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Always verify and check cross-signatures #265

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f1d63b8
Require and verify cross-signature for certification subkeys
twiss Jan 15, 2025
a51c34f
Check cross-signature details when signing/certifying
twiss Jan 15, 2025
838ee0a
Require and verify cross-signature for v6 authentication subkeys
twiss Jan 27, 2025
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6 changes: 3 additions & 3 deletions openpgp/packet/public_key.go
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Original file line number Diff line number Diff line change
Expand Up @@ -908,9 +908,9 @@ func (pk *PublicKey) VerifyKeySignature(signed *PublicKey, sig *Signature) error
return err
}

if sig.FlagSign {
// Signing subkeys must be cross-signed. See
// https://www.gnupg.org/faq/subkey-cross-certify.html.
if sig.FlagSign || sig.FlagCertify || (sig.FlagAuthenticate && sig.Version == 6) {
// Subkeys that can issue signatures must be cross-signed. See
// https://www.rfc-editor.org/rfc/rfc9580.html#section-10.1.5-8.
if sig.EmbeddedSignature == nil {
return errors.StructuralError("signing subkey is missing cross-signature")
}
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30 changes: 17 additions & 13 deletions openpgp/v2/subkeys.go
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Expand Up @@ -181,25 +181,29 @@ func (s *Subkey) Revoke(reason packet.ReasonForRevocation, reasonText string, co
// Returns a StructuralError if no valid self-signature is found.
func (s *Subkey) LatestValidBindingSignature(date time.Time, config *packet.Config) (selectedSig *packet.Signature, err error) {
for sigIdx := len(s.Bindings) - 1; sigIdx >= 0; sigIdx-- {
sig := s.Bindings[sigIdx]
if (date.IsZero() || date.Unix() >= sig.Packet.CreationTime.Unix()) &&
(selectedSig == nil || selectedSig.CreationTime.Unix() < sig.Packet.CreationTime.Unix()) {
if sig.Valid == nil {
err := s.Primary.PrimaryKey.VerifyKeySignature(s.PublicKey, sig.Packet)
binding := s.Bindings[sigIdx]
sig := binding.Packet
if (date.IsZero() || date.Unix() >= sig.CreationTime.Unix()) &&
(selectedSig == nil || selectedSig.CreationTime.Unix() < sig.CreationTime.Unix()) {
if binding.Valid == nil {
err := s.Primary.PrimaryKey.VerifyKeySignature(s.PublicKey, sig)
if err == nil {
err = checkSignatureDetails(s.Primary.PrimaryKey, sig.Packet, date, config)
err = checkSignatureDetails(s.Primary.PrimaryKey, sig, date, config)
}
if err == nil && (sig.FlagSign || sig.FlagCertify || (sig.FlagAuthenticate && sig.Version == 6)) {
err = checkSignatureDetails(s.PublicKey, sig.EmbeddedSignature, date, config)
}
valid := err == nil
sig.Valid = &valid
binding.Valid = &valid
}
mainSigExpired := !date.IsZero() &&
sig.Packet.SigExpired(date)
sig.SigExpired(date)
embeddedSigExpired := !date.IsZero() &&
sig.Packet.FlagSign &&
sig.Packet.EmbeddedSignature != nil &&
sig.Packet.EmbeddedSignature.SigExpired(date)
if *sig.Valid && !mainSigExpired && !embeddedSigExpired {
selectedSig = sig.Packet
sig.FlagSign &&
sig.EmbeddedSignature != nil &&
sig.EmbeddedSignature.SigExpired(date)
if *binding.Valid && !mainSigExpired && !embeddedSigExpired {
selectedSig = sig
}
}
}
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