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Add support for keyserver preferences and preferred keyserver (closes #…
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…206) (#232)

* Parse keyserver prefs and preferred keyserver subpackets; update refs to RFC 9580

* Serialise keyserver preferences and preferred keyserver; add tests
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@andrewgdotcom
andrewgdotcom authored Sep 23, 2024
1 parent 2add693 commit 7852179
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Showing 2 changed files with 155 additions and 44 deletions.
134 changes: 90 additions & 44 deletions openpgp/packet/signature.go
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Original file line number Diff line number Diff line change
Expand Up @@ -5,14 +5,14 @@
package packet

import (
"math/big"
"bytes"
"crypto"
"crypto/dsa"
"encoding/asn1"
"encoding/binary"
"hash"
"io"
"math/big"
"strconv"
"time"

Expand All @@ -26,7 +26,8 @@ import (
)

const (
// See RFC 4880, section 5.2.3.21 for details.
// First octet of key flags.
// See RFC 9580, section 5.2.3.29 for details.
KeyFlagCertify = 1 << iota
KeyFlagSign
KeyFlagEncryptCommunications
Expand All @@ -37,16 +38,29 @@ const (
KeyFlagGroupKey
)

const (
// First octet of keyserver preference flags.
// See RFC 9580, section 5.2.3.25 for details.
_ = 1 << iota
_
_
_
_
_
_
KeyserverPrefNoModify
)

const SaltNotationName = "[email protected]"

// Signature represents a signature. See RFC 4880, section 5.2.
// Signature represents a signature. See RFC 9580, section 5.2.
type Signature struct {
Version int
SigType SignatureType
PubKeyAlgo PublicKeyAlgorithm
Hash crypto.Hash
// salt contains a random salt value for v6 signatures
// See RFC the crypto refresh Section 5.2.3.
// See RFC 9580 Section 5.2.4.
salt []byte

// HashSuffix is extra data that is hashed in after the signed data.
Expand Down Expand Up @@ -87,27 +101,37 @@ type Signature struct {
// TrustLevel and TrustAmount can be set by the signer to assert that
// the key is not only valid but also trustworthy at the specified
// level.
// See RFC 4880, section 5.2.3.13 for details.
// See RFC 9580, section 5.2.3.21 for details.
TrustLevel TrustLevel
TrustAmount TrustAmount

// TrustRegularExpression can be used in conjunction with trust Signature
// packets to limit the scope of the trust that is extended.
// See RFC 4880, section 5.2.3.14 for details.
// See RFC 9580, section 5.2.3.22 for details.
TrustRegularExpression *string

// KeyserverPrefsValid is set if any keyserver preferences were given. See RFC 9580, section
// 5.2.3.25 for details.
KeyserverPrefsValid bool
KeyserverPrefNoModify bool

// PreferredKeyserver can be set to a URI where the latest version of the
// key that this signature is made over can be found. See RFC 9580, section
// 5.2.3.26 for details.
PreferredKeyserver string

// PolicyURI can be set to the URI of a document that describes the
// policy under which the signature was issued. See RFC 4880, section
// 5.2.3.20 for details.
// policy under which the signature was issued. See RFC 9580, section
// 5.2.3.28 for details.
PolicyURI string

// FlagsValid is set if any flags were given. See RFC 4880, section
// 5.2.3.21 for details.
// FlagsValid is set if any flags were given. See RFC 9580, section
// 5.2.3.29 for details.
FlagsValid bool
FlagCertify, FlagSign, FlagEncryptCommunications, FlagEncryptStorage, FlagSplitKey, FlagAuthenticate, FlagGroupKey bool

// RevocationReason is set if this signature has been revoked.
// See RFC 4880, section 5.2.3.23 for details.
// See RFC 9580, section 5.2.3.31 for details.
RevocationReason *ReasonForRevocation
RevocationReasonText string

Expand Down Expand Up @@ -147,7 +171,7 @@ func (sig *Signature) Salt() []byte {
}

func (sig *Signature) parse(r io.Reader) (err error) {
// RFC 4880, section 5.2.3
// RFC 9580, section 5.2.3
var buf [7]byte
_, err = readFull(r, buf[:1])
if err != nil {
Expand Down Expand Up @@ -319,7 +343,7 @@ func (sig *Signature) parse(r io.Reader) (err error) {
}

// parseSignatureSubpackets parses subpackets of the main signature packet. See
// RFC 4880, section 5.2.3.1.
// RFC 9580, section 5.2.3.1.
func parseSignatureSubpackets(sig *Signature, subpackets []byte, isHashed bool) (err error) {
for len(subpackets) > 0 {
subpackets, err = parseSignatureSubpacket(sig, subpackets, isHashed)
Expand Down Expand Up @@ -349,6 +373,8 @@ const (
notationDataSubpacket signatureSubpacketType = 20
prefHashAlgosSubpacket signatureSubpacketType = 21
prefCompressionSubpacket signatureSubpacketType = 22
keyserverPrefsSubpacket signatureSubpacketType = 23
prefKeyserverSubpacket signatureSubpacketType = 24
primaryUserIdSubpacket signatureSubpacketType = 25
policyUriSubpacket signatureSubpacketType = 26
keyFlagsSubpacket signatureSubpacketType = 27
Expand All @@ -363,7 +389,7 @@ const (

// parseSignatureSubpacket parses a single subpacket. len(subpacket) is >= 1.
func parseSignatureSubpacket(sig *Signature, subpacket []byte, isHashed bool) (rest []byte, err error) {
// RFC 4880, section 5.2.3.1
// RFC 9580, section 5.2.3.7
var (
length uint32
packetType signatureSubpacketType
Expand Down Expand Up @@ -421,7 +447,7 @@ func parseSignatureSubpacket(sig *Signature, subpacket []byte, isHashed bool) (r
t := binary.BigEndian.Uint32(subpacket)
sig.CreationTime = time.Unix(int64(t), 0)
case signatureExpirationSubpacket:
// Signature expiration time, section 5.2.3.10
// Signature expiration time, section 5.2.3.18
if len(subpacket) != 4 {
err = errors.StructuralError("expiration subpacket with bad length")
return
Expand All @@ -438,15 +464,15 @@ func parseSignatureSubpacket(sig *Signature, subpacket []byte, isHashed bool) (r
err = errors.StructuralError("trust subpacket with bad length")
return
}
// Trust level and amount, section 5.2.3.13
// Trust level and amount, section 5.2.3.21
sig.TrustLevel = TrustLevel(subpacket[0])
sig.TrustAmount = TrustAmount(subpacket[1])
case regularExpressionSubpacket:
if len(subpacket) == 0 {
err = errors.StructuralError("regexp subpacket with bad length")
return
}
// Trust regular expression, section 5.2.3.14
// Trust regular expression, section 5.2.3.22
// RFC specifies the string should be null-terminated; remove a null byte from the end
if subpacket[len(subpacket)-1] != 0x00 {
err = errors.StructuralError("expected regular expression to be null-terminated")
Expand All @@ -455,19 +481,19 @@ func parseSignatureSubpacket(sig *Signature, subpacket []byte, isHashed bool) (r
trustRegularExpression := string(subpacket[:len(subpacket)-1])
sig.TrustRegularExpression = &trustRegularExpression
case keyExpirationSubpacket:
// Key expiration time, section 5.2.3.6
// Key expiration time, section 5.2.3.13
if len(subpacket) != 4 {
err = errors.StructuralError("key expiration subpacket with bad length")
return
}
sig.KeyLifetimeSecs = new(uint32)
*sig.KeyLifetimeSecs = binary.BigEndian.Uint32(subpacket)
case prefSymmetricAlgosSubpacket:
// Preferred symmetric algorithms, section 5.2.3.7
// Preferred symmetric algorithms, section 5.2.3.14
sig.PreferredSymmetric = make([]byte, len(subpacket))
copy(sig.PreferredSymmetric, subpacket)
case issuerSubpacket:
// Issuer, section 5.2.3.5
// Issuer, section 5.2.3.12
if sig.Version > 4 && isHashed {
err = errors.StructuralError("issuer subpacket found in v6 key")
return
Expand All @@ -481,7 +507,7 @@ func parseSignatureSubpacket(sig *Signature, subpacket []byte, isHashed bool) (r
*sig.IssuerKeyId = binary.BigEndian.Uint64(subpacket)
}
case notationDataSubpacket:
// Notation data, section 5.2.3.16
// Notation data, section 5.2.3.24
if len(subpacket) < 8 {
err = errors.StructuralError("notation data subpacket with bad length")
return
Expand All @@ -503,15 +529,27 @@ func parseSignatureSubpacket(sig *Signature, subpacket []byte, isHashed bool) (r

sig.Notations = append(sig.Notations, &notation)
case prefHashAlgosSubpacket:
// Preferred hash algorithms, section 5.2.3.8
// Preferred hash algorithms, section 5.2.3.16
sig.PreferredHash = make([]byte, len(subpacket))
copy(sig.PreferredHash, subpacket)
case prefCompressionSubpacket:
// Preferred compression algorithms, section 5.2.3.9
// Preferred compression algorithms, section 5.2.3.17
sig.PreferredCompression = make([]byte, len(subpacket))
copy(sig.PreferredCompression, subpacket)
case keyserverPrefsSubpacket:
// Keyserver preferences, section 5.2.3.25
sig.KeyserverPrefsValid = true
if len(subpacket) == 0 {
return
}
if subpacket[0]&KeyserverPrefNoModify != 0 {
sig.KeyserverPrefNoModify = true
}
case prefKeyserverSubpacket:
// Preferred keyserver, section 5.2.3.26
sig.PreferredKeyserver = string(subpacket)
case primaryUserIdSubpacket:
// Primary User ID, section 5.2.3.19
// Primary User ID, section 5.2.3.27
if len(subpacket) != 1 {
err = errors.StructuralError("primary user id subpacket with bad length")
return
Expand All @@ -521,7 +559,7 @@ func parseSignatureSubpacket(sig *Signature, subpacket []byte, isHashed bool) (r
*sig.IsPrimaryId = true
}
case keyFlagsSubpacket:
// Key flags, section 5.2.3.21
// Key flags, section 5.2.3.29
sig.FlagsValid = true
if len(subpacket) == 0 {
return
Expand Down Expand Up @@ -551,7 +589,7 @@ func parseSignatureSubpacket(sig *Signature, subpacket []byte, isHashed bool) (r
userId := string(subpacket)
sig.SignerUserId = &userId
case reasonForRevocationSubpacket:
// Reason For Revocation, section 5.2.3.23
// Reason For Revocation, section 5.2.3.31
if len(subpacket) == 0 {
err = errors.StructuralError("empty revocation reason subpacket")
return
Expand All @@ -560,7 +598,7 @@ func parseSignatureSubpacket(sig *Signature, subpacket []byte, isHashed bool) (r
*sig.RevocationReason = NewReasonForRevocation(subpacket[0])
sig.RevocationReasonText = string(subpacket[1:])
case featuresSubpacket:
// Features subpacket, section 5.2.3.24 specifies a very general
// Features subpacket, section 5.2.3.32 specifies a very general
// mechanism for OpenPGP implementations to signal support for new
// features.
if len(subpacket) > 0 {
Expand All @@ -574,24 +612,21 @@ func parseSignatureSubpacket(sig *Signature, subpacket []byte, isHashed bool) (r
}
case embeddedSignatureSubpacket:
// Only usage is in signatures that cross-certify
// signing subkeys. section 5.2.3.26 describes the
// signing subkeys. section 5.2.3.34 describes the
// format, with its usage described in section 11.1
if sig.EmbeddedSignature != nil {
err = errors.StructuralError("Cannot have multiple embedded signatures")
return
}
sig.EmbeddedSignature = new(Signature)
// Embedded signatures are required to be v4 signatures see
// section 12.1. However, we only parse v4 signatures in this
// file anyway.
if err := sig.EmbeddedSignature.parse(bytes.NewBuffer(subpacket)); err != nil {
return nil, err
}
if sigType := sig.EmbeddedSignature.SigType; sigType != SigTypePrimaryKeyBinding {
return nil, errors.StructuralError("cross-signature has unexpected type " + strconv.Itoa(int(sigType)))
}
case policyUriSubpacket:
// Policy URI, section 5.2.3.20
// Policy URI, section 5.2.3.28
sig.PolicyURI = string(subpacket)
case issuerFingerprintSubpacket:
if len(subpacket) == 0 {
Expand All @@ -611,8 +646,7 @@ func parseSignatureSubpacket(sig *Signature, subpacket []byte, isHashed bool) (r
*sig.IssuerKeyId = binary.BigEndian.Uint64(subpacket[13:21])
}
case intendedRecipientSubpacket:
// Intended Recipient Fingerprint
// https://datatracker.ietf.org/doc/html/draft-ietf-openpgp-crypto-refresh#name-intended-recipient-fingerpr
// Intended Recipient Fingerprint, section 5.2.3.36
if len(subpacket) < 1 {
return nil, errors.StructuralError("invalid intended recipient fingerpring length")
}
Expand All @@ -624,8 +658,7 @@ func parseSignatureSubpacket(sig *Signature, subpacket []byte, isHashed bool) (r
copy(fingerprint, subpacket[1:])
sig.IntendedRecipients = append(sig.IntendedRecipients, &Recipient{int(version), fingerprint})
case prefCipherSuitesSubpacket:
// Preferred AEAD cipher suites
// See https://www.ietf.org/archive/id/draft-ietf-openpgp-crypto-refresh-07.html#name-preferred-aead-ciphersuites
// Preferred AEAD cipher suites, section 5.2.3.15
if len(subpacket)%2 != 0 {
err = errors.StructuralError("invalid aead cipher suite length")
return
Expand Down Expand Up @@ -676,7 +709,7 @@ func (sig *Signature) CheckKeyIdOrFingerprintExplicit(fingerprint []byte, keyId

// serializeSubpacketLength marshals the given length into to.
func serializeSubpacketLength(to []byte, length int) int {
// RFC 4880, Section 4.2.2.
// RFC 9580, Section 4.2.1.
if length < 192 {
to[0] = byte(length)
return 1
Expand Down Expand Up @@ -819,7 +852,7 @@ func (sig *Signature) signPrepareHash(h hash.Hash) (digest []byte, err error) {
// The created hash object initially hashes a randomly generated salt
// as required by v6 signatures. The generated salt is stored in sig. If the signature is not v6,
// the method returns an empty hash object.
// See RFC the crypto refresh Section 3.2.4.
// See RFC 9580 Section 5.2.4.
func (sig *Signature) PrepareSign(config *Config) (hash.Hash, error) {
if !sig.Hash.Available() {
return nil, errors.UnsupportedError("hash function")
Expand All @@ -843,7 +876,7 @@ func (sig *Signature) PrepareSign(config *Config) (hash.Hash, error) {
// If the signature is not v6, the method ignores the salt.
// Use PrepareSign whenever possible instead of generating and
// hashing the salt externally.
// See RFC the crypto refresh Section 3.2.4.
// See RFC 9580 Section 5.2.4.
func (sig *Signature) SetSalt(salt []byte) error {
if sig.Version == 6 {
expectedSaltLength, err := SaltLengthForHash(sig.Hash)
Expand All @@ -861,7 +894,7 @@ func (sig *Signature) SetSalt(salt []byte) error {
// PrepareVerify must be called to create a hash object before verifying v6 signatures.
// The created hash object initially hashes the internally stored salt.
// If the signature is not v6, the method returns an empty hash object.
// See crypto refresh Section 3.2.4.
// See RFC 9580 Section 5.2.4.
func (sig *Signature) PrepareVerify() (hash.Hash, error) {
if !sig.Hash.Available() {
return nil, errors.UnsupportedError("hash function")
Expand Down Expand Up @@ -1276,6 +1309,19 @@ func (sig *Signature) buildSubpackets(issuer PublicKey) (subpackets []outputSubp
if len(sig.PreferredCompression) > 0 {
subpackets = append(subpackets, outputSubpacket{true, prefCompressionSubpacket, false, sig.PreferredCompression})
}
// Keyserver Preferences
// Keyserver preferences may only appear in self-signatures or certification signatures.
if sig.KeyserverPrefsValid {
var prefs byte
if sig.KeyserverPrefNoModify {
prefs |= KeyserverPrefNoModify
}
subpackets = append(subpackets, outputSubpacket{true, keyserverPrefsSubpacket, false, []byte{prefs}})
}
// Preferred Keyserver
if len(sig.PreferredKeyserver) > 0 {
subpackets = append(subpackets, outputSubpacket{true, prefKeyserverSubpacket, false, []uint8(sig.PreferredKeyserver)})
}
// Primary User ID
if sig.IsPrimaryId != nil && *sig.IsPrimaryId {
subpackets = append(subpackets, outputSubpacket{true, primaryUserIdSubpacket, false, []byte{1}})
Expand Down Expand Up @@ -1316,7 +1362,7 @@ func (sig *Signature) buildSubpackets(issuer PublicKey) (subpackets []outputSubp
subpackets = append(subpackets, outputSubpacket{true, signerUserIdSubpacket, false, []byte(*sig.SignerUserId)})
}
// Reason for Revocation
// Revocation reason appears only in revocation signatures and is serialized as per section 5.2.3.23.
// Revocation reason appears only in revocation signatures and is serialized as per section 5.2.3.31.
if sig.RevocationReason != nil {
subpackets = append(subpackets, outputSubpacket{true, reasonForRevocationSubpacket, true,
append([]uint8{uint8(*sig.RevocationReason)}, []uint8(sig.RevocationReasonText)...)})
Expand All @@ -1333,7 +1379,7 @@ func (sig *Signature) buildSubpackets(issuer PublicKey) (subpackets []outputSubp
subpackets = append(subpackets, outputSubpacket{true, featuresSubpacket, false, []byte{features}})
}
// Embedded Signature
// EmbeddedSignature appears only in subkeys capable of signing and is serialized as per section 5.2.3.26.
// EmbeddedSignature appears only in subkeys capable of signing and is serialized as per section 5.2.3.34.
if sig.EmbeddedSignature != nil {
var buf bytes.Buffer
err = sig.EmbeddedSignature.serializeBody(&buf)
Expand Down Expand Up @@ -1418,7 +1464,7 @@ func (sig *Signature) AddMetadataToHashSuffix() {

// SaltLengthForHash selects the required salt length for the given hash algorithm,
// as per Table 23 (Hash algorithm registry) of the crypto refresh.
// See https://datatracker.ietf.org/doc/html/draft-ietf-openpgp-crypto-refresh#section-9.5|Crypto Refresh Section 9.5.
// See RFC 9580 Section 9.5.
func SaltLengthForHash(hash crypto.Hash) (int, error) {
switch hash {
case crypto.SHA256, crypto.SHA224, crypto.SHA3_256:
Expand All @@ -1434,7 +1480,7 @@ func SaltLengthForHash(hash crypto.Hash) (int, error) {

// SignatureSaltForHash generates a random signature salt
// with the length for the given hash algorithm.
// See https://datatracker.ietf.org/doc/html/draft-ietf-openpgp-crypto-refresh#section-9.5|Crypto Refresh Section 9.5.
// See RFC 9580 Section 9.5.
func SignatureSaltForHash(hash crypto.Hash, randReader io.Reader) ([]byte, error) {
saltLength, err := SaltLengthForHash(hash)
if err != nil {
Expand Down
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