tapchannel: add HTLC index tweak to sign desc tweak

If we're signing a first-level HTLC leaf, we need to add the HTLC index
tweak to the single tweak.

tapchannel/aux_leaf_signer.go

@@ -414,12 +414,20 @@ func verifyHtlcSignature(chainParams *address.ChainParams,
 				"verify second level: %w", err)
 		}
 
+		// Because we're verifying an HTLC first-level output, we need
+		// to apply the same tweak to the key as we did when creating
+		// the script key in the allocation. That tweak was applied to
+		// the script key to guarantee uniqueness on the asset level.
+		verifyKey := TweakPubKeyWithIndex(
+			keyRing.RemoteHtlcKey, baseJob.HTLC.HtlcIndex,
+		)
+
 		leafToVerify := txscript.TapLeaf{
 			Script:      htlcScript.WitnessScriptToSign(),
 			LeafVersion: txscript.BaseLeafVersion,
 		}
 		validator := &schnorrSigValidator{
-			pubKey:     *keyRing.RemoteHtlcKey,
+			pubKey:     *verifyKey,
 			tapLeaf:    lfn.Some(leafToVerify),
 			signMethod: input.TaprootScriptSpendSignMethod,
 		}
@@ -439,8 +447,9 @@ func verifyHtlcSignature(chainParams *address.ChainParams,
 // that should be used to verify the generated signature, and also the leaf to
 // be signed.
 func applySignDescToVIn(signDesc input.SignDescriptor, vIn *tappsbt.VInput,
-	chainParams *address.ChainParams,
-	tapscriptRoot []byte) (btcec.PublicKey, txscript.TapLeaf) {
+	chainParams *address.ChainParams, tapscriptRoot []byte,
+	index input.HtlcIndex, tweakWithIndex bool) (btcec.PublicKey,
+	txscript.TapLeaf) {
 
 	leafToSign := txscript.TapLeaf{
 		Script:      signDesc.WitnessScript,
@@ -466,20 +475,26 @@ func applySignDescToVIn(signDesc input.SignDescriptor, vIn *tappsbt.VInput,
 	vIn.SighashType = signDesc.HashType
 	vIn.TaprootMerkleRoot = tapscriptRoot
 
+	// We might need to apply another tweak to the public key if this is an
+	// HTLC first-level output, where we've applied a tweak to the script
+	// key to guarantee uniqueness.
+	singleTweak := signDesc.SingleTweak
+	if tweakWithIndex {
+		singleTweak = AddTweakWithIndex(singleTweak, index)
+	}
+
 	// Apply single or double tweaks if present in the sign
 	// descriptor. At the same time, we apply the tweaks to a copy
 	// of the public key, so we can validate the produced signature.
 	signingKey := signDesc.KeyDesc.PubKey
-	if len(signDesc.SingleTweak) > 0 {
+	if len(singleTweak) > 0 {
 		key := btcwallet.PsbtKeyTypeInputSignatureTweakSingle
 		vIn.Unknowns = append(vIn.Unknowns, &psbt.Unknown{
 			Key:   key,
-			Value: signDesc.SingleTweak,
+			Value: singleTweak,
 		})
 
-		signingKey = input.TweakPubKeyWithTweak(
-			signingKey, signDesc.SingleTweak,
-		)
+		signingKey = input.TweakPubKeyWithTweak(signingKey, singleTweak)
 	}
 	if signDesc.DoubleTweak != nil {
 		key := btcwallet.PsbtKeyTypeInputSignatureTweakDouble
@@ -542,6 +557,7 @@ func (s *AuxLeafSigner) generateHtlcSignature(chanState lnwallet.AuxChanState,
 
 		signingKey, leafToSign := applySignDescToVIn(
 			signDesc, vIn, s.cfg.ChainParams, tapscriptRoot,
+			baseJob.HTLC.HtlcIndex, true,
 		)
 
 		// We can now sign this virtual packet, as we've given the
@@ -832,3 +848,23 @@ func TweakHtlcTree(tree input.ScriptTree,
 		TapscriptRoot: tree.TapscriptRoot,
 	}
 }
+
+// AddTweakWithIndex adds the given index to the given tweak. If the tweak is
+// empty, the index is used as the tweak directly. The value of 1 is always
+// added to the index to make sure this value is always non-zero.
+func AddTweakWithIndex(maybeTweak []byte, index input.HtlcIndex) []byte {
+	indexTweak := HtlcIndexAsScriptKeyTweak(index)
+
+	// If we don't already have a tweak, we just use the index as the tweak.
+	if len(maybeTweak) == 0 {
+		return fn.ByteSlice(indexTweak.Bytes())
+	}
+
+	// If we have a tweak, we need to parse/decode it as a scalar, then add
+	// the index as a scalar, and encode it back to a byte slice.
+	tweak := new(secp256k1.ModNScalar)
+	_ = tweak.SetByteSlice(maybeTweak)
+	newTweak := tweak.Add(indexTweak)
+
+	return fn.ByteSlice(newTweak.Bytes())
+}

tapchannel/aux_leaf_signer_test.go

@@ -7,12 +7,14 @@ import (
 	"encoding/hex"
 	"fmt"
 	"math"
+	"math/big"
 	"testing"
 
 	"github.com/btcsuite/btcd/btcec/v2"
 	"github.com/btcsuite/btcd/txscript"
 	"github.com/decred/dcrd/dcrec/secp256k1/v4"
 	"github.com/lightninglabs/taproot-assets/asset"
+	"github.com/lightninglabs/taproot-assets/fn"
 	"github.com/lightninglabs/taproot-assets/internal/test"
 	cmsg "github.com/lightninglabs/taproot-assets/tapchannelmsg"
 	"github.com/lightningnetwork/lnd/input"
@@ -394,3 +396,61 @@ func TestTweakHtlcTree(t *testing.T) {
 		})
 	}
 }
+
+// TestAddTweakWithIndex tests the AddTweakWithIndex function.
+func TestAddTweakWithIndex(t *testing.T) {
+	var (
+		bufMaxUint64 = make([]byte, 8)
+		maxUint64    = new(secp256k1.ModNScalar)
+	)
+	binary.BigEndian.PutUint64(bufMaxUint64, math.MaxUint64)
+	_ = maxUint64.SetByteSlice(bufMaxUint64)
+	maxUint64Double := new(secp256k1.ModNScalar).
+		Set(maxUint64).Add(maxUint64)
+
+	testCases := []struct {
+		name   string
+		tweak  []byte
+		index  uint64
+		result *secp256k1.ModNScalar
+	}{
+		{
+			name:   "empty tweak, index 0",
+			index:  0,
+			result: new(secp256k1.ModNScalar).SetInt(1),
+		},
+		{
+			name:   "five as tweak, index 123",
+			tweak:  []byte{0x05},
+			index:  123,
+			result: new(secp256k1.ModNScalar).SetInt(129),
+		},
+		{
+			name:   "all zero tweak, index 123",
+			tweak:  bytes.Repeat([]byte{0}, 32),
+			index:  123,
+			result: new(secp256k1.ModNScalar).SetInt(124),
+		},
+		{
+			name:   "tweak math.MaxUint64, index math.MaxUint64-1",
+			tweak:  fn.ByteSlice(maxUint64.Bytes()),
+			index:  math.MaxUint64 - 1,
+			result: maxUint64Double,
+		},
+	}
+
+	for _, tc := range testCases {
+		t.Run(tc.name, func(t *testing.T) {
+			tweak := AddTweakWithIndex(tc.tweak, tc.index)
+			resultBytes := tc.result.Bytes()
+			resultBigInt := new(big.Int).SetBytes(resultBytes[:])
+			tweakBigInt := new(big.Int).SetBytes(tweak)
+			// 36893488147419103230
+			// 55340232221128654845
+
+			require.Equalf(t, resultBytes[:], tweak, "expected: "+
+				"%s, got: %s", resultBigInt.String(),
+				tweakBigInt.String())
+		})
+	}
+}

tapchannel/aux_sweeper.go

@@ -365,8 +365,7 @@ func (a *AuxSweeper) signSweepVpackets(vPackets []*tappsbt.VPacket,
 		// tweaks to generate the key we'll use to verify the
 		// signature.
 		signingKey, leafToSign := applySignDescToVIn(
-			signDesc, vIn, &a.cfg.ChainParams,
-			tapTweak,
+			signDesc, vIn, &a.cfg.ChainParams, tapTweak, 0, false,
 		)
 
 		// In this case, the witness isn't special, so we'll set the