loopout: use psbt when obtaining the MuSig2 signature

In this commit we deprecate the sighash field from the
MuSig2SignSweepReq request in favour of using a psbt serialized sweep
transaction instead. This way the sever gains full transparency about
client sweep requests and can assemble the sighash to sign on its own.

Author: bhandras

loopout.go

@@ -1353,7 +1353,7 @@ func (s *loopOutSwap) createMuSig2SweepTxn(
 
 	// First assemble our taproot keyspend sweep transaction and get the
 	// sig hash.
-	sweepTx, sigHash, err := s.sweeper.CreateUnsignedTaprootKeySpendSweepTx(
+	sweepTx, sweepTxPsbt, sigHash, err := s.sweeper.CreateUnsignedTaprootKeySpendSweepTx(
 		ctx, uint32(s.height), s.htlc, htlcOutpoint, htlcValue, fee,
 		s.DestAddr,
 	)
@@ -1387,7 +1387,7 @@ func (s *loopOutSwap) createMuSig2SweepTxn(
 	serverNonce, serverSig, err := s.swapKit.server.MuSig2SignSweep(
 		ctx, s.SwapContract.ProtocolVersion, s.hash,
 		s.swapInvoicePaymentAddr, musig2SessionInfo.PublicNonce[:],
-		sigHash,
+		sweepTxPsbt,
 	)
 	if err != nil {
 		return nil, err

server_mock_test.go

@@ -256,7 +256,8 @@ func (s *serverMock) ReportRoutingResult(_ context.Context, _ lntypes.Hash,
 }
 
 func (s *serverMock) MuSig2SignSweep(_ context.Context, _ loopdb.ProtocolVersion,
-	_ lntypes.Hash, _ [32]byte, _ []byte, _ []byte) ([]byte, []byte, error) {
+	_ lntypes.Hash, _ [32]byte, _ []byte, _ []byte) ([]byte,
+	[]byte, error) {
 
 	return nil, nil, nil
 }

swap_server_client.go

@@ -126,7 +126,7 @@ type swapServerClient interface {
 	// htlc spend. Returns the server's nonce and partial signature.
 	MuSig2SignSweep(ctx context.Context,
 		protocolVersion loopdb.ProtocolVersion, swapHash lntypes.Hash,
-		paymentAddr [32]byte, nonce []byte, sigHash []byte) (
+		paymentAddr [32]byte, nonce []byte, sweepTxPsbt []byte) (
 		[]byte, []byte, error)
 }
 
@@ -731,15 +731,15 @@ func (s *grpcSwapServerClient) ReportRoutingResult(ctx context.Context,
 // spend. Returns the server's nonce and partial signature.
 func (s *grpcSwapServerClient) MuSig2SignSweep(ctx context.Context,
 	protocolVersion loopdb.ProtocolVersion, swapHash lntypes.Hash,
-	paymentAddr [32]byte, nonce []byte, sigHash []byte) (
+	paymentAddr [32]byte, nonce []byte, sweepTxPsbt []byte) (
 	[]byte, []byte, error) {
 
 	req := &looprpc.MuSig2SignSweepReq{
 		ProtocolVersion: looprpc.ProtocolVersion(protocolVersion),
 		SwapHash:        swapHash[:],
 		PaymentAddress:  paymentAddr[:],
 		Nonce:           nonce,
-		SigHash:         sigHash,
+		SweepTxPsbt:     sweepTxPsbt,
 	}
 
 	rpcCtx, rpcCancel := context.WithTimeout(ctx, globalCallTimeout)