that the `epoch` field in the GroupInfo is equal to 1.
 
   * For usage `reinit`, verify that the last Commit to the referenced group
@@ -4836,27 +4445,27 @@ welcome_key = ExpandWithLabel(welcome_secret, "key", "", AEAD.Nk)
 External Commits are a mechanism for new members (external parties that want to
 become members of the group) to add themselves to a group, without requiring
 that an existing member has to come online to issue a Commit that references an
-Add Proposal.
+Add proposal.
 
-Whether existing members of the group will accept or reject an External Commit
+Whether existing members of the group will accept or reject an external Commit
 follows the same rules that are applied to other handshake messages.
 
-New members can create and issue an External Commit if they have access to the
+New members can create and issue an external Commit if they have access to the
 following information for the group's current epoch:
 
 * group ID
 * epoch ID
-* ciphersuite
+* cipher suite
 * public tree hash
 * confirmed transcript hash
 * confirmation tag of the most recent Commit
 * group extensions
 * external public key
 
-In other words, to join a group via an External Commit, a new member needs a
+In other words, to join a group via an external Commit, a new member needs a
 GroupInfo with an `external_pub` extension present in its `extensions` field.
 
-~~~ tls
+~~~ tls-presentation
 struct {
     HPKEPublicKey external_pub;
 } ExternalPub;
@@ -4870,13 +4479,13 @@ will cause the epoch to change.
 
 Note that the `tree_hash` field is used the same way as in the Welcome message.
 The full tree can be included via the `ratchet_tree` extension
-{{ratchet-tree-extension}}.
+(see {{ratchet-tree-extension}}).
 
 The information in a GroupInfo is not generally public information, but applications
 can choose to make it available to new members in order to allow External
 Commits.
 
-In principle, External Commits work like regular Commits. However, their content
+In principle, external Commits work like regular Commits. However, their content
 has to meet a specific set of requirements:
 
 * External Commits MUST contain a `path` field (and is therefore a "full"
@@ -4884,23 +4493,23 @@ has to meet a specific set of requirements:
   were added with an Add proposal), and the path is calculated relative to that
   leaf node.
 * The Commit MUST NOT include any proposals by reference, since an external
-  joiner cannot determine the validity of proposals sent within the group
+  joiner cannot determine the validity of proposals sent within the group.
 * External Commits MUST be signed by the new member.  In particular, the
   signature on the enclosing AuthenticatedContent MUST verify using the public key for
   the credential in the `leaf_node` of the `path` field.
 * When processing a Commit, both existing and new members MUST use the external
   init secret as described in {{external-initialization}}.
-* The sender type for the AuthenticatedContent encapsulating the External Commit MUST be
+* The sender type for the AuthenticatedContent encapsulating the external Commit MUST be
   `new_member_commit`.
 
-External Commits come in two "flavors" -- a "join" commit that
-adds the sender to the group or a "resync" commit that replaces a member's prior
+External Commits come in two "flavors" -- a "join" Commit that
+adds the sender to the group or a "resync" Commit that replaces a member's prior
 appearance with a new one.
 
 Note that the "resync" operation allows an attacker that has compromised a
 member's signature private key to introduce themselves into the group and remove the
 prior, legitimate member in a single Commit.  Without resync, this
-can still be done, but requires two operations, the external Commit to join and
+can still be done, but it requires two operations: the external Commit to join and
 a second Commit to remove the old appearance.  Applications for whom this
 distinction is salient can choose to disallow external commits that contain a
 Remove, or to allow such resync commits only if they contain a "reinit" PSK
@@ -4916,13 +4525,13 @@ the group's ratchet tree.  In order to process or generate handshake
 messages, the joiner will need to get a copy of the ratchet tree from some other
 source.  (For example, the DS might provide a cached copy.)  The inclusion of
 the tree hash in the GroupInfo message means that the source of the ratchet
-tree need not be trusted to maintain the integrity of tree.
+tree need not be trusted to maintain the integrity of the tree.
 
 In cases where the application does not wish to provide such an external source,
 the whole public state of the ratchet tree can be provided in an extension of
 type `ratchet_tree`, containing a `ratchet_tree` object of the following form:
 
-~~~ tls
+~~~ tls-presentation
 struct {
     NodeType node_type;
     select (Node.node_type) {
@@ -4942,25 +4551,25 @@ traversal of the ratchet tree. Each node is listed between its left subtree and
 its right subtree.  (This is the same ordering as specified for the array-based
 trees outlined in {{array-based-trees}}.)
 
-If the tree has `2^d` leaves, then it has `2^(d+1) - 1` nodes.  The
+If the tree has 2<sup>d</sup> leaves, then it has 2<sup>d+1</sup> - 1 nodes.  The
 `ratchet_tree` vector logically has this number of entries, but the sender
 MUST NOT include blank nodes after the last non-blank node.  The receiver MUST
-check that the last node in `ratchet_tree` is non-blank, and extend it to the
-right until it has a length of the form `2^(d+1) - 1`, adding the minimum number
+check that the last node in `ratchet_tree` is non-blank, and then extend the tree to the
+right until it has a length of the form 2<sup>d+1</sup> - 1, adding the minimum number
 of blank values possible.  (Obviously, this may be done "virtually", by
 synthesizing blank nodes when required, as opposed to actually changing the
 structure in memory.)
 
 The leaves of the tree are stored in even-numbered entries in the array (the
 leaf with index `L` in array position `2*L`). The root node of the tree is at
-position `2^d - 1` of the array. Intermediate parent nodes can be identified by
+position 2<sup>d</sup> - 1 of the array. Intermediate parent nodes can be identified by
 performing the same calculation to the subarrays to the left and right of the
 root, following something like the following algorithm:
 
 ~~~ python
 # Assuming a class Node that has left and right members
 def subtree_root(nodes):
-    # If there is only one node in the array return it
+    # If there is only one node in the array, return it
     if len(nodes) == 1:
         return Node(nodes[0])
 
@@ -5007,7 +4616,7 @@ A   B   C   D   E   F   _   _
 <---> R <--->   <---> R <--->
 - R -   - R -   - R -   - R -
 ~~~
-{: title="Left-to-right in-order traversal of a six-member tree" }
+{: title="Left-to-Right In-Order Traversal of a Six-Member Tree" }
 
 The presence of a `ratchet_tree` extension in a GroupInfo message does not
 result in any changes to the GroupContext extensions for the group.  The ratchet
@@ -5027,36 +4636,36 @@ using the tree for MLS operations.
 
 # Extensibility
 
-The base MLS protocol can be extended in a few ways.  New ciphersuites can be
+The base MLS protocol can be extended in a few ways.  New cipher suites can be
 added to enable the use of new cryptographic algorithms.  New types of proposals
 can be used to perform new actions within an epoch.  Extension fields can be
 used to add additional information to the protocol.  In this section, we discuss
 some constraints on these extensibility mechanisms that are necessary to ensure
 broad interoperability.
 
-## Additional Ciphersuites
+## Additional Cipher Suites
 
-As discussed in {{ciphersuites}}, MLS allows the participants in a group to
+As discussed in {{cipher-suites}}, MLS allows the participants in a group to
 negotiate the cryptographic algorithms used within the group.  This
 extensibility is important for maintaining the security of the protocol over
 time {{?RFC7696}}.  It also creates a risk of interoperability failure due to
-clients not supporting a common ciphersuite.
+clients not supporting a common cipher suite.
 
-The ciphersuite registry defined in {{mls-ciphersuites}} attempts to strike a
+The cipher suite registry defined in {{mls-cipher-suites}} attempts to strike a
 balance on this point.  On the one hand, the base policy for the registry is
 Specification Required, a fairly low bar designed to avoid the need for
 standards work in cases where different ciphers are needed for niche
-applications.  There is a higher bar (Standards Action) for ciphers to set the
+applications.  On the other hand, there is a higher bar (Standards Action) for ciphers to set the
 Recommended field in the registry.  This higher bar is there in part to ensure
-that the interoperability implications of new ciphersuites are considered.
+that the interoperability implications of new cipher suites are considered.
 
-MLS ciphersuites are defined independent of MLS versions, so that in principle
-the same ciphersuite can be used across versions.  Standards work defining new
+MLS cipher suites are defined independent of MLS versions, so that in principle,
+the same cipher suite can be used across versions.  Standards work defining new
 versions of MLS should consider whether it is desirable for the new version to
-be compatible with existing ciphersuites, or whether the new version should rule
-out some ciphersuites. For example, a new version could follow the example of
-HTTP/2, which restricted the set of allowed TLS ciphers (see Section 9.2.2 of
-{{?RFC9113}}.
+be compatible with existing cipher suites, or whether the new version should rule
+out some cipher suites. For example, a new version could follow the example of
+HTTP/2, which restricted the set of allowed TLS ciphers (see {{Section 9.2.2 of
+?RFC9113}}).
 
 ## Proposals
 
@@ -5067,12 +4676,12 @@ all the members of the group that will process the Commit.
 
 ## Credential Extensibility
 
-In order to ensure that MLS provides meaningful authentication it is important
+In order to ensure that MLS provides meaningful authentication, it is important
 that each member is able to authenticate some identity information for each
 other member.  Identity information is encoded in Credentials, so this property
 is provided by ensuring that members use compatible credential types.
 
-The types of credential that may be used in a group is restricted to what all
+The only types of credential that may be used in a group are those that all
 members of the group support, as specified by the `capabilities` field of each
 LeafNode in the ratchet tree. An application can introduce new credential types
 by choosing an unallocated identifier from the registry in
@@ -5125,11 +4734,11 @@ negotiation of extensions.  For this to work, implementations MUST correctly
 handle extensible fields:
 
 * A client that posts a KeyPackage MUST support all parameters advertised in
-  it.  Otherwise, another client might fail to interoperate by selecting one of
+  it. Otherwise, another client might fail to interoperate by selecting one of
   those parameters.
 
 * A client processing a KeyPackage object MUST ignore all unrecognized values
-  in the `capabilities` field of the `LeafNode`, and all unknown extensions in
+  in the `capabilities` field of the LeafNode and all unknown extensions in
   the `extensions` and `leaf_node.extensions` fields.  Otherwise, it could fail
   to interoperate with newer clients.
 
@@ -5159,18 +4768,18 @@ by reinitializing the group ({{reinitialization}}).
 
 As described in {{extensions}}, clients are required to ignore unknown values
 for certain parameters.  To help ensure that other clients implement this
-behavior, a client can follow the “Generate Random Extensions And Sustain
-Extensibility” or GREASE approach described in {{?RFC8701}}.  In the context of
+behavior, a client can follow the "Generate Random Extensions And Sustain
+Extensibility" or GREASE approach described in {{?RFC8701}}.  In the context of
 MLS, this means that a client generating a KeyPackage, LeafNode, or GroupInfo object includes
 random values in certain fields which would be ignored by a
-correctly-implemented client processing the message.  A client that incorrectly
+correctly implemented client processing the message.  A client that incorrectly
 rejects unknown code points will fail to process such a message, providing a
 signal to its implementer that the client needs to be fixed.
 
 When generating the following fields, an MLS client SHOULD include a random
 selection of values chosen from these GREASE values:
 
-* `LeafNode.capabilities.ciphersuites`
+* `LeafNode.capabilities.cipher_suites`
 * `LeafNode.capabilities.extensions`
 * `LeafNode.capabilities.proposals`
 * `LeafNode.capabilities.credentials`
@@ -5180,8 +4789,8 @@ selection of values chosen from these GREASE values:
 
 For the KeyPackage and GroupInfo extensions, the `extension_data` for GREASE
 extensions MAY have any contents selected by the sender, since they will be
-ignored by a correctly-implemented receiver.  For example, a sender might
-populate these extensions with a randomly-sized amount of random data.
+ignored by a correctly implemented receiver.  For example, a sender might
+populate these extensions with a randomly sized amount of random data.
 
 Note that any GREASE values added to `LeafNode.extensions` need to be reflected
 in `LeafNode.capabilities.extensions`, since the LeafNode validation process
@@ -5189,7 +4798,7 @@ described in {{leaf-node-validation}} requires that these two fields be
 consistent.
 
 GREASE values MUST NOT be sent in the following fields, because an unsupported
-value in one these fields (including a GREASE value), will cause the enclosing
+value in one these fields (including a GREASE value) will cause the enclosing
 message to be rejected:
 
 * `Proposal.proposal_type`
@@ -5197,7 +4806,7 @@ message to be rejected:
 * `GroupContext.extensions`
 * `GroupContextExtensions.extensions`
 
-A set of values reserved for GREASE have been registered in the various
+Values reserved for GREASE have been registered in the various
 registries in {{iana-considerations}}.  This prevents conflict between GREASE
 and real future values.  The following values are reserved in each registry:
 `0x0A0A`, `0x1A1A`, `0x2A2A`, `0x3A3A`, `0x4A4A`, `0x5A5A`, `0x6A6A`, `0x7A7A`,
@@ -5209,7 +4818,7 @@ for how to handle these values when receiving them, since this negates their
 utility for detecting extensibility failures.
 
 GREASE values MUST be handled using normal logic for processing unsupported
-values.  When comparing lists of capabilities to identify mutually-supported
+values.  When comparing lists of capabilities to identify mutually supported
 capabilities, clients MUST represent their own capabilities with a list
 containing only the capabilities actually supported, without any GREASE values.
 In other words, lists including GREASE values are only sent to other clients;
@@ -5250,11 +4859,11 @@ on the dynamics of the application.
 
 # Application Messages
 
-The primary purpose of handshake messages are to provide an authenticated group
+The primary purpose of handshake messages is to provide an authenticated group
 key exchange to clients. In order to protect application messages sent among the
 members of a group, the `encryption_secret` provided by the key schedule is used
 to derive a sequence of nonces and keys for message encryption. Every epoch
-moves the key schedule forward which triggers the creation of a new secret
+moves the key schedule forward, which triggers the creation of a new secret
 tree, as described in {{secret-tree}}, along with a new set of symmetric
 ratchets of nonces and keys for each member.
 
@@ -5265,7 +4874,7 @@ ones as soon as they have been used.
 
 The group identifier and epoch allow a recipient to know which group secrets
 should be used and from which `epoch_secret` to start computing other secrets.
-The sender identifier and content type is used to identify which
+The sender identifier and content type are used to identify which
 symmetric ratchet to use from the secret tree. The
 `generation` counter determines how far into the ratchet to iterate in
 order to produce the required nonce and key for encryption or decryption.
@@ -5282,7 +4891,7 @@ the attacker enough information to mount an attack. If Alice asks Bob
 "When are we going to the movie?", then the answer "Wednesday" could be leaked
 to an adversary solely by the ciphertext length.
 
-The length of the `padding` field in `PrivateMessageContent` can be
+The length of the `padding` field in PrivateMessageContent can be
 chosen by the sender at the time of message encryption. Senders may use padding
 to reduce the ability of attackers outside the group to infer the size of the
 encrypted content.  Note, however, that the transports used to carry MLS
@@ -5302,13 +4911,13 @@ removed from the group can no longer receive messages and to (potentially)
 recover confidentiality and authenticity for future messages despite a past
 state compromise.
 
-## Delayed and Reordered Application messages
+## Delayed and Reordered Application Messages
 
 Since each application message contains the group identifier, the epoch, and a
 generation counter, a client can receive messages out of order. When messages
 are received out of order, the client moves the sender ratchet forward to match
 the received generation counter. Any unused nonce and key pairs from the ratchet
-are potentially stored so that they can be used to decrypt the messages which
+are potentially stored so that they can be used to decrypt the messages that
 were delayed or reordered.
 
 Applications SHOULD define a policy on how long to keep unused nonce and key
@@ -5317,7 +4926,7 @@ ensuring that these secrets are deleted according to the deletion schedule
 defined in {{deletion-schedule}}. Applications SHOULD also define a policy
 limiting the maximum number of steps that clients will move the ratchet forward
 in response to a new message.  Messages received with a generation counter
-that's too much higher than the last message received would then be rejected.
+that is too much higher than the last message received would then be rejected.
 This avoids causing a denial-of-service attack by requiring the recipient to
 perform an excessive number of key derivations. For example, a malicious group
 member could send a message with `generation = 0xffffffff` at the beginning of a
@@ -5345,20 +4954,19 @@ selectively removing messages in transit. In order to prevent this
 form of attack, it is RECOMMENDED that all MLS messages be carried
 over a secure transport such as TLS {{?RFC8446}} or QUIC {{?RFC9000}}.
 
-
-## Confidentiality of the Group Secrets
+## Confidentiality of Group Secrets
 
 Group secrets are partly derived from the output of a ratchet tree. Ratchet
 trees work by assigning each member of the group to a leaf in the tree and
 maintaining the following property: the private key of a node in the tree is
 known only to members of the group that are assigned a leaf in the node's
-subtree. This is called the *ratchet tree invariant* and it makes it possible to
+subtree. This is called the *tree invariant*, and it makes it possible to
 encrypt to all group members except one, with a number of ciphertexts that is
 logarithmic in the number of group members.
 
 The ability to efficiently encrypt to all members except one allows members to
 be securely removed from a group. It also allows a member to rotate their
-keypair such that the old private key can no longer be used to decrypt new
+key pair such that the old private key can no longer be used to decrypt new
 messages.
 
 ## Confidentiality of Sender Data
@@ -5370,16 +4978,16 @@ is a variant of the HN1 construction analyzed in {{NAN}}.  A sample of the
 ciphertext is combined with a `sender_data_secret` to derive a key and nonce
 that are used for AEAD encryption of the sender data.
 
-``` pseudocode
+~~~ pseudocode
 (key, nonce) = PRF(sender_data_secret, sample)
 encrypted_sender_data =
   AEAD.Seal(key, nonce, sender_data_aad, sender_data)
-```
+~~~
 
-The only differences between this construction and HN1 as described in {{NAN}} are
-(1) that it uses authenticated encryption instead of unauthenticated encryption
-and (2) that it protects information used to derive a nonce instead of the nonce
-itself.
+The only differences between this construction and HN1 as described in {{NAN}}
+are that it (1) uses authenticated encryption instead of unauthenticated
+encryption and (2) protects information used to derive a nonce instead of the
+nonce itself.
 
 Since the `sender_data_secret` is distinct from the content encryption key, it
 follows that the sender data encryption scheme achieves AE2 security as defined
@@ -5393,10 +5001,10 @@ indistinguishable from random data (i.e., the AEAD is AE1-secure in the phrasing
 of {{NAN}}), the odds of two ciphertext samples being identical is roughly
 2<sup>-L/2</sup>, i.e., the birthday bound.
 
-The AEAD algorithms for ciphersuites defined in this document all provide this
-property. The size of the sample depends on the ciphersuite's hash function, but
+The AEAD algorithms for cipher suites defined in this document all provide this
+property. The size of the sample depends on the cipher suite's hash function, but
 in all cases, the probability of collision is no more than 2<sup>-128</sup>.
-Any future ciphersuite MUST use an AE1-secure AEAD algorithm.
+Any future cipher suite MUST use an AE1-secure AEAD algorithm.
 
 ## Confidentiality of Group Metadata
 
@@ -5447,7 +5055,7 @@ will cause decryption failure.
 ### Group Extensions
 
 A group's extensions are first set by the group's creator and then updated by
-GroupContextExtensions proposals.  A GroupContextExtension proposal sent as
+GroupContextExtensions proposals.  A GroupContextExtensions proposal sent as
 a PublicMessage leaks the group's extensions.
 
 A new member learns the group's extensions via a GroupInfo object.  When the new
@@ -5478,7 +5086,7 @@ member.  If the party can also associate the Welcome with a group, then the
 party can infer that the identified new member was added to that group.
 
 Note that these information leaks reveal the group's membership only to the degree
-that that membership is revealed by the contents of a member's LeafNode in the
+that membership is revealed by the contents of a member's LeafNode in the
 ratchet tree.  In some cases, this may be quite direct, e.g., due to credentials
 attesting to identifiers such as email addresses.  An application could
 construct a member's leaf node to be less identifying, e.g., by using a
@@ -5490,7 +5098,7 @@ The first form of authentication we provide is that group members can verify a
 message originated from one of the members of the group. For encrypted messages,
 this is guaranteed because messages are encrypted with an AEAD under a key
 derived from the group secrets. For plaintext messages, this is guaranteed by
-the use of a `membership_tag` which constitutes a MAC over the message, under a
+the use of a `membership_tag`, which constitutes a MAC over the message, under a
 key derived from the group secrets.
 
 The second form of authentication is that group members can verify a message
@@ -5503,8 +5111,8 @@ attacker can create LeafNodes and KeyPackages impersonating the member; dependin
 application, this can then allow the attacker to join the group with the
 compromised member's identity.  For example, if a group has enabled external
 parties to join via external commits, then an attacker that has compromised a
-member's signature key could use an external commit to insert themselves into
-the group -- even using a "resync"-style external commit to replace the
+member's signature key could use an external Commit to insert themselves into
+the group -- even using a "resync"-style external Commit to replace the
 compromised member in the group.
 
 Applications can mitigate the risks of signature key compromise using pre-shared
@@ -5523,7 +5131,7 @@ point in time are secure in the face of later compromise of a group member.
 Post-compromise security means that messages are secure even if a group member
 was compromised at some point in the past.
 
-~~~aasvg
+~~~ aasvg
                    Compromise
                        |
                        |
@@ -5534,7 +5142,7 @@ was compromised at some point in the past.
   Forward Secrecy |         | Post-Compromise
                   |         |   Security
 ~~~
-{: title="Forward secrecy and post-compromise security" }
+{: title="Forward Secrecy and Post-Compromise Security" }
 
 Post-compromise security is provided between epochs by members
 regularly updating their leaf key in the ratchet tree. Updating their
@@ -5552,14 +5160,14 @@ versions of the ratchet tree, as this prevents old group secrets from being
 re-derived. Forward secrecy *within* an epoch is provided by deleting message
 encryption keys once they've been used to encrypt or decrypt a message.
 Note that group secrets and message encryption keys are shared by the
-group, and thus there is a risk to forward secrecy as long as any
+group.  There is thus a risk to forward secrecy as long as any
 member has not deleted these keys. This is a particular risk if a member
 is offline for a long period of time. Applications SHOULD have mechanisms
-for evicting group members which are offline for too long (i.e., have
+for evicting group members that are offline for too long (i.e., have
 not changed their key within some period).
 
 New groups are also at risk of using previously compromised keys (as with
-post-compromise security), if a member is added to a new group via an old
+post-compromise security) if a member is added to a new group via an old
 KeyPackage whose corresponding private key has been compromised.  This risk can
 be mitigated by having clients regularly generate new KeyPackages and upload
 them to the Delivery Service.  This way, the key material used to add a member
@@ -5573,12 +5181,12 @@ If two members' leaf nodes have the same signature key, for example, then the
 data origin authentication properties afforded by signatures within the group
 are degraded.
 
-Uniqueness of keys in leaf nodes is assured by explicit checks on leaf nodes
-being added to the tree by Add or Update proposals, or in the `path` field of a
+Uniqueness of keys in leaf nodes is assured by explicitly checking each leaf node
+as it is added to the tree, whether in an Add proposal, in an Update proposal, or in the `path` field of a
 Commit.  Details can be found in {{leaf-node-validation}},
 {{proposal-list-validation}}, and {{processing-a-commit}}.  Uniqueness of
 encryption keys in parent nodes is assured by checking that the keys in an
-UpdatePath are not found elsewhere in the tree (see {{processing-a-commit}}.
+UpdatePath are not found elsewhere in the tree (see {{processing-a-commit}}).
 
 ## KeyPackage Reuse
 
@@ -5589,7 +5197,7 @@ to replay attacks.
 
 An application MAY allow for reuse of a "last resort" KeyPackage in order to
 prevent denial-of-service attacks.  Since a KeyPackage is needed to add a
-client to a new group, an attacker could prevent a client being added to new
+client to a new group, an attacker could prevent a client from being added to new
 groups by exhausting all available KeyPackages. To prevent such a denial-of-service
 attack, the KeyPackage publication system SHOULD rate-limit KeyPackage
 requests, especially if not authenticated.
@@ -5599,7 +5207,7 @@ requests, especially if not authenticated.
 MLS is designed to protect the confidentiality and integrity of
 the group data even in the face of a compromised DS. However, a compromised
 DS can still mount some attacks. While it cannot forge messages,
-it can selectively delay or remove them. This can in some cases be
+it can selectively delay or remove them. In some cases, this can be
 observed by detecting gaps in the per-sender generation counter,
 though it may not always be possible to distinguish an attack from message
 loss. In addition, the DS can permanently block messages to and from
@@ -5625,7 +5233,7 @@ signature key and identity pair of its choice, thus allowing
 impersonation of a given user. This ability is sufficient to allow the
 AS to join new groups as if it were that user. Depending on the
 application architecture, it may also be sufficient to allow the
-compromised AS to join the group as an existing user, for instance as
+compromised AS to join the group as an existing user, for instance, as
 if it were a new device associated with the same user. If
 the application uses a transparency mechanism such as CONIKS
 {{CONIKS}} or Key Transparency {{KT}}, then it may be possible for end
@@ -5658,7 +5266,7 @@ key provided for that node.
 
 Both of these types of corruption will cause processing of a Commit to fail for
 some members of the group.  If the public key for a node does not match the path
-secret, then the members that decrypt that path secret will reject the commit
+secret, then the members that decrypt that path secret will reject the Commit
 based on this mismatch.  If the path secret sequence is incorrect at some point,
 then members that can decrypt nodes before that point will compute a different
 public key for the mismatched node than the one in the UpdatePath, which also
@@ -5667,14 +5275,14 @@ commits to be reported, so that group members who were not able to recognize the
 error themselves can reinitialize the group if necessary.
 
 Even with such an error reporting mechanism in place, however, it is still
-possible for members to get locked out of the group by a malformed commit.
+possible for members to get locked out of the group by a malformed Commit.
 Since malformed Commits can only be recognized by certain members of the group,
 in an asynchronous application, it may be the case that all members that could
 detect a fault in a Commit are offline.  In such a case, the Commit will be
-accepted by the group, and the resulting state possibly used as the basis for
+accepted by the group, and the resulting state will possibly be used as the basis for
 further Commits.  When the affected members come back online, they will reject
-the first commit, and thus be unable to catch up with the group. These members
-will either need to add themselves back with an external Commit, or reinitialize
+the first Commit, and thus be unable to catch up with the group. These members
+will need to either add themselves back with an external Commit or reinitialize
 the group from scratch.
 
 Applications can address this risk by requiring certain members of the group to
@@ -5688,9 +5296,9 @@ be added at the application layer.
 
 # IANA Considerations
 
-This document requests the creation of the following new IANA registries:
+IANA has created the following registries:
 
-* MLS Ciphersuites ({{mls-ciphersuites}})
+* MLS Cipher Suites ({{mls-cipher-suites}})
 * MLS Wire Formats ({{mls-wire-formats}})
 * MLS Extension Types ({{mls-extension-types}})
 * MLS Proposal Types ({{mls-proposal-types}})
@@ -5699,27 +5307,24 @@ This document requests the creation of the following new IANA registries:
 * MLS Public Key Encryption Labels ({{mls-public-key-encryption-labels}})
 * MLS Exporter Labels ({{mls-exporter-labels}})
 
-All of these registries should be under a heading of "Messaging Layer Security",
+All of these registries are under the "Messaging Layer Security" group registry heading,
 and assignments are made via the Specification Required policy {{!RFC8126}}. See
 {{de}} for additional information about the MLS Designated Experts (DEs).
 
-RFC EDITOR: Please replace XXXX throughout with the RFC number assigned to
-this document
-
-## MLS Ciphersuites
+## MLS Cipher Suites
 
-A ciphersuite is a combination of a protocol version and the set of
+A cipher suite is a combination of a protocol version and the set of
 cryptographic algorithms that should be used.
 
-Ciphersuite names follow the naming convention:
+Cipher suite names follow the naming convention:
 
 ~~~ pseudocode
 CipherSuite MLS_LVL_KEM_AEAD_HASH_SIG = VALUE;
 ~~~
 
-Where VALUE is represented as a sixteen-bit integer:
+Where VALUE is represented as a 16-bit integer:
 
-~~~ tls
+~~~ tls-presentation
 uint16 CipherSuite;
 ~~~
 
@@ -5729,19 +5334,15 @@ uint16 CipherSuite;
 | KEM       | The KEM algorithm used for HPKE in ratchet tree operations             |
 | AEAD      | The AEAD algorithm used for HPKE and message protection                |
 | HASH      | The hash algorithm used for HPKE and the MLS transcript hash           |
-| SIG       | The Signature algorithm used for message authentication                |
+| SIG       | The signature algorithm used for message authentication                |
 
 The columns in the registry are as follows:
 
-* Value: The numeric value of the ciphersuite
+* Value: The numeric value of the cipher suite
 
-* Name: The name of the ciphersuite
+* Name: The name of the cipher suite
 
-[[ RFC EDITOR: This section should be the same as the corresponding text in
-draft-ietf-tls-rfc8447bis.  Please align the two documents if they have diverged
-in the approval process. ]]
-
-* Recommended: Whether support for this ciphersuite is recommended by the IETF.
+* Recommended: Whether support for this cipher suite is recommended by the IETF.
   Valid values are "Y", "N", and "D", as described below.  The default
   value of the "Recommended" column is "N".  Setting the Recommended item to "Y"
   or "D", or changing an item whose current value is "Y" or "D", requires
@@ -5751,7 +5352,7 @@ in the approval process. ]]
     only means that the associated mechanism is fit for the purpose for which it
     was defined. Careful reading of the documentation for the mechanism is
     necessary to understand the applicability of that mechanism. The IETF could
-    recommend mechanisms that have limited applicability, but will provide
+    recommend mechanisms that have limited applicability, but it will provide
     applicability statements that describe any limitations of the mechanism or
     necessary constraints on its use.
 
@@ -5767,41 +5368,42 @@ in the approval process. ]]
     problems if they are used, such as a weak cryptographic algorithm or a
     mechanism that might cause interoperability problems in deployment.
 
-* Reference: The document where this ciphersuite is defined
+* Reference: The document where this cipher suite is defined
 
 Initial contents:
 
 | Value           | Name                                                | R | Ref      |
 |:----------------|:----------------------------------------------------|:--|:---------|
-| 0x0000          | RESERVED                                            | - | RFC XXXX |
-| 0x0001          | MLS_128_DHKEMX25519_AES128GCM_SHA256_Ed25519        | Y | RFC XXXX |
-| 0x0002          | MLS_128_DHKEMP256_AES128GCM_SHA256_P256             | Y | RFC XXXX |
-| 0x0003          | MLS_128_DHKEMX25519_CHACHA20POLY1305_SHA256_Ed25519 | Y | RFC XXXX |
-| 0x0004          | MLS_256_DHKEMX448_AES256GCM_SHA512_Ed448            | Y | RFC XXXX |
-| 0x0005          | MLS_256_DHKEMP521_AES256GCM_SHA512_P521             | Y | RFC XXXX |
-| 0x0006          | MLS_256_DHKEMX448_CHACHA20POLY1305_SHA512_Ed448     | Y | RFC XXXX |
-| 0x0007          | MLS_256_DHKEMP384_AES256GCM_SHA384_P384.            | Y | RFC XXXX |
-| 0x0A0A          | GREASE                                              | Y | RFC XXXX |
-| 0x1A1A          | GREASE                                              | Y | RFC XXXX |
-| 0x2A2A          | GREASE                                              | Y | RFC XXXX |
-| 0x3A3A          | GREASE                                              | Y | RFC XXXX |
-| 0x4A4A          | GREASE                                              | Y | RFC XXXX |
-| 0x5A5A          | GREASE                                              | Y | RFC XXXX |
-| 0x6A6A          | GREASE                                              | Y | RFC XXXX |
-| 0x7A7A          | GREASE                                              | Y | RFC XXXX |
-| 0x8A8A          | GREASE                                              | Y | RFC XXXX |
-| 0x9A9A          | GREASE                                              | Y | RFC XXXX |
-| 0xAAAA          | GREASE                                              | Y | RFC XXXX |
-| 0xBABA          | GREASE                                              | Y | RFC XXXX |
-| 0xCACA          | GREASE                                              | Y | RFC XXXX |
-| 0xDADA          | GREASE                                              | Y | RFC XXXX |
-| 0xEAEA          | GREASE                                              | Y | RFC XXXX |
-| 0xF000 - 0xFFFF | Reserved for Private Use                            | - | RFC XXXX |
-
-All of these ciphersuites use HMAC {{!RFC2104}} as their MAC function, with
-different hashes per ciphersuite.  The mapping of ciphersuites to HPKE
-primitives, HMAC hash functions, and TLS signature schemes is as follows
-{{RFC9180}} {{RFC8446}}:
+| 0x0000          | RESERVED                                            | - | RFC 9420 |
+| 0x0001          | MLS_128_DHKEMX25519_AES128GCM_SHA256_Ed25519        | Y | RFC 9420 |
+| 0x0002          | MLS_128_DHKEMP256_AES128GCM_SHA256_P256             | Y | RFC 9420 |
+| 0x0003          | MLS_128_DHKEMX25519_CHACHA20POLY1305_SHA256_Ed25519 | Y | RFC 9420 |
+| 0x0004          | MLS_256_DHKEMX448_AES256GCM_SHA512_Ed448            | Y | RFC 9420 |
+| 0x0005          | MLS_256_DHKEMP521_AES256GCM_SHA512_P521             | Y | RFC 9420 |
+| 0x0006          | MLS_256_DHKEMX448_CHACHA20POLY1305_SHA512_Ed448     | Y | RFC 9420 |
+| 0x0007          | MLS_256_DHKEMP384_AES256GCM_SHA384_P384             | Y | RFC 9420 |
+| 0x0A0A          | GREASE                                              | Y | RFC 9420 |
+| 0x1A1A          | GREASE                                              | Y | RFC 9420 |
+| 0x2A2A          | GREASE                                              | Y | RFC 9420 |
+| 0x3A3A          | GREASE                                              | Y | RFC 9420 |
+| 0x4A4A          | GREASE                                              | Y | RFC 9420 |
+| 0x5A5A          | GREASE                                              | Y | RFC 9420 |
+| 0x6A6A          | GREASE                                              | Y | RFC 9420 |
+| 0x7A7A          | GREASE                                              | Y | RFC 9420 |
+| 0x8A8A          | GREASE                                              | Y | RFC 9420 |
+| 0x9A9A          | GREASE                                              | Y | RFC 9420 |
+| 0xAAAA          | GREASE                                              | Y | RFC 9420 |
+| 0xBABA          | GREASE                                              | Y | RFC 9420 |
+| 0xCACA          | GREASE                                              | Y | RFC 9420 |
+| 0xDADA          | GREASE                                              | Y | RFC 9420 |
+| 0xEAEA          | GREASE                                              | Y | RFC 9420 |
+| 0xF000 - 0xFFFF | Reserved for Private Use                            | - | RFC 9420 |
+{: title="MLS Extension Types Registry" }
+
+All of the non-GREASE cipher suites use HMAC {{!RFC2104}} as their MAC function, with
+different hashes per cipher suite.  The mapping of cipher suites to HPKE
+primitives {{RFC9180}}, HMAC hash functions, and TLS signature schemes
+{{RFC8446}} is as follows:
 
 | Value  | KEM    | KDF    | AEAD   | Hash   | Signature              |
 |:-------|:-------|:-------|:-------|:-------|:-----------------------|
@@ -5814,36 +5416,34 @@ primitives, HMAC hash functions, and TLS signature schemes is as follows
 | 0x0007 | 0x0011 | 0x0002 | 0x0002 | SHA384 | ecdsa_secp384r1_sha384 |
 
 The hash used for the MLS transcript hash is the one referenced in the
-ciphersuite name.  In the ciphersuites defined above, "SHA256", "SHA384", and "SHA512"
-refer to the SHA-256, SHA-384, and SHA-512 functions defined in {{SHS}}.
+cipher suite name.  In the cipher suites defined above, "SHA256", "SHA384", and
+"SHA512" refer, respectively, to the SHA-256, SHA-384, and SHA-512 functions
+defined in {{SHS}}.
 
-In addition to the general requirements of {{additional-ciphersuites}}, future
-ciphersuites MUST meet the requirements of {{confidentiality-of-sender-data}}.
+In addition to the general requirements of {{additional-cipher-suites}}, future
+cipher suites MUST meet the requirements of {{confidentiality-of-sender-data}}.
 
-It is advisable to keep the number of ciphersuites low to increase the chances
-clients can interoperate in a federated environment, therefore the ciphersuites
-only include modern, yet well-established algorithms.  Depending on their
+It is advisable to keep the number of cipher suites low to increase the likelihood
+that clients can interoperate in a federated environment. The cipher suites therefore
+include only modern, yet well-established algorithms.  Depending on their
 requirements, clients can choose between two security levels (roughly 128-bit
-and 256-bit). Within the security levels clients can choose between faster
-X25519/X448 curves and FIPS 140-2 compliant curves for Diffie-Hellman key
+and 256-bit). Within the security levels, clients can choose between faster
+X25519/X448 curves and curves compliant with FIPS 140-2 for Diffie-Hellman key
 negotiations. Clients may also choose ChaCha20Poly1305 or AES-GCM, e.g., for
-performance reasons. Since ChaCha20Poly1305 is not listed by FIPS 140-2 it is
-not paired with FIPS 140-2 compliant curves. The security level of symmetric
+performance reasons. Since ChaCha20Poly1305 is not listed by FIPS 140-2, it is
+not paired with curves compliant with FIPS 140-2. The security level of symmetric
 encryption algorithms and hash functions is paired with the security level of
 the curves.
 
 
-The mandatory-to-implement ciphersuite for MLS 1.0 is
-`MLS_128_DHKEMX25519_AES128GCM_SHA256_Ed25519` which uses
+The mandatory-to-implement cipher suite for MLS 1.0 is
+`MLS_128_DHKEMX25519_AES128GCM_SHA256_Ed25519`, which uses
 Curve25519 for key exchange, AES-128-GCM for HPKE, HKDF over SHA2-256, and
-Ed25519 for signatures.  MLS clients MUST implement this ciphersuite.
-
-New ciphersuite values are assigned by IANA as described in
-{{iana-considerations}}.
+Ed25519 for signatures.  MLS clients MUST implement this cipher suite.
 
 ## MLS Wire Formats
 
-This registry lists identifiers for the types of messages that can be sent in
+The "MLS Wire Formats" registry lists identifiers for the types of messages that can be sent in
 MLS.  The wire format field is two bytes wide, so the valid wire format values
 are in the range 0x0000 to 0xFFFF.
 
@@ -5853,7 +5453,7 @@ Template:
 
 * Name: The name of the wire format
 
-* Recommended: Same as in {{mls-ciphersuites}}
+* Recommended: Same as in {{mls-cipher-suites}}
 
 * Reference: The document where this wire format is defined
 
@@ -5861,17 +5461,18 @@ Initial contents:
 
 | Value           | Name                     | R | Ref       |
 |:----------------|:-------------------------|:--|:----------|
-| 0x0000          | RESERVED                 | - | RFC XXXX  |
-| 0x0001          | mls_public_message       | Y | RFC XXXX  |
-| 0x0002          | mls_private_message      | Y | RFC XXXX  |
-| 0x0003          | mls_welcome              | Y | RFC XXXX  |
-| 0x0004          | mls_group_info           | Y | RFC XXXX  |
-| 0x0005          | mls_key_package          | Y | RFC XXXX  |
-| 0xF000 - 0xFFFF | Reserved for Private Use | - | RFC XXXX  |
+| 0x0000          | RESERVED                 | - | RFC 9420  |
+| 0x0001          | mls_public_message       | Y | RFC 9420  |
+| 0x0002          | mls_private_message      | Y | RFC 9420  |
+| 0x0003          | mls_welcome              | Y | RFC 9420  |
+| 0x0004          | mls_group_info           | Y | RFC 9420  |
+| 0x0005          | mls_key_package          | Y | RFC 9420  |
+| 0xF000 - 0xFFFF | Reserved for Private Use | - | RFC 9420  |
+{: title="MLS Wire Formats Registry" }
 
 ## MLS Extension Types
 
-This registry lists identifiers for extensions to the MLS protocol.  The
+The "MLS Extension Types" registry lists identifiers for extensions to the MLS protocol.  The
 extension type field is two bytes wide, so valid extension type values are in
 the range 0x0000 to 0xFFFF.
 
@@ -5889,7 +5490,7 @@ Template:
   * GC: GroupContext objects
   * GI: GroupInfo objects
 
-* Recommended: Same as in {{mls-ciphersuites}}
+* Recommended: Same as in {{mls-cipher-suites}}
 
 * Reference: The document where this extension is defined
 
@@ -5897,32 +5498,33 @@ Initial contents:
 
 | Value            | Name                     | Message(s) | R | Ref      |
 |:-----------------|:-------------------------|:-----------|:--|:---------|
-| 0x0000           | RESERVED                 | N/A        | - | RFC XXXX |
-| 0x0001           | application_id           | LN         | Y | RFC XXXX |
-| 0x0002           | ratchet_tree             | GI         | Y | RFC XXXX |
-| 0x0003           | required_capabilities    | GC         | Y | RFC XXXX |
-| 0x0004           | external_pub             | GI         | Y | RFC XXXX |
-| 0x0005           | external_senders         | GC         | Y | RFC XXXX |
-| 0x0A0A           | GREASE                   | KP, GI, LN | Y | RFC XXXX |
-| 0x1A1A           | GREASE                   | KP, GI, LN | Y | RFC XXXX |
-| 0x2A2A           | GREASE                   | KP, GI, LN | Y | RFC XXXX |
-| 0x3A3A           | GREASE                   | KP, GI, LN | Y | RFC XXXX |
-| 0x4A4A           | GREASE                   | KP, GI, LN | Y | RFC XXXX |
-| 0x5A5A           | GREASE                   | KP, GI, LN | Y | RFC XXXX |
-| 0x6A6A           | GREASE                   | KP, GI, LN | Y | RFC XXXX |
-| 0x7A7A           | GREASE                   | KP, GI, LN | Y | RFC XXXX |
-| 0x8A8A           | GREASE                   | KP, GI, LN | Y | RFC XXXX |
-| 0x9A9A           | GREASE                   | KP, GI, LN | Y | RFC XXXX |
-| 0xAAAA           | GREASE                   | KP, GI, LN | Y | RFC XXXX |
-| 0xBABA           | GREASE                   | KP, GI, LN | Y | RFC XXXX |
-| 0xCACA           | GREASE                   | KP, GI, LN | Y | RFC XXXX |
-| 0xDADA           | GREASE                   | KP, GI, LN | Y | RFC XXXX |
-| 0xEAEA           | GREASE                   | KP, GI, LN | Y | RFC XXXX |
-| 0xF000  - 0xFFFF | Reserved for Private Use | N/A        | - | RFC XXXX |
+| 0x0000           | RESERVED                 | N/A        | - | RFC 9420 |
+| 0x0001           | application_id           | LN         | Y | RFC 9420 |
+| 0x0002           | ratchet_tree             | GI         | Y | RFC 9420 |
+| 0x0003           | required_capabilities    | GC         | Y | RFC 9420 |
+| 0x0004           | external_pub             | GI         | Y | RFC 9420 |
+| 0x0005           | external_senders         | GC         | Y | RFC 9420 |
+| 0x0A0A           | GREASE                   | KP, GI, LN | Y | RFC 9420 |
+| 0x1A1A           | GREASE                   | KP, GI, LN | Y | RFC 9420 |
+| 0x2A2A           | GREASE                   | KP, GI, LN | Y | RFC 9420 |
+| 0x3A3A           | GREASE                   | KP, GI, LN | Y | RFC 9420 |
+| 0x4A4A           | GREASE                   | KP, GI, LN | Y | RFC 9420 |
+| 0x5A5A           | GREASE                   | KP, GI, LN | Y | RFC 9420 |
+| 0x6A6A           | GREASE                   | KP, GI, LN | Y | RFC 9420 |
+| 0x7A7A           | GREASE                   | KP, GI, LN | Y | RFC 9420 |
+| 0x8A8A           | GREASE                   | KP, GI, LN | Y | RFC 9420 |
+| 0x9A9A           | GREASE                   | KP, GI, LN | Y | RFC 9420 |
+| 0xAAAA           | GREASE                   | KP, GI, LN | Y | RFC 9420 |
+| 0xBABA           | GREASE                   | KP, GI, LN | Y | RFC 9420 |
+| 0xCACA           | GREASE                   | KP, GI, LN | Y | RFC 9420 |
+| 0xDADA           | GREASE                   | KP, GI, LN | Y | RFC 9420 |
+| 0xEAEA           | GREASE                   | KP, GI, LN | Y | RFC 9420 |
+| 0xF000  - 0xFFFF | Reserved for Private Use | N/A        | - | RFC 9420 |
+{: title="MLS Extension Types Registry" }
 
 ## MLS Proposal Types
 
-This registry lists identifiers for types of proposals that can be made for
+The "MLS Proposal Types" registry lists identifiers for types of proposals that can be made for
 changes to an MLS group.  The extension type field is two bytes wide, so valid
 extension type values are in the range 0x0000 to 0xFFFF.
 
@@ -5932,13 +5534,13 @@ Template:
 
 * Name: The name of the proposal type
 
-* Recommended: Same as in {{mls-ciphersuites}}
+* Recommended: Same as in {{mls-cipher-suites}}
 
 * External: Whether a proposal of this type may be sent by an `external` sender
-  (see {{external-proposals}}).
+  (see {{external-proposals}})
 
 * Path Required: Whether a Commit covering a proposal of this type is required
-  to have its `path` field populated (see {{commit}}).
+  to have its `path` field populated (see {{commit}})
 
 * Reference: The document where this extension is defined
 
@@ -5946,34 +5548,35 @@ Initial contents:
 
 | Value            | Name                     | R | Ext | Path | Ref      |
 |:-----------------|:-------------------------|:--|:----|:-----|:---------|
-| 0x0000           | RESERVED                 | - | -   | -    | RFC XXXX |
-| 0x0001           | add                      | Y | Y   | N    | RFC XXXX |
-| 0x0002           | update                   | Y | N   | Y    | RFC XXXX |
-| 0x0003           | remove                   | Y | Y   | Y    | RFC XXXX |
-| 0x0004           | psk                      | Y | Y   | N    | RFC XXXX |
-| 0x0005           | reinit                   | Y | Y   | N    | RFC XXXX |
-| 0x0006           | external_init            | Y | N   | Y    | RFC XXXX |
-| 0x0007           | group_context_extensions | Y | Y   | Y    | RFC XXXX |
-| 0x0A0A           | GREASE                   | Y | -   | -    | RFC XXXX |
-| 0x1A1A           | GREASE                   | Y | -   | -    | RFC XXXX |
-| 0x2A2A           | GREASE                   | Y | -   | -    | RFC XXXX |
-| 0x3A3A           | GREASE                   | Y | -   | -    | RFC XXXX |
-| 0x4A4A           | GREASE                   | Y | -   | -    | RFC XXXX |
-| 0x5A5A           | GREASE                   | Y | -   | -    | RFC XXXX |
-| 0x6A6A           | GREASE                   | Y | -   | -    | RFC XXXX |
-| 0x7A7A           | GREASE                   | Y | -   | -    | RFC XXXX |
-| 0x8A8A           | GREASE                   | Y | -   | -    | RFC XXXX |
-| 0x9A9A           | GREASE                   | Y | -   | -    | RFC XXXX |
-| 0xAAAA           | GREASE                   | Y | -   | -    | RFC XXXX |
-| 0xBABA           | GREASE                   | Y | -   | -    | RFC XXXX |
-| 0xCACA           | GREASE                   | Y | -   | -    | RFC XXXX |
-| 0xDADA           | GREASE                   | Y | -   | -    | RFC XXXX |
-| 0xEAEA           | GREASE                   | Y | -   | -    | RFC XXXX |
-| 0xF000  - 0xFFFF | Reserved for Private Use | - | -   | -    | RFC XXXX |
+| 0x0000           | RESERVED                 | - | -   | -    | RFC 9420 |
+| 0x0001           | add                      | Y | Y   | N    | RFC 9420 |
+| 0x0002           | update                   | Y | N   | Y    | RFC 9420 |
+| 0x0003           | remove                   | Y | Y   | Y    | RFC 9420 |
+| 0x0004           | psk                      | Y | Y   | N    | RFC 9420 |
+| 0x0005           | reinit                   | Y | Y   | N    | RFC 9420 |
+| 0x0006           | external_init            | Y | N   | Y    | RFC 9420 |
+| 0x0007           | group_context_extensions | Y | Y   | Y    | RFC 9420 |
+| 0x0A0A           | GREASE                   | Y | -   | -    | RFC 9420 |
+| 0x1A1A           | GREASE                   | Y | -   | -    | RFC 9420 |
+| 0x2A2A           | GREASE                   | Y | -   | -    | RFC 9420 |
+| 0x3A3A           | GREASE                   | Y | -   | -    | RFC 9420 |
+| 0x4A4A           | GREASE                   | Y | -   | -    | RFC 9420 |
+| 0x5A5A           | GREASE                   | Y | -   | -    | RFC 9420 |
+| 0x6A6A           | GREASE                   | Y | -   | -    | RFC 9420 |
+| 0x7A7A           | GREASE                   | Y | -   | -    | RFC 9420 |
+| 0x8A8A           | GREASE                   | Y | -   | -    | RFC 9420 |
+| 0x9A9A           | GREASE                   | Y | -   | -    | RFC 9420 |
+| 0xAAAA           | GREASE                   | Y | -   | -    | RFC 9420 |
+| 0xBABA           | GREASE                   | Y | -   | -    | RFC 9420 |
+| 0xCACA           | GREASE                   | Y | -   | -    | RFC 9420 |
+| 0xDADA           | GREASE                   | Y | -   | -    | RFC 9420 |
+| 0xEAEA           | GREASE                   | Y | -   | -    | RFC 9420 |
+| 0xF000  - 0xFFFF | Reserved for Private Use | - | -   | -    | RFC 9420 |
+{: title="MLS Proposal Types Registry" }
 
 ## MLS Credential Types
 
-This registry lists identifiers for types of credentials that can be used for
+The "MLS Credential Types" registry lists identifiers for types of credentials that can be used for
 authentication in the MLS protocol.  The credential type field is two bytes wide,
 so valid credential type values are in the range 0x0000 to 0xFFFF.
 
@@ -5983,7 +5586,7 @@ Template:
 
 * Name: The name of the credential type
 
-* Recommended: Same as in {{mls-ciphersuites}}
+* Recommended: Same as in {{mls-cipher-suites}}
 
 * Reference: The document where this credential is defined
 
@@ -5991,92 +5594,95 @@ Initial contents:
 
 | Value            | Name                     | R | Ref      |
 |:-----------------|:-------------------------|:--|:---------|
-| 0x0000           | RESERVED                 | - | RFC XXXX |
-| 0x0001           | basic                    | Y | RFC XXXX |
-| 0x0002           | x509                     | Y | RFC XXXX |
-| 0x0A0A           | GREASE                   | Y | RFC XXXX |
-| 0x1A1A           | GREASE                   | Y | RFC XXXX |
-| 0x2A2A           | GREASE                   | Y | RFC XXXX |
-| 0x3A3A           | GREASE                   | Y | RFC XXXX |
-| 0x4A4A           | GREASE                   | Y | RFC XXXX |
-| 0x5A5A           | GREASE                   | Y | RFC XXXX |
-| 0x6A6A           | GREASE                   | Y | RFC XXXX |
-| 0x7A7A           | GREASE                   | Y | RFC XXXX |
-| 0x8A8A           | GREASE                   | Y | RFC XXXX |
-| 0x9A9A           | GREASE                   | Y | RFC XXXX |
-| 0xAAAA           | GREASE                   | Y | RFC XXXX |
-| 0xBABA           | GREASE                   | Y | RFC XXXX |
-| 0xCACA           | GREASE                   | Y | RFC XXXX |
-| 0xDADA           | GREASE                   | Y | RFC XXXX |
-| 0xEAEA           | GREASE                   | Y | RFC XXXX |
-| 0xF000  - 0xFFFF | Reserved for Private Use | - | RFC XXXX |
+| 0x0000           | RESERVED                 | - | RFC 9420 |
+| 0x0001           | basic                    | Y | RFC 9420 |
+| 0x0002           | x509                     | Y | RFC 9420 |
+| 0x0A0A           | GREASE                   | Y | RFC 9420 |
+| 0x1A1A           | GREASE                   | Y | RFC 9420 |
+| 0x2A2A           | GREASE                   | Y | RFC 9420 |
+| 0x3A3A           | GREASE                   | Y | RFC 9420 |
+| 0x4A4A           | GREASE                   | Y | RFC 9420 |
+| 0x5A5A           | GREASE                   | Y | RFC 9420 |
+| 0x6A6A           | GREASE                   | Y | RFC 9420 |
+| 0x7A7A           | GREASE                   | Y | RFC 9420 |
+| 0x8A8A           | GREASE                   | Y | RFC 9420 |
+| 0x9A9A           | GREASE                   | Y | RFC 9420 |
+| 0xAAAA           | GREASE                   | Y | RFC 9420 |
+| 0xBABA           | GREASE                   | Y | RFC 9420 |
+| 0xCACA           | GREASE                   | Y | RFC 9420 |
+| 0xDADA           | GREASE                   | Y | RFC 9420 |
+| 0xEAEA           | GREASE                   | Y | RFC 9420 |
+| 0xF000  - 0xFFFF | Reserved for Private Use | - | RFC 9420 |
+{: title="MLS Credential Types Registry" }
 
 ## MLS Signature Labels
 
 The `SignWithLabel` function defined in {{signing}} avoids the risk of
 confusion between signatures in different contexts.  Each context is assigned a
-distinct label that is incorporated into the signature.  This registry records
-the labels defined in this document, and allows additional labels to be
-registered in case extensions add other types of signature using the same
+distinct label that is incorporated into the signature.  The "MLS Signature Labels" registry records
+the labels defined in this document and allows additional labels to be
+registered in case extensions add other types of signatures using the same
 signature keys used elsewhere in MLS.
 
 Template:
 
 * Label: The string to be used as the `Label` parameter to `SignWithLabel`
 
-* Recommended: Same as in {{mls-ciphersuites}}
+* Recommended: Same as in {{mls-cipher-suites}}
 
-* Reference: The document where this credential is defined
+* Reference: The document where this label is defined
 
 Initial contents:
 
 | Label              | R | Ref      |
 |:-------------------|:--|:---------|
-| "FramedContentTBS" | Y | RFC XXXX |
-| "LeafNodeTBS"      | Y | RFC XXXX |
-| "KeyPackageTBS"    | Y | RFC XXXX |
-| "GroupInfoTBS"     | Y | RFC XXXX |
+| "FramedContentTBS" | Y | RFC 9420 |
+| "LeafNodeTBS"      | Y | RFC 9420 |
+| "KeyPackageTBS"    | Y | RFC 9420 |
+| "GroupInfoTBS"     | Y | RFC 9420 |
+{: title="MLS Signature Labels Registry" }
 
 ## MLS Public Key Encryption Labels
 
 The `EncryptWithLabel` function defined in {{public-key-encryption}} avoids the
 risk of confusion between ciphertexts produced for different purposes in
 different contexts.  Each context is assigned a distinct label that is
-incorporated into the signature.  This registry records the labels defined in
-this document, and allows additional labels to be registered in case extensions
-add other types of public-key encryption using the same HPKE keys used elsewhere
+incorporated into the signature.  The "MLS Public Key Encryption Labels" registry records the labels defined in
+this document and allows additional labels to be registered in case extensions
+add other types of public key encryption using the same HPKE keys used elsewhere
 in MLS.
 
 Template:
 
 * Label: The string to be used as the `Label` parameter to `EncryptWithLabel`
 
-* Recommended: Same as in {{mls-ciphersuites}}
+* Recommended: Same as in {{mls-cipher-suites}}
 
-* Reference: The document where this credential is defined
+* Reference: The document where this label is defined
 
 Initial contents:
 
 | Label            | R | Ref      |
 |:-----------------|:--|:---------|
-| "UpdatePathNode" | Y | RFC XXXX |
-| "Welcome"        | Y | RFC XXXX |
+| "UpdatePathNode" | Y | RFC 9420 |
+| "Welcome"        | Y | RFC 9420 |
+{: title="MLS Public Key Encryption Labels Registry" }
 
 ## MLS Exporter Labels
 
 The exporter function defined in {{exporters}} allows applications to derive key
 material from the MLS key schedule.  Like the TLS exporter {{RFC8446}}, the MLS
 exporter uses a label to distinguish between different applications' use of the
-exporter.  This registry allows applications to register their usage to avoid
+exporter.  The "MLS Exporter Labels" registry allows applications to register their usage to avoid
 collisions.
 
 Template:
 
 * Label: The string to be used as the `Label` parameter to `MLS-Exporter`
 
-* Recommended: Same as in {{mls-ciphersuites}}
+* Recommended: Same as in {{mls-cipher-suites}}
 
-* Reference: The document where this credential is defined
+* Reference: The document where this label is defined
 
 The registry has no initial contents, since it is intended to be used by
 applications, not the core protocol.  The table below is intended only to show
@@ -6085,22 +5691,23 @@ the column layout of the registry.
 | Label | Recommended | Reference |
 |:------|:------------|:----------|
 | (N/A) | (N/A)       | (N/A)     |
+{: title="MLS Exporter Labels Registry" }
 
 ## MLS Designated Expert Pool {#de}
 
 Specification Required {{RFC8126}} registry requests are registered
-after a three-week review period on the MLS DEs' mailing list:
-<mls-reg-review@ietf.org>, on the advice of one or more of the MLS DEs. However,
+after a three-week review period on the MLS Designated Expert (DE) mailing list
+<mls-reg-review@ietf.org> on the advice of one or more of the MLS DEs. However,
 to allow for the allocation of values prior to publication, the MLS
 DEs may approve registration once they are satisfied that such a
 specification will be published.
 
-Registration requests sent to the MLS DEs mailing list for review
+Registration requests sent to the MLS DEs' mailing list for review
 SHOULD use an appropriate subject (e.g., "Request to register value
 in MLS Bar registry").
 
 Within the review period, the MLS DEs will either approve or deny
-the registration request, communicating this decision to the MLS DEs
+the registration request, communicating this decision to the MLS DEs'
 mailing list and IANA. Denials SHOULD include an explanation and, if
 applicable, suggestions as to how to make the request successful.
 Registration requests that are undetermined for a period longer than
@@ -6111,22 +5718,22 @@ Criteria that SHOULD be applied by the MLS DEs includes determining
 whether the proposed registration duplicates existing functionality,
 whether it is likely to be of general applicability or useful only
 for a single application, and whether the registration description
-is clear. For example, the MLS DEs will apply the ciphersuite-related
-advisory found in {{mls-ciphersuites}}.
+is clear. For example, for cipher suite registrations, the MLS DEs will apply the
+advisory found in {{mls-cipher-suites}}.
 
 IANA MUST only accept registry updates from the MLS DEs and SHOULD
 direct all requests for registration to the MLS DEs' mailing list.
 
-It is suggested that multiple MLS DEs be appointed who are able to
+It is suggested that multiple MLS DEs who are able to
 represent the perspectives of different applications using this
-specification, in order to enable broadly informed review of
+specification be appointed, in order to enable a broadly informed review of
 registration decisions. In cases where a registration decision could
 be perceived as creating a conflict of interest for a particular
 MLS DE, that MLS DE SHOULD defer to the judgment of the other MLS DEs.
 
-## The "message/mls" MIME Type
+## The "message/mls" Media Type
 
-This document registers the "message/mls" MIME media type in order to allow other
+This document registers the "message/mls" media type in the "message" registry in order to allow other
 protocols (e.g., HTTP {{?RFC9113}}) to convey MLS messages.
 
 Type name:
@@ -6142,27 +5749,27 @@ Optional parameters:
 : version
     version:
     : The MLS protocol version expressed as a string
-    `<major>.<minor>`.  If omitted the version is "1.0", which
+    `<major>.<minor>`.  If omitted, the version is "1.0", which
     corresponds to MLS ProtocolVersion mls10. If for some reason
-    the version number in the MIME type parameter differs from the
+    the version number in the media type parameter differs from the
     ProtocolVersion embedded in the protocol, the protocol takes
     precedence.
 
 Encoding considerations:
 : MLS messages are represented using the TLS
-  presentation language [RFC8446]. Therefore MLS messages need to be
+  presentation language [RFC8446]. Therefore, MLS messages need to be
   treated as binary data.
 
 Security considerations:
 : MLS is an encrypted messaging layer designed
-  to be transmitted over arbitrary lower layer protocols. The
-  security considerations in this document (RFC XXXX) also apply.
+  to be transmitted over arbitrary lower-layer protocols. The
+  security considerations in this document (RFC 9420) also apply.
 
 Interoperability considerations:
 : N/A
 
 Published specification:
-: RFC XXXX
+: RFC 9420
 
 Applications that use this media type:
 : MLS-based messaging applications
@@ -6171,11 +5778,19 @@ Fragment identifier considerations:
 : N/A
 
 Additional information:
+:
+
+    Deprecated alias names for this type:
+    : N/A
 
-  * Deprecated alias names for this type: N/A
-  * Magic number(s): N/A
-  * File extension(s): N/A
-  * Macintosh file type code(s): N/A
+    Magic number(s):
+    : N/A
+
+    File extension(s):
+    : N/A
+
+    Macintosh file type code(s):
+    : N/A
 
 Person & email address to contact for further information:
 : IETF MLS Working Group <mls@ietf.org>
@@ -6192,9 +5807,6 @@ Author:
 Change controller:
 : IESG
 
-Provisional registration? (standards tree only):
-: No
-
 
 --- back
 
@@ -6208,8 +5820,8 @@ in this document.
 To construct the tree in {{full-tree}}:
 
 * A creates a group with B, ..., G
-* F sends an empty Commit, setting X, Y, W
-* G removes C and D, blanking V, U, and setting Y, W
+* F sends an empty Commit, setting X, Y, and W
+* G removes C and D, blanking V, U, and setting Y and W
 * B sends an empty Commit, setting T and W
 
 To construct the tree in {{resolution-tree}}:
@@ -6224,7 +5836,7 @@ To construct the tree in {{resolution-tree}}:
 
 To construct the tree in {{evolution-tree}}:
 
-* A creates a group with B, C, D
+* A creates a group with B, C, and D
 * B sends a full Commit, setting X and Y
 * D removes C, setting Z and Y
 * B adds a new member at C with a full Commit
@@ -6247,7 +5859,7 @@ operations:
 1. A initializes a new group
 2. A adds B to the group with a full Commit
 3. B adds C and D to the group with a full Commit
-4. C sends an empty Commit.
+4. C sends an empty Commit
 
 ~~~ aasvg
                           Y                   Y'
@@ -6258,7 +5870,7 @@ operations:
          / \         / \     / \         / \     / \
 A       A   B       A   B   C   D       A   B   C   D
 ~~~
-{: title="Building a four-member tree to illustrate parent hashes" }
+{: title="Building a Four-Member Tree to Illustrate Parent Hashes" }
 
 Then the parent hashes associated to the nodes will be updated as follows (where
 we use the shorthand `ph` for parent hash, `th` for tree hash, and `osth` for
@@ -6268,7 +5880,7 @@ original sibling tree hash):
 
     * `A.parent_hash = ph(X) = H(X, ph="", osth=th(B))`
 
-2. B adds C, D: set B', X', Y
+2. B adds C, D: set B', X', and Y
 
     * `X'.parent_hash = ph(Y)  = H(Y, ph="", osth=th(Z))`,
       where `th(Z)` covers `(C, _, D)`
@@ -6280,9 +5892,9 @@ original sibling tree hash):
     * `C'.parent_hash = ph(Z') = H(Z', ph=Z'.parent_hash, osth=th(D))`
 
 When a new member joins, they will receive a tree that has the following parent
-hash values, and compute the indicated parent-hash validity relationships:
+hash values and compute the indicated parent hash validity relationships:
 
-| Node | Parent hash value                    | Valid?              |
+| Node | Parent Hash Value                    | Valid?              |
 |:-----|:-------------------------------------|:--------------------|
 | A    | H(X, ph="", osth=th(B))              | No, B changed       |
 | B'   | H(X', ph=X'.parent_hash, osth=th(A)) | Yes                 |
@@ -6310,7 +5922,7 @@ the tree is parent-hash valid.
 
 Note that this tree, though valid, contains invalid parent-hash links. If a
 client were checking parent hashes top-down from Y', for example, they would
-find that X' has an invalid parent hash relative to Y', but that Z' has valid
+find that X' has an invalid parent hash relative to Y', but that Z' has a valid
 parent hash.  Likewise, if the client were checking bottom-up, they would find
 that the chain from B' ends in an invalid link from X' to Y'.  These invalid
 links are the natural result of multiple clients having committed.
@@ -6319,7 +5931,7 @@ Note also the way the tree hash and the parent hash interact.  The parent hash
 of node C' includes the tree hash of node D.  The parent hash of node Z'
 includes the tree hash of X', which covers nodes A and B' (including the parent
 hash of B').  Although the tree hash and the parent hash depend on each other,
-the dependency relationships are structured so that there's never a circular
+the dependency relationships are structured so that there is never a circular
 dependency.
 
 In the particular case where a new member first receives the tree for a group
@@ -6327,7 +5939,7 @@ In the particular case where a new member first receives the tree for a group
 parent hashes will be expressed in the tree representation, but the tree hash
 need not be.  Instead, the new member will recompute the tree hashes for all the
 nodes in the tree, verifying that this matches the tree hash in the GroupInfo
-object.  Then, if the tree is valid, then the subtree hashes computed in this
+object.  If the tree is valid, then the subtree hashes computed in this
 way will align with the inputs needed for parent hash validation (except where
 recomputation is needed to account for unmerged leaves).
 
@@ -6357,7 +5969,7 @@ Node: 0  1  2  3  4  5  6  7  8  9 10 11 12 13 14
 
 Leaf: 0     1     2     3     4     5     6     7
 ~~~
-{: title="An 8-member tree represented as an array" }
+{: title="An Eight-Member Tree Represented as an Array" }
 
 This allows us to compute relationships between tree nodes simply by
 manipulating indices, rather than having to maintain complicated structures in
@@ -6504,10 +6116,10 @@ def common_ancestor_direct(x, y, _):
 
 An implementation may choose to store ratchet trees in a "link-based"
 representation, where each node stores references to its parents and/or
-children.   (As opposed to the array-based representation suggested above, where
+children (as opposed to the array-based representation suggested above, where
 these relationships are computed from relationships between nodes' indices in
-the array.)  Such an implementation needs to update these links to maintain the
-balanced structure of the tree as the tree is extended to add new members,
+the array). Such an implementation needs to update these links to maintain the
+balanced structure of the tree as the tree is extended to add new members
 or truncated when members are removed.
 
 The following code snippet shows how these algorithms could be implemented in
@@ -6531,7 +6143,7 @@ class Node:
 
     def empty(self):
         L_empty = (self.left == None) or self.left.empty()
-        R_empty = (self.left == None) or self.left.empty()
+        R_empty = (self.right == None) or self.right.empty()
         return (self.value == None) and L_empty and R_empty
 
 class Tree:
@@ -6552,8 +6164,8 @@ class Tree:
 
     # Truncate the right subtree
     def truncate(self):
-        if self.root == None or self.root.right == None:
-            raise Exception("Cannot truncate a tree with 0 or 1 nodes")
+        if self.root == None:
+            return
 
         if not self.root.right.empty():
             raise Exception("Cannot truncate non-blank subtree")