instantout: verify finalized MuSig2 witnesses

Run script validation for every combined signature before accepting a
finalized transaction, surfacing invalid witnesses immediately.

instantout/instantout.go

@@ -364,6 +364,9 @@ func (i *InstantOut) finalizeMusig2Transaction(ctx context.Context,
 			"expected %d, got %d", len(inputs), len(serverSigs))
 	}
 
+	prevOutFetcher := inputs.GetPrevoutFetcher()
+	sigHashes := txscript.NewTxSigHashes(tx, prevOutFetcher)
+
 	for idx := range inputs {
 		if musig2Sessions[idx] == nil {
 			return nil, fmt.Errorf("MuSig2 session %d is nil", idx)
@@ -382,6 +385,20 @@ func (i *InstantOut) finalizeMusig2Transaction(ctx context.Context,
 		}
 
 		tx.TxIn[idx].Witness = wire.TxWitness{finalSig}
+
+		vm, err := txscript.NewEngine(
+			inputs[idx].PkScript, tx, idx,
+			txscript.StandardVerifyFlags, nil, sigHashes,
+			int64(inputs[idx].Value), prevOutFetcher,
+		)
+		if err != nil {
+			return nil, fmt.Errorf("unable to verify final MuSig2 "+
+				"signature for input %d: %w", idx, err)
+		}
+		if err := vm.Execute(); err != nil {
+			return nil, fmt.Errorf("invalid final MuSig2 signature "+
+				"for input %d: %w", idx, err)
+		}
 	}
 
 	return tx, nil

instantout/instantout_test.go

@@ -7,11 +7,22 @@ import (
 	"github.com/btcsuite/btcd/btcec/v2"
 	"github.com/btcsuite/btcd/btcutil"
 	"github.com/btcsuite/btcd/wire"
+	"github.com/lightninglabs/lndclient"
 	"github.com/lightninglabs/loop/instantout/reservation"
 	"github.com/lightningnetwork/lnd/input"
 	"github.com/stretchr/testify/require"
 )
 
+type invalidFinalSigSigner struct {
+	lndclient.SignerClient
+}
+
+func (s *invalidFinalSigSigner) MuSig2CombineSig(context.Context, [32]byte,
+	[][]byte) (bool, []byte, error) {
+
+	return true, make([]byte, 64), nil
+}
+
 // TestMuSig2VectorLengthValidation verifies that malformed server-controlled
 // vectors are rejected before they can be indexed.
 func TestMuSig2VectorLengthValidation(t *testing.T) {
@@ -45,3 +56,29 @@ func TestMuSig2VectorLengthValidation(t *testing.T) {
 		require.ErrorContains(t, err, "server signatures")
 	})
 }
+
+// TestFinalizeMuSig2TransactionVerifiesSignature verifies that a combined
+// signature is validated locally before the transaction can be used as the
+// instant-out safety net.
+func TestFinalizeMuSig2TransactionVerifiesSignature(t *testing.T) {
+	_, pubKey := btcec.PrivKeyFromBytes([]byte{1})
+	res := &reservation.Reservation{
+		ClientPubkey: pubKey,
+		ServerPubkey: pubKey,
+		Value:        btcutil.Amount(100_000),
+		Expiry:       200,
+		Outpoint:     &wire.OutPoint{},
+	}
+	instantOut := &InstantOut{
+		Reservations: []*reservation.Reservation{res},
+	}
+	tx := wire.NewMsgTx(2)
+	tx.AddTxIn(&wire.TxIn{PreviousOutPoint: *res.Outpoint})
+	tx.AddTxOut(&wire.TxOut{Value: 90_000})
+
+	_, err := instantOut.finalizeMusig2Transaction(
+		context.Background(), &invalidFinalSigSigner{},
+		[]*input.MuSig2SessionInfo{{}}, tx, [][]byte{{1}},
+	)
+	require.ErrorContains(t, err, "invalid final MuSig2 signature")
+}