multi: add tag option to SignMessageSchnorr

keychain/btcwallet.go

@@ -458,8 +458,8 @@ func (b *BtcWalletKeyRing) SignMessageCompact(keyLoc KeyLocator,
 //
 // NOTE: This is part of the keychain.MessageSignerRing interface.
 func (b *BtcWalletKeyRing) SignMessageSchnorr(keyLoc KeyLocator,
-	msg []byte, doubleHash bool, taprootTweak []byte) (*schnorr.Signature,
+	msg []byte, doubleHash bool, taprootTweak []byte,
-	error) {
+	tag []byte) (*schnorr.Signature, error) {
 
 	privKey, err := b.DerivePrivKey(KeyDescriptor{
 		KeyLocator: keyLoc,
@@ -472,10 +472,15 @@ func (b *BtcWalletKeyRing) SignMessageSchnorr(keyLoc KeyLocator,
 		privKey = txscript.TweakTaprootPrivKey(*privKey, taprootTweak)
 	}
 
+	// If a tag was provided, we need to take the tagged hash of the input.
 	var digest []byte
-	if doubleHash {
+	switch {
+	case len(tag) > 0:
+		taggedHash := chainhash.TaggedHash(tag, msg)
+		digest = taggedHash[:]
+	case doubleHash:
 		digest = chainhash.DoubleHashB(msg)
-	} else {
+	default:
 		digest = chainhash.HashB(msg)
 	}
 	return schnorr.Sign(privKey, digest)

keychain/derivation.go

@@ -239,8 +239,8 @@ type MessageSignerRing interface {
 	// hashing it first, with the private key described in the key locator
 	// and the optional Taproot tweak applied to the private key.
 	SignMessageSchnorr(keyLoc KeyLocator, msg []byte,
-		doubleHash bool, taprootTweak []byte) (*schnorr.Signature,
+		doubleHash bool, taprootTweak []byte,
-		error)
+		tag []byte) (*schnorr.Signature, error)
 }
 
 // SingleKeyMessageSigner is an abstraction interface that hides the

lnrpc/signrpc/signer_server.go

@@ -633,7 +633,7 @@ func (s *Server) SignMessage(_ context.Context,
 	if in.SchnorrSig {
 		sig, err := s.cfg.KeyRing.SignMessageSchnorr(
 			keyLocator, in.Msg, in.DoubleHash,
-			in.SchnorrSigTapTweak,
+			in.SchnorrSigTapTweak, in.Tag,
 		)
 		if err != nil {
 			return nil, fmt.Errorf("can't sign the hash: %v", err)

lntest/mock/secretkeyring.go

@@ -74,13 +74,17 @@ func (s *SecretKeyRing) SignMessageCompact(_ keychain.KeyLocator,
 
 // SignMessageSchnorr signs the passed message and ignores the KeyDescriptor.
 func (s *SecretKeyRing) SignMessageSchnorr(_ keychain.KeyLocator,
-	msg []byte, doubleHash bool, taprootTweak []byte) (*schnorr.Signature,
+	msg []byte, doubleHash bool, taprootTweak []byte,
-	error) {
+	tag []byte) (*schnorr.Signature, error) {
 
 	var digest []byte
-	if doubleHash {
+	switch {
+	case len(tag) > 0:
+		taggedHash := chainhash.TaggedHash(tag, msg)
+		digest = taggedHash[:]
+	case doubleHash:
 		digest = chainhash.DoubleHashB(msg)
-	} else {
+	default:
 		digest = chainhash.HashB(msg)
 	}
 

lnwallet/rpcwallet/rpcwallet.go

@@ -513,8 +513,8 @@ func (r *RPCKeyRing) SignMessageCompact(keyLoc keychain.KeyLocator,
 //
 // NOTE: This method is part of the keychain.MessageSignerRing interface.
 func (r *RPCKeyRing) SignMessageSchnorr(keyLoc keychain.KeyLocator,
-	msg []byte, doubleHash bool, taprootTweak []byte) (*schnorr.Signature,
+	msg []byte, doubleHash bool, taprootTweak []byte,
-	error) {
+	tag []byte) (*schnorr.Signature, error) {
 
 	ctxt, cancel := context.WithTimeout(context.Background(), r.rpcTimeout)
 	defer cancel()
@@ -528,6 +528,7 @@ func (r *RPCKeyRing) SignMessageSchnorr(keyLoc keychain.KeyLocator,
 		DoubleHash:         doubleHash,
 		SchnorrSig:         true,
 		SchnorrSigTapTweak: taprootTweak,
+		Tag:                tag,
 	})
 	if err != nil {
 		considerShutdown(err)