lnwire: update Sig to support both ECDSA and schnorr sigs

In this commit, we update the Sig type to support ECDSA and schnorr
signatures. We need to do this as the HTLC signatures will become
schnorr sigs for taproot channels. The current spec draft opts to
overload this field since both the sigs are actually 64 bytes in length.
The only consideration with this move is that callers need to "coerce" a
sig to the proper type if they need schnorr signatures.

watchtower/blob/justice_kit.go

@@ -398,7 +398,7 @@ func (b *JusticeKit) encodeV0(w io.Writer) error {
 	}
 
 	// Write 64-byte revocation signature for commit to-local output.
-	_, err = w.Write(b.CommitToLocalSig[:])
+	_, err = w.Write(b.CommitToLocalSig.RawBytes())
 	if err != nil {
 		return err
 	}
@@ -410,7 +410,7 @@ func (b *JusticeKit) encodeV0(w io.Writer) error {
 	}
 
 	// Write 64-byte commit to-remote signature, which may be blank.
-	_, err = w.Write(b.CommitToRemoteSig[:])
+	_, err = w.Write(b.CommitToRemoteSig.RawBytes())
 	return err
 }
 
@@ -472,14 +472,20 @@ func (b *JusticeKit) decodeV0(r io.Reader) error {
 	}
 
 	// Read 64-byte revocation signature for commit to-local output.
-	_, err = io.ReadFull(r, b.CommitToLocalSig[:])
+	var localSig [64]byte
+	_, err = io.ReadFull(r, localSig[:])
+	if err != nil {
+		return err
+	}
+
+	b.CommitToLocalSig, err = lnwire.NewSigFromWireECDSA(localSig[:])
 	if err != nil {
 		return err
 	}
 
 	var (
 		commitToRemotePubkey PubKey
-		commitToRemoteSig    lnwire.Sig
+		commitToRemoteSig    [64]byte
 	)
 
 	// Read 33-byte commit to-remote public key, which may be discarded.
@@ -498,7 +504,12 @@ func (b *JusticeKit) decodeV0(r io.Reader) error {
 	// valid compressed public key was read from the reader.
 	if btcec.IsCompressedPubKey(commitToRemotePubkey[:]) {
 		b.CommitToRemotePubKey = commitToRemotePubkey
-		b.CommitToRemoteSig = commitToRemoteSig
+		b.CommitToRemoteSig, err = lnwire.NewSigFromWireECDSA(
+			commitToRemoteSig[:],
+		)
+		if err != nil {
+			return err
+		}
 	}
 
 	return nil

watchtower/blob/justice_kit_test.go

@@ -28,8 +28,10 @@ func makePubKey(i uint64) blob.PubKey {
 }
 
 func makeSig(i int) lnwire.Sig {
-	var sig lnwire.Sig
-	binary.BigEndian.PutUint64(sig[:8], uint64(i))
+	var sigBytes [64]byte
+	binary.BigEndian.PutUint64(sigBytes[:8], uint64(i))
+
+	sig, _ := lnwire.NewSigFromWireECDSA(sigBytes[:])
 	return sig
 }