Only verify and get the time stamp of the first signature packet

If the detached signature has several packets,
GetVerifiedSignatureTimestamp() only verifies and returns
the timestamp of the first packet

crypto/keyring_message.go

@@ -122,11 +122,30 @@ func (keyRing *KeyRing) VerifyDetachedEncrypted(message *PlainMessage, encrypted
 // returns the creation time of the signature if it succeeds
 // and returns a SignatureVerificationError if fails.
 func (keyRing *KeyRing) GetVerifiedSignatureTimestamp(message *PlainMessage, signature *PGPSignature, verifyTime int64) (int64, error) {
-	err := keyRing.VerifyDetached(message, signature, verifyTime)
+	packets := packet.NewReader(bytes.NewReader(signature.Data))
+	p, err := packets.Next()
+	if err != nil {
+		return 0, errors.Wrap(err, "gopenpgp: can't parse signature")
+	}
+	sigPacket, ok := p.(*packet.Signature)
+	if !ok {
+		return 0, errors.New("gopenpgp: non signature packet found")
+	}
+	var outBuf bytes.Buffer
+	err = sigPacket.Serialize(&outBuf)
+	if err != nil {
+		return 0, errors.Wrap(err, "gopenpgp: can't serialize signature packet")
+	}
+	err = verifySignature(
+		keyRing.entities,
+		message.NewReader(),
+		outBuf.Bytes(),
+		verifyTime,
+	)
 	if err != nil {
 		return 0, err
 	}
-	return signature.getCreationTime()
+	return sigPacket.CreationTime.Unix(), nil
 }
 
 // ------ INTERNAL FUNCTIONS -------