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ZIP 316: clarify requirements, especially for Unified Viewing Keys #536

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ZIP 316: clarify requirements, especially for Unified Viewing Keys #536

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af1bee0
ZIP 316: clarify requirements, especially for Unified Viewing Keys.
daira Jul 12, 2021
fed2bc0
ZIP 316: more instances of Sender that should be Consumer.
daira Jul 12, 2021
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60 changes: 33 additions & 27 deletions zip-0316.html
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<dd>A wallet or other software that can receive transfers of assets (such as ZEC) or in the future potentially other transaction-based state changes.</dd>
<dt>Producer</dt>
<dd>A wallet or other software that can create an Address (normally also a Recipient).</dd>
<dt>Consumer</dt>
<dd>A wallet or other software that can make use of an Address that it is given.</dd>
<dt>Sender</dt>
<dd>A wallet or other software that can send transfers of assets, or other consensus state side-effects defined in future.</dd>
<dd>A wallet or other software that can send transfers of assets, or other consensus state side-effects defined in future. Senders are a subset of Consumers.</dd>
<dt>Receiver</dt>
<dd>The necessary information to transfer an asset to a Recipient that generated that Receiver using a specific Transfer Protocol. Each Receiver is associated unambiguously with a specific Receiver Type, identified by an integer Typecode.</dd>
<dt>Receiver Encoding</dt>
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</ul>
<p>Each of these has its own Address Encodings, as a string and as a QR code. (Since the QR code is derivable from the string encoding, for many purposes it suffices to consider the string encoding.)</p>
<p>The Orchard proposal <a id="id2" class="footnote_reference" href="#zip-0224">6</a> adds a new Address type, Orchard Addresses.</p>
<p>The difficulty with defining new Address Encodings for each Address type, is that end-users are forced to be aware of the various types, and in particular which types are supported by a given Sender or Recipient. In order to make sure that transfers are completed successfully, users may be forced to explicitly generate Addresses of different types and re-distribute encodings of them, which adds significant friction and cognitive overhead to understanding and using Zcash.</p>
<p>The difficulty with defining new Address Encodings for each Address type, is that end-users are forced to be aware of the various types, and in particular which types are supported by a given Consumer or Recipient. In order to make sure that transfers are completed successfully, users may be forced to explicitly generate Addresses of different types and re-distribute encodings of them, which adds significant friction and cognitive overhead to understanding and using Zcash.</p>
<p>The goals for a Unified Address standard are as follows:</p>
<ul>
<li>Simplify coordination between Recipients and Senders by removing complexity from negotiating Address types.</li>
<li>Simplify coordination between Recipients and Consumers by removing complexity from negotiating Address types.</li>
<li>Provide a “bridging mechanism” to allow shielded wallets to successfully interact with conformant Transparent-Only wallets.</li>
<li>Allow older conformant wallets to interact seamlessly with newer wallets.</li>
<li>Enable users of newer wallets to upgrade to newer transaction technologies and/or pools while maintaining seamless interactions with counterparties using older wallets.</li>
Expand All @@ -89,11 +91,11 @@
<li>A Producer <em>generates</em> an Address.</li>
<li>The Producer <em>encodes</em> the Address.</li>
<li>The Producer wallet or human user <em>distributes</em> this Address Encoding, This ZIP leaves distribution mechanisms out of scope.</li>
<li>A Sender wallet or user <em>imports</em> the Address Encoding through any of a variety of mechanisms (QR Code scanning, Payment URIs, cut-and-paste, or “in-band” protocols like <code>Reply-To</code> memos).</li>
<li>A Sender wallet <em>decodes</em> the Address Encoding and performs validity checks.</li>
<li>(Perhaps later in time) the Sender wallet executes a transfer of ZEC (or other assets or protocol state changes) to the Address.</li>
<li>A Consumer wallet or user <em>imports</em> the Address Encoding through any of a variety of mechanisms (QR Code scanning, Payment URIs, cut-and-paste, or “in-band” protocols like <code>Reply-To</code> memos).</li>
<li>A Consumer wallet <em>decodes</em> the Address Encoding and performs validity checks.</li>
<li>(Perhaps later in time) if the Consumer wallet is a Sender, it can execute a transfer of ZEC (or other assets or protocol state changes) to the Address.</li>
</ol>
<p>Encodings of the same Address may be distributed zero or more times through different means. Zero or more Senders may import Addresses. Zero or more of those may execute a Transfer. A single Sender may execute multiple Transfers over time from a single import.</p>
<p>Encodings of the same Address may be distributed zero or more times through different means. Zero or more Consumers may import Addresses. Zero or more of those (that are Senders) may execute a Transfer. A single Sender may execute multiple Transfers over time from a single import.</p>
<p>Steps 1 to 5 inclusive also apply to Interaction Flows for Unified Full Viewing Keys and Unified Incoming Viewing Keys.</p>
</section>
<section id="addresses"><h3><span class="section-heading">Addresses</span><span class="section-anchor"> <a rel="bookmark" href="#addresses"><img width="24" height="24" src="assets/images/section-anchor.png" alt=""></a></span></h3>
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</section>
<section id="specification"><h2><span class="section-heading">Specification</span><span class="section-anchor"> <a rel="bookmark" href="#specification"><img width="24" height="24" src="assets/images/section-anchor.png" alt=""></a></span></h2>
<section id="encoding-of-unified-addresses"><h3><span class="section-heading">Encoding of Unified Addresses</span><span class="section-anchor"> <a rel="bookmark" href="#encoding-of-unified-addresses"><img width="24" height="24" src="assets/images/section-anchor.png" alt=""></a></span></h3>
<p>Rather than defining a Bech32 string encoding of Orchard Shielded Payment Addresses, we instead define a Unified Address format that is able to encode a set of Receivers of different types. This enables the consumer of a Unified Address (i.e. the Sender) to choose the Receiver of the best type it supports, providing a better user experience as new Receiver Types are added in the future.</p>
<p>Rather than defining a Bech32 string encoding of Orchard Shielded Payment Addresses, we instead define a Unified Address format that is able to encode a set of Receivers of different types. This enables the Consumer of a Unified Address to choose the Receiver of the best type it supports, providing a better user experience as new Receiver Types are added in the future.</p>
<p>Assume that we are given a set of one or more Receiver Encodings for distinct types. That is, the set may optionally contain one Receiver of each of the Receiver Types in the following fixed Priority List:</p>
<ul>
<li>Typecode
Expand All @@ -150,9 +152,6 @@
— a Transparent P2PKH address.</li>
</ul>
<p>We say that a Receiver Type is “preferred” over another when it appears earlier in this Priority List.</p>
<p>A Unified Address MUST contain at least one shielded Receiver (Typecodes
<span class="math">\(\geq \mathtt{0x02}\)</span>
).</p>
<p>The Sender of a payment to a Unified Address MUST use the Receiver of the most preferred Receiver Type that it supports from the set.</p>
<p>For example, consider a wallet that supports sending funds to Orchard Receivers, and does not support sending to any Receiver Type that is preferred over Orchard. If that wallet is given a UA that includes an Orchard Receiver and possibly other Receivers, it MUST send to the Orchard Receiver.</p>
<p>The raw encoding of a Unified Address is a concatenation of
Expand All @@ -179,7 +178,7 @@
<p>We append 16 zero bytes to the concatenated encodings, and then apply the
<span class="math">\(\mathsf{F4Jumble}\)</span>
algorithm as described in <a href="#address-hardening">Address Hardening</a>. The output is then encoded with Bech32m <a id="id10" class="footnote_reference" href="#bip-0350">8</a>, ignoring any length restrictions. This is chosen over Bech32 in order to better handle variable-length inputs.</p>
<p>To decode a Unified Address Encoding, a Sender MUST use the following procedure:</p>
<p>To decode a Unified Address Encoding, a Consumer MUST use the following procedure:</p>
<ul>
<li>Decode using Bech32m, rejecting any address with an incorrect checksum.</li>
<li>Apply
Expand All @@ -189,29 +188,36 @@
<li>Parse the result as a raw encoding as described above, rejecting the entire Unified Address if it does not parse correctly.</li>
</ul>
<p>For Unified Addresses on Mainnet, the Human-Readable Part (as defined in <a id="id11" class="footnote_reference" href="#bip-0350">8</a>) is “<code>u</code>”. For Unified Addresses on Testnet, the Human-Readable Part is “<code>utest</code>”.</p>
<p>Notes:</p>
<p>A wallet MAY allow its user(s) to configure which Receiver Types it can send to. It MUST NOT allow the user(s) to change the order of the Priority List used to choose the Receiver Type.</p>
</section>
<section id="encoding-of-unified-full-incoming-viewing-keys"><h3><span class="section-heading">Encoding of Unified Full/Incoming Viewing Keys</span><span class="section-anchor"> <a rel="bookmark" href="#encoding-of-unified-full-incoming-viewing-keys"><img width="24" height="24" src="assets/images/section-anchor.png" alt=""></a></span></h3>
<p>Unified Full or Incoming Viewing Keys are encoded and decoded analogously to Unified Addresses. A Consumer MUST use the decoding procedure from the previous section. The same Priority List and the same Typecodes are used. The Priority List only applies to situations in which a Consumer needs to choose a particular Receiver.</p>
<p>For Shielded Addresses, the encoding used in place of the
<span class="math">\(\mathtt{addr}\)</span>
field is the raw encoding of the Full Viewing Key or Incoming Viewing Key.</p>
<p>Transparent Addresses do not have separate corresponding viewing keys, but the address itself can effectively be used as a viewing key. Therefore, a UFVK or UIVK MAY include a Transparent Address, which is encoded using the same Typecode and Receiver Encoding as in a Unified Address.</p>
</section>
<section id="requirements-for-both-unified-addresses-and-unified-viewing-keys"><h3><span class="section-heading">Requirements for both Unified Addresses and Unified Viewing Keys</span><span class="section-anchor"> <a rel="bookmark" href="#requirements-for-both-unified-addresses-and-unified-viewing-keys"><img width="24" height="24" src="assets/images/section-anchor.png" alt=""></a></span></h3>
<ul>
<li>A Unified Address or Unified Viewing Key MUST NOT contain only transparent P2SH or P2PKH addresses (Typecodes
<span class="math">\(\mathtt{0x00}\)</span>
and
<span class="math">\(\mathtt{0x01}\)</span>
). The rationale is that the existing P2SH and P2PKH transparent-only address formats suffice for this purpose and are already supported by the existing ecosystem.</li>
<li>The
<span class="math">\(\mathtt{length}\)</span>
field is always encoded as a single byte, <em>not</em> as a
<span class="math">\(\mathtt{compactSize}\)</span>
.</li>
<li>For Transparent Addresses, the Receiver Encoding does not include the first two bytes of a raw encoding.</li>
<li>There is intentionally no Typecode defined for a Sprout Shielded Payment Address. Since it is no longer possible (since activation of ZIP 211 in the Canopy network upgrade <a id="id12" class="footnote_reference" href="#zip-0211">5</a>) to send funds into the Sprout chain value pool, this would not be generally useful.</li>
<li>Senders MUST ignore constituent Addresses with Typecodes they do not recognize.</li>
<li>Senders MUST reject Unified Addresses in which the same Typecode appears more than once, or that include both P2SH and P2PKH Transparent Addresses, or that contain only a Transparent Address.</li>
<li>Senders MUST reject Unified Addresses in which <em>any</em> constituent address does not meet the validation requirements of its Receiver Encoding, as specified in the Zcash Protocol Specification <a id="id13" class="footnote_reference" href="#protocol-nu5">2</a>.</li>
<li>Producers SHOULD order the constituent Addresses in the same order as in the Priority List above. However, Senders MUST NOT assume this ordering, and it does not affect which Address should be used by a Sender.</li>
<li>There is intentionally no Typecode defined for a Sprout Shielded Payment Address or Sprout Incoming Viewing Key. Since it is no longer possible (since activation of ZIP 211 in the Canopy network upgrade <a id="id12" class="footnote_reference" href="#zip-0211">5</a>) to send funds into the Sprout chain value pool, this would not be generally useful.</li>
<li>Consumers MUST ignore constituent Addresses/Viewing Keys with Typecodes they do not recognize.</li>
<li>Consumers MUST reject Unified Addresses/Viewing Keys in which the same Typecode appears more than once, or that include both P2SH and P2PKH Transparent Addresses, or that contain only a Transparent Address.</li>
<li>Consumers MUST reject Unified Addresses/Viewing Keys in which <em>any</em> constituent address does not meet the validation requirements of its Receiver Encoding, as specified in the Zcash Protocol Specification <a id="id13" class="footnote_reference" href="#protocol-nu5">2</a>.</li>
<li>Producers SHOULD order the constituent Addresses/Viewing Keys in the same order as in the Priority List above. However, Consumers MUST NOT assume this ordering, and it does not affect which Address should be used by a Sender.</li>
<li>There MUST NOT be additional bytes at the end of the raw encoding that cannot be interpreted as specified above.</li>
<li>A wallet MAY allow its user(s) to configure which Receiver Types it can send to. It MUST NOT allow the user(s) to change the order of the Priority List used to choose the Receiver Type.</li>
</ul>
</section>
<section id="encoding-of-unified-full-incoming-viewing-keys"><h3><span class="section-heading">Encoding of Unified Full/Incoming Viewing Keys</span><span class="section-anchor"> <a rel="bookmark" href="#encoding-of-unified-full-incoming-viewing-keys"><img width="24" height="24" src="assets/images/section-anchor.png" alt=""></a></span></h3>
<p>Unified Full or Incoming Viewing Keys are encoded analogously to Unified Addresses. The same Priority List and the same Typecodes are used. For Shielded Addresses, the encoding used in place of the
<span class="math">\(\mathtt{addr}\)</span>
field is the raw encoding of the Full Viewing Key or Incoming Viewing Key.</p>
<p>Transparent Addresses do not have separate corresponding viewing keys, but the address itself can effectively be used as a viewing key. Therefore, a UFVK or UIVK MAY include a Transparent Address, which is encoded using the same Typecode and Receiver Encoding as in a Unified Address.</p>
</section>
<section id="address-hardening"><h3><span class="section-heading">Address hardening</span><span class="section-anchor"> <a rel="bookmark" href="#address-hardening"><img width="24" height="24" src="assets/images/section-anchor.png" alt=""></a></span></h3>
<p>Security goal (<strong>near second preimage resistance</strong>):</p>
<ul>
Expand Down Expand Up @@ -354,7 +360,7 @@
<p>In order to prevent the generic attack against nonmalleability, there needs to be some redundancy in the encoding. Therefore, the Producer of a Unified Address, UFVK, or UIVK appends 16 zero bytes to the raw encoding, then applies
<span class="math">\(\mathsf{F4Jumble}\)</span>
before encoding the result with Bech32m.</p>
<p>The Sender rejects any Bech32m-decoded byte sequence that is less than 48 bytes or greater than 16448 bytes; otherwise it applies
<p>The Consumer rejects any Bech32m-decoded byte sequence that is less than 48 bytes or greater than 16448 bytes; otherwise it applies
<span class="math">\(\mathsf{F4Jumble}^{-1}\)</span>
. It rejects any result that does not end in 16 zero bytes, before stripping these 16 bytes and parsing the result.</p>
<p>(48 bytes is the minimum size of a valid UA, UFVK, or UIVK raw encoding plus 16 zero bytes, corresponding to a single Sapling Incoming Viewing Key. 16448 bytes is the largest input/output size supported by
Expand Down Expand Up @@ -436,7 +442,7 @@
, all four pieces are randomized.</li>
</ul>
<p>Note that the size of each piece is at least 24 bytes.</p>
<p>It would be possible to make an attack more expensive by making the work done by a Producer more expensive. (This wouldn't necessarily have to increase the work done by the Sender.) However, given that Unified Addresses may need to be produced on constrained computing platforms, this was not considered to be beneficial overall.</p>
<p>It would be possible to make an attack more expensive by making the work done by a Producer more expensive. (This wouldn't necessarily have to increase the work done by the Consumer.) However, given that Unified Addresses may need to be produced on constrained computing platforms, this was not considered to be beneficial overall.</p>
</section>
<section id="efficiency"><h4><span class="section-heading">Efficiency</span><span class="section-anchor"> <a rel="bookmark" href="#efficiency"><img width="24" height="24" src="assets/images/section-anchor.png" alt=""></a></span></h4>
<p>The cost is dominated by 4 BLAKE2b compressions for
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