tapchannel: tweak HTLC script key internal key to enforce uniqueness

This commit tweaks the internal key of the asset-level script key with
the HTLC index to enforce uniqueness of script keys across multiple
HTLCs with the same payment hash and timeout (MPP shards of the same
payment).
The internal key we tweak is the revocation key. So we must also apply
that same tweak when doing a revocation/breach sweep transaction. But
since that functionality is not yet implemented, we'll just leave a TODO
in a follow-up commit for that.
Therefore, no tweak will currently need to be applied at _sign_ time.

tapchannel/aux_leaf_signer.go

@@ -3,12 +3,14 @@ package tapchannel
 import (
 	"bytes"
 	"fmt"
+	"math/big"
 	"sync"
 
 	"github.com/btcsuite/btcd/btcec/v2"
 	"github.com/btcsuite/btcd/btcutil/psbt"
 	"github.com/btcsuite/btcd/txscript"
 	"github.com/btcsuite/btcd/wire"
+	"github.com/decred/dcrd/dcrec/secp256k1/v4"
 	"github.com/lightninglabs/taproot-assets/address"
 	"github.com/lightninglabs/taproot-assets/asset"
 	"github.com/lightninglabs/taproot-assets/commitment"
@@ -751,3 +753,102 @@ func (v *schnorrSigValidator) validateSchnorrSig(virtualTx *wire.MsgTx,
 
 	return nil
 }
+
+// HtlcIndexAsScriptKeyTweak converts the given HTLC index into a modulo N
+// scalar that can be used to tweak the internal key of the HTLC script key on
+// the asset level. The value of 1 is always added to the index to make sure
+// this value is always non-zero.
+func HtlcIndexAsScriptKeyTweak(index input.HtlcIndex) *secp256k1.ModNScalar {
+	// We need to avoid the tweak being zero, so we always add 1 to the
+	// index. Otherwise, we'd multiply G by zero.
+	index++
+
+	// If we wrapped around from math.MaxUint64 to 0, we need to make sure
+	// the tweak is 1 to not cause a multiplication by zero. This should
+	// never happen, as it would mean we have more than math.MaxUint64
+	// updates in a channel, which exceeds the protocol's maximum.
+	if index == 0 {
+		return new(secp256k1.ModNScalar).SetInt(1)
+	}
+
+	indexAsBytes := new(big.Int).SetUint64(index).Bytes()
+	indexAsScalar := new(secp256k1.ModNScalar)
+	_ = indexAsScalar.SetByteSlice(indexAsBytes)
+
+	return indexAsScalar
+}
+
+// TweakPubKeyWithIndex tweaks the given internal public key with the given
+// HTLC index. The tweak is derived from the index in a way that never results
+// in a zero tweak. The value of 1 is always added to the index to make sure
+// this value is always non-zero. The public key is tweaked like this:
+//
+//	tweakedKey = key + (index+1) * G
+func TweakPubKeyWithIndex(pubKey *btcec.PublicKey,
+	index input.HtlcIndex) *btcec.PublicKey {
+
+	// We need to operate on Jacobian points, which is just a different
+	// representation of the public key that allows us to do scalar
+	// multiplication.
+	var (
+		pubKeyJacobian, tweakTimesG, tweakedKey btcec.JacobianPoint
+	)
+	pubKey.AsJacobian(&pubKeyJacobian)
+
+	// Derive the tweak from the HTLC index in a way that never results in
+	// a zero tweak. Then we multiply G by the tweak.
+	tweak := HtlcIndexAsScriptKeyTweak(index)
+	secp256k1.ScalarBaseMultNonConst(tweak, &tweakTimesG)
+
+	// And finally we add the result to the key to get the tweaked key.
+	secp256k1.AddNonConst(&pubKeyJacobian, &tweakTimesG, &tweakedKey)
+
+	// Convert the tweaked key back to an affine point and create a new
+	// taproot key from it.
+	tweakedKey.ToAffine()
+	return btcec.NewPublicKey(&tweakedKey.X, &tweakedKey.Y)
+}
+
+// TweakHtlcTree tweaks the internal key of the given HTLC script tree with the
+// given index, then returns the tweaked tree with the updated taproot key.
+// The tapscript tree and tapscript root are not modified.
+// The internal key is tweaked like this:
+//
+//	tweakedInternalKey = internalKey + (index+1) * G
+func TweakHtlcTree(tree input.ScriptTree,
+	index input.HtlcIndex) input.ScriptTree {
+
+	// The tapscript tree and root are not modified, only the internal key
+	// is tweaked, which inherently modifies the taproot key.
+	tweakedInternalPubKey := TweakPubKeyWithIndex(tree.InternalKey, index)
+	newTaprootKey := txscript.ComputeTaprootOutputKey(
+		tweakedInternalPubKey, tree.TapscriptRoot,
+	)
+
+	return input.ScriptTree{
+		InternalKey:   tweakedInternalPubKey,
+		TaprootKey:    newTaprootKey,
+		TapscriptTree: tree.TapscriptTree,
+		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())
+}