sweepbatcher: add option WithCustomSignMuSig2

It is needed to provide a custom MuSig2 signer.

sweepbatcher/sweep_batch.go

@@ -137,6 +137,12 @@ type batchConfig struct {
 	// the caller has to update it in the source of SweepInfo (interface
 	// SweepFetcher) and re-add the sweep by calling AddSweep.
 	noBumping bool
+
+	// customMuSig2Signer is a custom signer. If it is set, it is used to
+	// create musig2 signatures instead of musig2SignSweep and signerClient.
+	// Note that musig2SignSweep must be nil in this case, however signer
+	// client must still be provided, as it is used for non-coop spendings.
+	customMuSig2Signer SignMuSig2
 }
 
 // rbfCache stores data related to our last fee bump.
@@ -503,9 +509,12 @@ func (b *batch) Run(ctx context.Context) error {
 		close(b.finished)
 	}()
 
-	if b.muSig2SignSweep == nil {
+	if b.muSig2SignSweep == nil && b.cfg.customMuSig2Signer == nil {
 		return fmt.Errorf("no musig2 signer available")
 	}
+	if b.muSig2SignSweep != nil && b.cfg.customMuSig2Signer != nil {
+		return fmt.Errorf("both musig2 signers provided")
+	}
 
 	blockChan, blockErrChan, err :=
 		b.chainNotifier.RegisterBlockEpochNtfn(runCtx)
@@ -1008,6 +1017,36 @@ func (b *batch) musig2sign(ctx context.Context, inputIndex int, sweep sweep,
 		return nil, fmt.Errorf("invalid htlc script version")
 	}
 
+	var digest [32]byte
+	copy(digest[:], sigHash)
+
+	// If a custom signer is installed, use it instead of b.signerClient
+	// and b.muSig2SignSweep.
+	if b.cfg.customMuSig2Signer != nil {
+		// Produce a signature.
+		finalSig, err := b.cfg.customMuSig2Signer(
+			ctx, muSig2Version, sweep.swapHash,
+			htlcScript.RootHash, digest,
+		)
+		if err != nil {
+			return nil, fmt.Errorf("customMuSig2Signer failed: %w",
+				err)
+		}
+
+		// To be sure that we're good, parse and validate that the
+		// combined signature is indeed valid for the sig hash and the
+		// internal pubkey.
+		err = b.verifySchnorrSig(
+			htlcScript.TaprootKey, sigHash, finalSig,
+		)
+		if err != nil {
+			return nil, fmt.Errorf("verifySchnorrSig failed: %w",
+				err)
+		}
+
+		return finalSig, nil
+	}
+
 	// Now we're creating a local MuSig2 session using the receiver key's
 	// key locator and the htlc's root hash.
 	keyLocator := &sweep.htlcKeys.ClientScriptKeyLocator
@@ -1052,9 +1091,6 @@ func (b *batch) musig2sign(ctx context.Context, inputIndex int, sweep sweep,
 			"nonces missing")
 	}
 
-	var digest [32]byte
-	copy(digest[:], sigHash)
-
 	// Since our MuSig2 session has all nonces, we can now create
 	// the local partial signature by signing the sig hash.
 	_, err = b.signerClient.MuSig2Sign(

sweepbatcher/sweep_batcher.go

@@ -10,6 +10,7 @@ import (
 	"github.com/btcsuite/btcd/btcec/v2"
 	"github.com/btcsuite/btcd/btcutil"
 	"github.com/btcsuite/btcd/chaincfg"
+	"github.com/btcsuite/btcd/chaincfg/chainhash"
 	"github.com/btcsuite/btcd/wire"
 	"github.com/lightninglabs/lndclient"
 	"github.com/lightninglabs/loop/loopdb"
@@ -140,6 +141,12 @@ type MuSig2SignSweep func(ctx context.Context,
 	prevoutMap map[wire.OutPoint]*wire.TxOut) (
 	[]byte, []byte, error)
 
+// SignMuSig2 is a function that can be used to sign a sweep transaction in a
+// custom way.
+type SignMuSig2 func(ctx context.Context, muSig2Version input.MuSig2Version,
+	swapHash lntypes.Hash, rootHash chainhash.Hash, sigHash [32]byte,
+) ([]byte, error)
+
 // VerifySchnorrSig is a function that can be used to verify a schnorr
 // signature.
 type VerifySchnorrSig func(pubKey *btcec.PublicKey, hash, sig []byte) error
@@ -245,6 +252,12 @@ type Batcher struct {
 	// the caller has to update it in the source of SweepInfo (interface
 	// SweepFetcher) and re-add the sweep by calling AddSweep.
 	noBumping bool
+
+	// customMuSig2Signer is a custom signer. If it is set, it is used to
+	// create musig2 signatures instead of musig2SignSweep and signerClient.
+	// Note that musig2SignSweep must be nil in this case, however signer
+	// client must still be provided, as it is used for non-coop spendings.
+	customMuSig2Signer SignMuSig2
 }
 
 // BatcherConfig holds batcher configuration.
@@ -254,6 +267,12 @@ type BatcherConfig struct {
 	// the caller has to update it in the source of SweepInfo (interface
 	// SweepFetcher) and re-add the sweep by calling AddSweep.
 	noBumping bool
+
+	// customMuSig2Signer is a custom signer. If it is set, it is used to
+	// create musig2 signatures instead of musig2SignSweep and signerClient.
+	// Note that musig2SignSweep must be nil in this case, however signer
+	// client must still be provided, as it is used for non-coop spendings.
+	customMuSig2Signer SignMuSig2
 }
 
 // BatcherOption configures batcher behaviour.
@@ -269,6 +288,17 @@ func WithNoBumping() BatcherOption {
 	}
 }
 
+// WithCustomSignMuSig2 instructs sweepbatcher to use a custom function to
+// produce MuSig2 signatures. If it is set, it is used to create
+// musig2 signatures instead of musig2SignSweep and signerClient. Note
+// that musig2SignSweep must be nil in this case, however signerClient
+// must still be provided, as it is used for non-coop spendings.
+func WithCustomSignMuSig2(customMuSig2Signer SignMuSig2) BatcherOption {
+	return func(cfg *BatcherConfig) {
+		cfg.customMuSig2Signer = customMuSig2Signer
+	}
+}
+
 // NewBatcher creates a new Batcher instance.
 func NewBatcher(wallet lndclient.WalletKitClient,
 	chainNotifier lndclient.ChainNotifierClient,
@@ -282,21 +312,27 @@ func NewBatcher(wallet lndclient.WalletKitClient,
 		opt(&cfg)
 	}
 
+	if cfg.customMuSig2Signer != nil && musig2ServerSigner != nil {
+		panic("customMuSig2Signer must not be used with " +
+			"musig2ServerSigner")
+	}
+
 	return &Batcher{
-		batches:          make(map[int32]*batch),
-		sweepReqs:        make(chan SweepRequest),
-		errChan:          make(chan error, 1),
-		quit:             make(chan struct{}),
-		initDone:         make(chan struct{}),
-		wallet:           wallet,
-		chainNotifier:    chainNotifier,
-		signerClient:     signerClient,
-		musig2ServerSign: musig2ServerSigner,
-		VerifySchnorrSig: verifySchnorrSig,
-		chainParams:      chainparams,
-		store:            store,
-		sweepStore:       sweepStore,
-		noBumping:        cfg.noBumping,
+		batches:            make(map[int32]*batch),
+		sweepReqs:          make(chan SweepRequest),
+		errChan:            make(chan error, 1),
+		quit:               make(chan struct{}),
+		initDone:           make(chan struct{}),
+		wallet:             wallet,
+		chainNotifier:      chainNotifier,
+		signerClient:       signerClient,
+		musig2ServerSign:   musig2ServerSigner,
+		VerifySchnorrSig:   verifySchnorrSig,
+		chainParams:        chainparams,
+		store:              store,
+		sweepStore:         sweepStore,
+		noBumping:          cfg.noBumping,
+		customMuSig2Signer: cfg.customMuSig2Signer,
 	}
 }
 
@@ -456,6 +492,7 @@ func (b *Batcher) spinUpBatch(ctx context.Context) (*batch, error) {
 	cfg := batchConfig{
 		maxTimeoutDistance: defaultMaxTimeoutDistance,
 		noBumping:          b.noBumping,
+		customMuSig2Signer: b.customMuSig2Signer,
 	}
 
 	switch b.chainParams {
@@ -574,6 +611,7 @@ func (b *Batcher) spinUpBatchFromDB(ctx context.Context, batch *batch) error {
 	cfg := batchConfig{
 		maxTimeoutDistance: batch.cfg.maxTimeoutDistance,
 		noBumping:          b.noBumping,
+		customMuSig2Signer: b.customMuSig2Signer,
 	}
 
 	newBatch, err := NewBatchFromDB(cfg, batchKit)
@@ -637,6 +675,7 @@ func (b *Batcher) FetchUnconfirmedBatches(ctx context.Context) ([]*batch,
 		bchCfg := batchConfig{
 			maxTimeoutDistance: bch.MaxTimeoutDistance,
 			noBumping:          b.noBumping,
+			customMuSig2Signer: b.customMuSig2Signer,
 		}
 		batch.cfg = &bchCfg
 

sweepbatcher/sweep_batcher_test.go

@@ -16,6 +16,7 @@ import (
 	"github.com/lightninglabs/loop/test"
 	"github.com/lightninglabs/loop/utils"
 	"github.com/lightningnetwork/lnd/chainntnfs"
+	"github.com/lightningnetwork/lnd/input"
 	"github.com/lightningnetwork/lnd/keychain"
 	"github.com/lightningnetwork/lnd/lntypes"
 	"github.com/lightningnetwork/lnd/lnwallet/chainfee"
@@ -68,6 +69,18 @@ func testMuSig2SignSweep(ctx context.Context,
 	return nil, nil, nil
 }
 
+var customSignature = func() []byte {
+	sig := [64]byte{10, 20, 30}
+	return sig[:]
+}()
+
+func testSignMuSig2func(ctx context.Context, muSig2Version input.MuSig2Version,
+	swapHash lntypes.Hash, rootHash chainhash.Hash,
+	sigHash [32]byte) ([]byte, error) {
+
+	return customSignature, nil
+}
+
 var dummyNotifier = SpendNotifier{
 	SpendChan:    make(chan *SpendDetail, ntfnBufferSize),
 	SpendErrChan: make(chan error, ntfnBufferSize),
@@ -1985,6 +1998,89 @@ func testSweepBatcherCloseDuringAdding(t *testing.T, store testStore,
 	<-registrationChan
 }
 
+// testCustomSignMuSig2 tests the operation with custom musig2 signer.
+func testCustomSignMuSig2(t *testing.T, store testStore,
+	batcherStore testBatcherStore) {
+
+	defer test.Guard(t)()
+
+	lnd := test.NewMockLnd()
+	ctx, cancel := context.WithCancel(context.Background())
+
+	sweepStore, err := NewSweepFetcherFromSwapStore(store, lnd.ChainParams)
+	require.NoError(t, err)
+
+	// Use custom MuSig2 signer function.
+	batcher := NewBatcher(lnd.WalletKit, lnd.ChainNotifier, lnd.Signer,
+		nil, testVerifySchnorrSig, lnd.ChainParams, batcherStore,
+		sweepStore, WithCustomSignMuSig2(testSignMuSig2func))
+
+	var wg sync.WaitGroup
+	wg.Add(1)
+
+	var runErr error
+	go func() {
+		defer wg.Done()
+		runErr = batcher.Run(ctx)
+	}()
+
+	// Wait for the batcher to be initialized.
+	<-batcher.initDone
+
+	// Create a sweep request.
+	sweepReq := SweepRequest{
+		SwapHash: lntypes.Hash{1, 1, 1},
+		Value:    111,
+		Outpoint: wire.OutPoint{
+			Hash:  chainhash.Hash{1, 1},
+			Index: 1,
+		},
+		Notifier: &dummyNotifier,
+	}
+
+	swap := &loopdb.LoopOutContract{
+		SwapContract: loopdb.SwapContract{
+			CltvExpiry:      111,
+			AmountRequested: 111,
+			ProtocolVersion: loopdb.ProtocolVersionMuSig2,
+			HtlcKeys: loopdb.HtlcKeys{
+				SenderScriptKey:        senderKey,
+				ReceiverScriptKey:      receiverKey,
+				SenderInternalPubKey:   senderKey,
+				ReceiverInternalPubKey: receiverKey,
+			},
+		},
+
+		DestAddr:        destAddr,
+		SwapInvoice:     swapInvoice,
+		SweepConfTarget: 111,
+	}
+
+	err = store.CreateLoopOut(ctx, sweepReq.SwapHash, swap)
+	require.NoError(t, err)
+	store.AssertLoopOutStored()
+
+	// Deliver sweep request to batcher.
+	require.NoError(t, batcher.AddSweep(&sweepReq))
+
+	// Since a batch was created we check that it registered for its primary
+	// sweep's spend.
+	<-lnd.RegisterSpendChannel
+
+	// Wait for tx to be published.
+	tx := <-lnd.TxPublishChannel
+
+	// Check the signature.
+	gotSig := tx.TxIn[0].Witness[0]
+	require.Equal(t, customSignature, gotSig, "signatures don't match")
+
+	// Now make the batcher quit by canceling the context.
+	cancel()
+	wg.Wait()
+
+	checkBatcherError(t, runErr)
+}
+
 // TestSweepBatcherBatchCreation tests that sweep requests enter the expected
 // batch based on their timeout distance.
 func TestSweepBatcherBatchCreation(t *testing.T) {
@@ -2070,6 +2166,11 @@ func TestSweepBatcherCloseDuringAdding(t *testing.T) {
 	runTests(t, testSweepBatcherCloseDuringAdding)
 }
 
+// TestCustomSignMuSig2 tests the operation with custom musig2 signer.
+func TestCustomSignMuSig2(t *testing.T) {
+	runTests(t, testCustomSignMuSig2)
+}
+
 // testBatcherStore is BatcherStore used in tests.
 type testBatcherStore interface {
 	BatcherStore