Correctly serialize critical flag of signature subpackets (#83)

The signature subpackets are already setting the isCritical field to
true for various signature subpacket types. This commit changes the
serializeSubpackets function to take this field into account and set
the critical bit accordingly.

openpgp/packet/signature.go

@@ -507,6 +507,9 @@ func serializeSubpackets(to []byte, subpackets []outputSubpacket, hashed bool) {
 		if subpacket.hashed == hashed {
 			n := serializeSubpacketLength(to, len(subpacket.contents)+1)
 			to[n] = byte(subpacket.subpacketType)
+			if subpacket.isCritical {
+				to[n] |= 0x80
+			}
 			to = to[1+n:]
 			n = copy(to, subpacket.contents)
 			to = to[n:]

openpgp/packet/signature_test.go

@@ -76,4 +76,56 @@ func TestSignUserId(t *testing.T) {
 	}
 }
 
+func TestSignatureWithLifetime(t *testing.T) {
+	lifeTime := uint32(3600 * 24 * 30) // 30 days
+	sig := &Signature{
+		SigType:         SigTypeGenericCert,
+		PubKeyAlgo:      PubKeyAlgoRSA,
+		Hash:            crypto.SHA256,
+		SigLifetimeSecs: &lifeTime,
+	}
+
+	packet, err := Read(readerFromHex(rsaPkDataHex))
+	if err != nil {
+		t.Fatalf("failed to deserialize public key: %v", err)
+	}
+	pubKey := packet.(*PublicKey)
+
+	packet, err = Read(readerFromHex(privKeyRSAHex))
+	if err != nil {
+		t.Fatalf("failed to deserialize private key: %v", err)
+	}
+	privKey := packet.(*PrivateKey)
+
+	err = privKey.Decrypt([]byte("testing"))
+	if err != nil {
+		t.Fatalf("failed to decrypt private key: %v", err)
+	}
+
+	err = sig.SignUserId("", pubKey, privKey, nil)
+	if err != nil {
+		t.Errorf("failed to sign user id: %v", err)
+	}
+
+	buf := bytes.NewBuffer([]byte{})
+	err = sig.Serialize(buf)
+	if err != nil {
+		t.Errorf("failed to serialize signature: %v", err)
+	}
+
+	packet, _ = Read(bytes.NewReader(buf.Bytes()))
+	sig = packet.(*Signature)
+	if sig.SigLifetimeSecs == nil || *sig.SigLifetimeSecs != lifeTime {
+		t.Errorf("signature lifetime is wrong: %d instead of %d", *sig.SigLifetimeSecs, lifeTime)
+	}
+
+	for _, subPacket := range sig.rawSubpackets {
+		if subPacket.subpacketType == signatureExpirationSubpacket {
+			if !subPacket.isCritical {
+				t.Errorf("signature expiration subpacket is not marked as critical")
+			}
+		}
+	}
+}
+
 const signatureDataHex = "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"