go.crypto/openpgp: fix user id signing.

Entity.SignIdentity used the wrong signature function and produced invalid
certification signatures. Fixed.

VerifyUserIdSignature was used for checking the self-signature on an identity,
but didn't support anything but a self-signature. This change alters the
low-level API so that checking cross-signatures is possible.

Fixes golang/go#7371.

LGTM=bradfitz
R=bradfitz
CC=golang-codereviews
https://golang.org/cl/96240043

openpgp/packet/public_key.go

@@ -620,9 +620,9 @@ func userIdSignatureHash(id string, pk *PublicKey, hashFunc crypto.Hash) (h hash
 }
 
 // VerifyUserIdSignature returns nil iff sig is a valid signature, made by this
-// public key, of id.
-func (pk *PublicKey) VerifyUserIdSignature(id string, sig *Signature) (err error) {
-	h, err := userIdSignatureHash(id, pk, sig.Hash)
+// public key, that id is the identity of pub.
+func (pk *PublicKey) VerifyUserIdSignature(id string, pub *PublicKey, sig *Signature) (err error) {
+	h, err := userIdSignatureHash(id, pub, sig.Hash)
 	if err != nil {
 		return err
 	}
@@ -630,9 +630,9 @@ func (pk *PublicKey) VerifyUserIdSignature(id string, sig *Signature) (err error
 }
 
 // VerifyUserIdSignatureV3 returns nil iff sig is a valid signature, made by this
-// public key, of id.
-func (pk *PublicKey) VerifyUserIdSignatureV3(id string, sig *SignatureV3) (err error) {
-	h, err := userIdSignatureV3Hash(id, pk, sig.Hash)
+// public key, that id is the identity of pub.
+func (pk *PublicKey) VerifyUserIdSignatureV3(id string, pub *PublicKey, sig *SignatureV3) (err error) {
+	h, err := userIdSignatureV3Hash(id, pub, sig.Hash)
 	if err != nil {
 		return err
 	}

openpgp/packet/public_key_test.go

@@ -133,7 +133,7 @@ func TestEcc384Serialize(t *testing.T) {
 			t.Error(err)
 		}
 		uidSig := p.(*Signature)
-		err = pubkey.VerifyUserIdSignature(uid.Id, uidSig)
+		err = pubkey.VerifyUserIdSignature(uid.Id, pubkey, uidSig)
 		if err != nil {
 			t.Error(err, ": UID")
 		}

openpgp/packet/public_key_v3.go

@@ -215,8 +215,8 @@ func (pk *PublicKeyV3) VerifySignatureV3(signed hash.Hash, sig *SignatureV3) (er
 }
 
 // VerifyUserIdSignatureV3 returns nil iff sig is a valid signature, made by this
-// public key, of id.
-func (pk *PublicKeyV3) VerifyUserIdSignatureV3(id string, sig *SignatureV3) (err error) {
+// public key, that id is the identity of pub.
+func (pk *PublicKeyV3) VerifyUserIdSignatureV3(id string, pub *PublicKeyV3, sig *SignatureV3) (err error) {
 	h, err := userIdSignatureV3Hash(id, pk, sig.Hash)
 	if err != nil {
 		return err

openpgp/packet/signature.go

@@ -515,7 +515,7 @@ func (sig *Signature) SignUserId(id string, pub *PublicKey, priv *PrivateKey, co
 	return sig.Sign(h, priv, config)
 }
 
-// SignKey computes a signature from priv, asserting that pub is a subkey.  On
+// SignKey computes a signature from priv, asserting that pub is a subkey. On
 // success, the signature is stored in sig. Call Serialize to write it out.
 // If config is nil, sensible defaults will be used.
 func (sig *Signature) SignKey(pub *PublicKey, priv *PrivateKey, config *Config) error {