loop/swap/htlc.go

package swap

import (
	"bytes"
	"crypto/sha256"
	"errors"
	"fmt"

	btcec "github.com/btcsuite/btcd/btcec/v2"
	"github.com/btcsuite/btcd/btcec/v2/schnorr"
	"github.com/btcsuite/btcd/btcutil"
	"github.com/btcsuite/btcd/chaincfg"
	"github.com/btcsuite/btcd/txscript"
	"github.com/btcsuite/btcd/wire"
	secp "github.com/decred/dcrd/dcrec/secp256k1/v4"
	"github.com/lightningnetwork/lnd/input"
	"github.com/lightningnetwork/lnd/lntypes"
)

// HtlcOutputType defines the output type of the htlc that is published.
type HtlcOutputType uint8

const (
	// HtlcP2WSH is a pay-to-witness-script-hash output (segwit only)
	HtlcP2WSH HtlcOutputType = iota

	// HtlcNP2WSH is a nested pay-to-witness-script-hash output that can be
	// paid to be legacy wallets.
	HtlcNP2WSH

	// HtlcP2TR is a pay-to-taproot output with three separate spend paths.
	HtlcP2TR
)

// ScriptVersion defines the HTLC script version.
type ScriptVersion uint8

const (
	// HtlcV1 refers to the original version of the HTLC script.
	HtlcV1 ScriptVersion = iota

	// HtlcV2 refers to the improved version of the HTLC script.
	HtlcV2

	// HtlcV3 refers to an upgraded version of HtlcV2 implemented with
	// tapscript.
	HtlcV3
)

// htlcScript defines an interface for the different HTLC implementations.
type HtlcScript interface {
	// genSuccessWitness returns the success script to spend this htlc with
	// the preimage.
	genSuccessWitness(receiverSig []byte,
		preimage lntypes.Preimage) (wire.TxWitness, error)

	// GenTimeoutWitness returns the timeout script to spend this htlc after
	// timeout.
	GenTimeoutWitness(senderSig []byte) (wire.TxWitness, error)

	// IsSuccessWitness checks whether the given stack is valid for
	// redeeming the htlc.
	IsSuccessWitness(witness wire.TxWitness) bool

	// Script returns the htlc script.
	Script() []byte

	// MaxSuccessWitnessSize returns the maximum witness size for the
	// success case witness.
	MaxSuccessWitnessSize() int

	// MaxTimeoutWitnessSize returns the maximum witness size for the
	// timeout case witness.
	MaxTimeoutWitnessSize() int

	// SuccessSequence returns the sequence to spend this htlc in the
	// success case.
	SuccessSequence() uint32
}

// Htlc contains relevant htlc information from the receiver perspective.
type Htlc struct {
	HtlcScript

	Version     ScriptVersion
	PkScript    []byte
	Hash        lntypes.Hash
	OutputType  HtlcOutputType
	ChainParams *chaincfg.Params
	Address     btcutil.Address
	SigScript   []byte
}

var (
	quoteKey [33]byte

	quoteHash lntypes.Hash

	// QuoteHtlc is a template script just used for fee estimation. It uses
	// the maximum value for cltv expiry to get the maximum (worst case)
	// script size.
	QuoteHtlc, _ = NewHtlc(
		HtlcV2,
		^int32(0), quoteKey, quoteKey, nil, quoteHash, HtlcP2WSH,
		&chaincfg.MainNetParams,
	)

	// ErrInvalidScriptVersion is returned when an unknown htlc version
	// is provided to NewHtlc. The supported version are HtlcV1, HtlcV2,
	// HtlcV3 as enums.
	ErrInvalidScriptVersion = fmt.Errorf("invalid script version")

	// ErrInvalidOutputSelected is returned when a taproot output is
	// selected for a v1 or v2 script.
	ErrInvalidOutputSelected = fmt.Errorf("taproot output selected for " +
		"non taproot htlc")

	// ErrSharedKeyNotNeeded is returned when a shared key is provided for
	// either the v1 or v2 script. Shared key is only necessary for the v3
	// script.
	ErrSharedKeyNotNeeded = fmt.Errorf("shared key not supported for " +
		"script version")

	// ErrSharedKeyRequired is returned when a script version requires a
	// shared key.
	ErrSharedKeyRequired = fmt.Errorf("shared key required for script " +
		"version")
)

// String returns the string value of HtlcOutputType.
func (h HtlcOutputType) String() string {
	switch h {
	case HtlcP2WSH:
		return "P2WSH"

	case HtlcNP2WSH:
		return "NP2WSH"

	case HtlcP2TR:
		return "P2TR"

	default:
		return "unknown"
	}
}

// NewHtlc returns a new instance. For v3 scripts, an internal pubkey generated
// by both participants must be provided.
func NewHtlc(version ScriptVersion, cltvExpiry int32,
	senderKey, receiverKey [33]byte, sharedKey *btcec.PublicKey,
	hash lntypes.Hash, outputType HtlcOutputType,
	chainParams *chaincfg.Params) (*Htlc, error) {

	var (
		err  error
		htlc HtlcScript
	)

	switch version {
	case HtlcV1:
		if sharedKey != nil {
			return nil, ErrSharedKeyNotNeeded
		}
		htlc, err = newHTLCScriptV1(
			cltvExpiry, senderKey, receiverKey, hash,
		)

	case HtlcV2:
		if sharedKey != nil {
			return nil, ErrSharedKeyNotNeeded
		}
		htlc, err = newHTLCScriptV2(
			cltvExpiry, senderKey, receiverKey, hash,
		)

	case HtlcV3:
		if sharedKey == nil {
			return nil, ErrSharedKeyRequired
		}
		htlc, err = newHTLCScriptV3(
			cltvExpiry, senderKey, receiverKey,
			sharedKey, hash,
		)

	default:
		return nil, ErrInvalidScriptVersion
	}

	if err != nil {
		return nil, err
	}

	var pkScript, sigScript []byte
	var address btcutil.Address

	switch outputType {
	case HtlcNP2WSH:
		p2wshPkScript, err := input.WitnessScriptHash(htlc.Script())
		if err != nil {
			return nil, err
		}

		// Generate p2sh script for p2wsh (nested).
		p2wshPkScriptHash := sha256.Sum256(p2wshPkScript)
		hash160 := input.Ripemd160H(p2wshPkScriptHash[:])

		builder := txscript.NewScriptBuilder()

		builder.AddOp(txscript.OP_HASH160)
		builder.AddData(hash160)
		builder.AddOp(txscript.OP_EQUAL)

		pkScript, err = builder.Script()
		if err != nil {
			return nil, err
		}

		// Generate a valid sigScript that will allow us to spend the
		// p2sh output. The sigScript will contain only a single push of
		// the p2wsh witness program corresponding to the matching
		// public key of this address.
		sigScript, err = txscript.NewScriptBuilder().
			AddData(p2wshPkScript).
			Script()
		if err != nil {
			return nil, err
		}

		address, err = btcutil.NewAddressScriptHash(
			p2wshPkScript, chainParams,
		)
		if err != nil {
			return nil, err
		}

	case HtlcP2WSH:
		pkScript, err = input.WitnessScriptHash(htlc.Script())
		if err != nil {
			return nil, err
		}

		address, err = btcutil.NewAddressWitnessScriptHash(
			pkScript[2:],
			chainParams,
		)
		if err != nil {
			return nil, err
		}

	case HtlcP2TR:
		// Confirm we have a v3 htlc.
		trHtlc, ok := htlc.(*HtlcScriptV3)
		if !ok {
			return nil, ErrInvalidOutputSelected
		}

		// Generate a tapscript address from our HTLC's taptree.
		address, err = btcutil.NewAddressTaproot(
			schnorr.SerializePubKey(trHtlc.TaprootKey), chainParams,
		)
		if err != nil {
			return nil, err
		}

		// Generate locking script.
		pkScript, err = txscript.PayToAddrScript(address)
		if err != nil {
			return nil, err
		}

	default:
		return nil, errors.New("unknown output type")
	}

	return &Htlc{
		HtlcScript:  htlc,
		Hash:        hash,
		Version:     version,
		PkScript:    pkScript,
		OutputType:  outputType,
		ChainParams: chainParams,
		Address:     address,
		SigScript:   sigScript,
	}, nil
}

// GenSuccessWitness returns the success script to spend this htlc with
// the preimage.
func (h *Htlc) GenSuccessWitness(receiverSig []byte,
	preimage lntypes.Preimage) (wire.TxWitness, error) {

	if h.Hash != preimage.Hash() {
		return nil, errors.New("preimage doesn't match hash")
	}

	return h.genSuccessWitness(receiverSig, preimage)
}

// AddSuccessToEstimator adds a successful spend to a weight estimator.
func (h *Htlc) AddSuccessToEstimator(estimator *input.TxWeightEstimator) error {
	maxSuccessWitnessSize := h.MaxSuccessWitnessSize()

	switch h.OutputType {
	case HtlcP2TR:
		// Generate tapscript.
		trHtlc, ok := h.HtlcScript.(*HtlcScriptV3)
		if !ok {
			return ErrInvalidOutputSelected
		}
		successLeaf := txscript.NewBaseTapLeaf(trHtlc.SuccessScript)
		timeoutLeaf := txscript.NewBaseTapLeaf(trHtlc.TimeoutScript)
		tapscript := input.TapscriptPartialReveal(
			trHtlc.InternalPubKey, successLeaf,
			timeoutLeaf.TapHash(),
		)

		estimator.AddTapscriptInput(maxSuccessWitnessSize, tapscript)

	case HtlcP2WSH:
		estimator.AddWitnessInput(maxSuccessWitnessSize)

	case HtlcNP2WSH:
		estimator.AddNestedP2WSHInput(maxSuccessWitnessSize)
	}

	return nil
}

// AddTimeoutToEstimator adds a timeout spend to a weight estimator.
func (h *Htlc) AddTimeoutToEstimator(estimator *input.TxWeightEstimator) error {
	maxTimeoutWitnessSize := h.MaxTimeoutWitnessSize()

	switch h.OutputType {
	case HtlcP2TR:
		// Generate tapscript.
		trHtlc, ok := h.HtlcScript.(*HtlcScriptV3)
		if !ok {
			return ErrInvalidOutputSelected
		}
		successLeaf := txscript.NewBaseTapLeaf(trHtlc.SuccessScript)
		timeoutLeaf := txscript.NewBaseTapLeaf(trHtlc.TimeoutScript)
		tapscript := input.TapscriptPartialReveal(
			trHtlc.InternalPubKey, timeoutLeaf,
			successLeaf.TapHash(),
		)

		estimator.AddTapscriptInput(maxTimeoutWitnessSize, tapscript)

	case HtlcP2WSH:
		estimator.AddWitnessInput(maxTimeoutWitnessSize)

	case HtlcNP2WSH:
		estimator.AddNestedP2WSHInput(maxTimeoutWitnessSize)
	}

	return nil
}

// HtlcScriptV1 encapsulates the htlc v1 script.
type HtlcScriptV1 struct {
	script []byte
}

// newHTLCScriptV1 constructs an HtlcScript with the HTLC V1 witness script.
//
// OP_SIZE 32 OP_EQUAL
// OP_IF
//    OP_HASH160 <ripemd160(swapHash)> OP_EQUALVERIFY
//    <receiverHtlcKey>
// OP_ELSE
//    OP_DROP
//    <cltv timeout> OP_CHECKLOCKTIMEVERIFY OP_DROP
//    <senderHtlcKey>
// OP_ENDIF
// OP_CHECKSIG
func newHTLCScriptV1(cltvExpiry int32, senderHtlcKey,
	receiverHtlcKey [33]byte, swapHash lntypes.Hash) (*HtlcScriptV1, error) {

	builder := txscript.NewScriptBuilder()

	builder.AddOp(txscript.OP_SIZE)
	builder.AddInt64(32)
	builder.AddOp(txscript.OP_EQUAL)

	builder.AddOp(txscript.OP_IF)

	builder.AddOp(txscript.OP_HASH160)
	builder.AddData(input.Ripemd160H(swapHash[:]))
	builder.AddOp(txscript.OP_EQUALVERIFY)

	builder.AddData(receiverHtlcKey[:])

	builder.AddOp(txscript.OP_ELSE)

	builder.AddOp(txscript.OP_DROP)

	builder.AddInt64(int64(cltvExpiry))
	builder.AddOp(txscript.OP_CHECKLOCKTIMEVERIFY)
	builder.AddOp(txscript.OP_DROP)

	builder.AddData(senderHtlcKey[:])

	builder.AddOp(txscript.OP_ENDIF)

	builder.AddOp(txscript.OP_CHECKSIG)

	script, err := builder.Script()
	if err != nil {
		return nil, err
	}

	return &HtlcScriptV1{
		script: script,
	}, nil
}

// genSuccessWitness returns the success script to spend this htlc with
// the preimage.
func (h *HtlcScriptV1) genSuccessWitness(receiverSig []byte,
	preimage lntypes.Preimage) (wire.TxWitness, error) {

	witnessStack := make(wire.TxWitness, 3)
	witnessStack[0] = append(receiverSig, byte(txscript.SigHashAll))
	witnessStack[1] = preimage[:]
	witnessStack[2] = h.script

	return witnessStack, nil
}

// GenTimeoutWitness returns the timeout script to spend this htlc after
// timeout.
func (h *HtlcScriptV1) GenTimeoutWitness(
	senderSig []byte) (wire.TxWitness, error) {

	witnessStack := make(wire.TxWitness, 3)
	witnessStack[0] = append(senderSig, byte(txscript.SigHashAll))
	witnessStack[1] = []byte{0}
	witnessStack[2] = h.script

	return witnessStack, nil
}

// IsSuccessWitness checks whether the given stack is valid for redeeming the
// htlc.
func (h *HtlcScriptV1) IsSuccessWitness(witness wire.TxWitness) bool {
	if len(witness) != 3 {
		return false
	}

	isTimeoutTx := bytes.Equal([]byte{0}, witness[1])
	return !isTimeoutTx
}

// Script returns the htlc script.
func (h *HtlcScriptV1) Script() []byte {
	return h.script
}

// MaxSuccessWitnessSize returns the maximum success witness size.
func (h *HtlcScriptV1) MaxSuccessWitnessSize() int {
	// Calculate maximum success witness size
	//
	// - number_of_witness_elements: 1 byte
	// - receiver_sig_length: 1 byte
	// - receiver_sig: 73 bytes
	// - preimage_length: 1 byte
	// - preimage: 32 bytes
	// - witness_script_length: 1 byte
	// - witness_script: len(script) bytes
	return 1 + 1 + 73 + 1 + 32 + 1 + len(h.script)
}

// MaxTimeoutWitnessSize return the maximum timeout witness size.
func (h *HtlcScriptV1) MaxTimeoutWitnessSize() int {
	// Calculate maximum timeout witness size
	//
	// - number_of_witness_elements: 1 byte
	// - sender_sig_length: 1 byte
	// - sender_sig: 73 bytes
	// - zero_length: 1 byte
	// - zero: 1 byte
	// - witness_script_length: 1 byte
	// - witness_script: len(script) bytes
	return 1 + 1 + 73 + 1 + 1 + 1 + len(h.script)
}

// SuccessSequence returns the sequence to spend this htlc in the success case.
func (h *HtlcScriptV1) SuccessSequence() uint32 {
	return 0
}

// HtlcScriptV2 encapsulates the htlc v2 script.
type HtlcScriptV2 struct {
	script    []byte
	senderKey [33]byte
}

// newHTLCScriptV2 construct an HtlcScipt with the HTLC V2 witness script.
//
// <receiverHtlcKey> OP_CHECKSIG OP_NOTIF
//   OP_DUP OP_HASH160 <HASH160(senderHtlcKey)> OP_EQUALVERIFY OP_CHECKSIGVERIFY
//   <cltv timeout> OP_CHECKLOCKTIMEVERIFY
// OP_ELSE
//   OP_SIZE <20> OP_EQUALVERIFY OP_HASH160 <ripemd(swapHash)> OP_EQUALVERIFY 1
//   OP_CHECKSEQUENCEVERIFY
// OP_ENDIF
func newHTLCScriptV2(cltvExpiry int32, senderHtlcKey,
	receiverHtlcKey [33]byte, swapHash lntypes.Hash) (*HtlcScriptV2, error) {

	builder := txscript.NewScriptBuilder()
	builder.AddData(receiverHtlcKey[:])
	builder.AddOp(txscript.OP_CHECKSIG)

	builder.AddOp(txscript.OP_NOTIF)

	builder.AddOp(txscript.OP_DUP)
	builder.AddOp(txscript.OP_HASH160)
	senderHtlcKeyHash := sha256.Sum256(senderHtlcKey[:])
	builder.AddData(input.Ripemd160H(senderHtlcKeyHash[:]))

	builder.AddOp(txscript.OP_EQUALVERIFY)
	builder.AddOp(txscript.OP_CHECKSIGVERIFY)

	builder.AddInt64(int64(cltvExpiry))
	builder.AddOp(txscript.OP_CHECKLOCKTIMEVERIFY)

	builder.AddOp(txscript.OP_ELSE)

	builder.AddOp(txscript.OP_SIZE)
	builder.AddInt64(0x20)
	builder.AddOp(txscript.OP_EQUALVERIFY)
	builder.AddOp(txscript.OP_HASH160)
	builder.AddData(input.Ripemd160H(swapHash[:]))
	builder.AddOp(txscript.OP_EQUALVERIFY)
	builder.AddOp(txscript.OP_1)

	builder.AddOp(txscript.OP_CHECKSEQUENCEVERIFY)

	builder.AddOp(txscript.OP_ENDIF)

	script, err := builder.Script()
	if err != nil {
		return nil, err
	}

	return &HtlcScriptV2{
		script:    script,
		senderKey: senderHtlcKey,
	}, nil
}

// genSuccessWitness returns the success script to spend this htlc with
// the preimage.
func (h *HtlcScriptV2) genSuccessWitness(receiverSig []byte,
	preimage lntypes.Preimage) (wire.TxWitness, error) {

	witnessStack := make(wire.TxWitness, 3)
	witnessStack[0] = preimage[:]
	witnessStack[1] = append(receiverSig, byte(txscript.SigHashAll))
	witnessStack[2] = h.script

	return witnessStack, nil
}

// IsSuccessWitness checks whether the given stack is valid for redeeming the
// htlc.
func (h *HtlcScriptV2) IsSuccessWitness(witness wire.TxWitness) bool {
	isTimeoutTx := len(witness) == 4

	return !isTimeoutTx
}

// GenTimeoutWitness returns the timeout script to spend this htlc after
// timeout.
func (h *HtlcScriptV2) GenTimeoutWitness(
	senderSig []byte) (wire.TxWitness, error) {

	witnessStack := make(wire.TxWitness, 4)
	witnessStack[0] = append(senderSig, byte(txscript.SigHashAll))
	witnessStack[1] = h.senderKey[:]
	witnessStack[2] = []byte{}
	witnessStack[3] = h.script

	return witnessStack, nil
}

// Script returns the htlc script.
func (h *HtlcScriptV2) Script() []byte {
	return h.script
}

// MaxSuccessWitnessSize returns maximum success witness size.
func (h *HtlcScriptV2) MaxSuccessWitnessSize() int {
	// Calculate maximum success witness size
	//
	// - number_of_witness_elements: 1 byte
	// - receiver_sig_length: 1 byte
	// - receiver_sig: 73 bytes
	// - preimage_length: 1 byte
	// - preimage: 32 bytes
	// - witness_script_length: 1 byte
	// - witness_script: len(script) bytes
	return 1 + 1 + 73 + 1 + 32 + 1 + len(h.script)
}

// MaxTimeoutWitnessSize returns maximum timeout witness size.
func (h *HtlcScriptV2) MaxTimeoutWitnessSize() int {
	// Calculate maximum timeout witness size
	//
	// - number_of_witness_elements: 1 byte
	// - sender_sig_length: 1 byte
	// - sender_sig: 73 bytes
	// - sender_key_length: 1 byte
	// - sender_key: 33 bytes
	// - zero: 1 byte
	// - witness_script_length: 1 byte
	// - witness_script: len(script) bytes
	return 1 + 1 + 73 + 1 + 33 + 1 + 1 + len(h.script)
}

// SuccessSequence returns the sequence to spend this htlc in the success case.
func (h *HtlcScriptV2) SuccessSequence() uint32 {
	return 1
}

// HtlcScriptV3 encapsulates the htlc v3 script.
type HtlcScriptV3 struct {
	// The final locking script for the timeout path which is available to
	// the sender after the set blockheight.
	TimeoutScript []byte

	// The final locking script for the success path in which the receiver
	// reveals the preimage.
	SuccessScript []byte

	// The public key for the keyspend path which bypasses the above two
	// locking scripts.
	InternalPubKey *btcec.PublicKey

	// The taproot public key which is created with the above 3 inputs.
	TaprootKey *btcec.PublicKey
}

// newHTLCScriptV3 constructs a HtlcScipt with the HTLC V3 taproot script.
func newHTLCScriptV3(cltvExpiry int32, senderHtlcKey,
	receiverHtlcKey [33]byte, sharedKey *btcec.PublicKey,
	swapHash lntypes.Hash) (*HtlcScriptV3, error) {

	receiverPubKey, err := btcec.ParsePubKey(
		receiverHtlcKey[:],
	)
	if err != nil {
		return nil, err
	}

	senderPubKey, err := btcec.ParsePubKey(
		senderHtlcKey[:],
	)
	if err != nil {
		return nil, err
	}

	var schnorrSenderKey, schnorrReceiverKey [32]byte
	copy(schnorrSenderKey[:], schnorr.SerializePubKey(senderPubKey))
	copy(schnorrReceiverKey[:], schnorr.SerializePubKey(receiverPubKey))

	// Create our success path script, we'll use this separately
	// to generate the success path leaf.
	successPathScript, err := GenSuccessPathScript(
		schnorrReceiverKey, swapHash,
	)
	if err != nil {
		return nil, err
	}

	// Create our timeout path leaf, we'll use this separately
	// to generate the timeout path leaf.
	timeoutPathScript, err := GenTimeoutPathScript(
		schnorrSenderKey, int64(cltvExpiry),
	)
	if err != nil {
		return nil, err
	}

	// Assemble our taproot script tree from our leaves.
	tree := txscript.AssembleTaprootScriptTree(
		txscript.NewBaseTapLeaf(successPathScript),
		txscript.NewBaseTapLeaf(timeoutPathScript),
	)

	rootHash := tree.RootNode.TapHash()

	// Calculate top level taproot key.
	taprootKey := txscript.ComputeTaprootOutputKey(
		sharedKey, rootHash[:],
	)

	return &HtlcScriptV3{
		TimeoutScript:  timeoutPathScript,
		SuccessScript:  successPathScript,
		InternalPubKey: sharedKey,
		TaprootKey:     taprootKey,
	}, nil
}

// GenTimeoutPathScript constructs an HtlcScript for the timeout payment path.
// Largest possible bytesize of the script is 32 + 1 + 2 + 1 = 36.
//
//	<senderHtlcKey> OP_CHECKSIGVERIFY <cltvExpiry> OP_CHECKLOCKTIMEVERIFY
func GenTimeoutPathScript(
	senderHtlcKey [32]byte, cltvExpiry int64) ([]byte, error) {

	builder := txscript.NewScriptBuilder()
	builder.AddData(senderHtlcKey[:])
	builder.AddOp(txscript.OP_CHECKSIGVERIFY)
	builder.AddInt64(cltvExpiry)
	builder.AddOp(txscript.OP_CHECKLOCKTIMEVERIFY)
	return builder.Script()
}

// GenSuccessPathScript constructs an HtlcScript for the success payment path.
// Largest possible bytesize of the script is 32 + 5*1 + 20 + 3*1 = 60.
//
//	<receiverHtlcKey> OP_CHECKSIGVERIFY
//	OP_SIZE 32 OP_EQUALVERIFY
//	OP_HASH160 <ripemd160h(swapHash)> OP_EQUALVERIFY
//	1 OP_CHECKSEQUENCEVERIFY
func GenSuccessPathScript(receiverHtlcKey [32]byte,
	swapHash lntypes.Hash) ([]byte, error) {

	builder := txscript.NewScriptBuilder()

	builder.AddData(receiverHtlcKey[:])
	builder.AddOp(txscript.OP_CHECKSIGVERIFY)
	builder.AddOp(txscript.OP_SIZE)
	builder.AddInt64(32)
	builder.AddOp(txscript.OP_EQUALVERIFY)
	builder.AddOp(txscript.OP_HASH160)
	builder.AddData(input.Ripemd160H(swapHash[:]))
	builder.AddOp(txscript.OP_EQUALVERIFY)
	builder.AddInt64(1)
	builder.AddOp(txscript.OP_CHECKSEQUENCEVERIFY)

	return builder.Script()
}

// genControlBlock constructs the control block with the depth 1 leaf of the
// unused path to compute the proof. For example if spending path a of (root ->
// a, root -> b), genControlBlock(b.Script) would be used to create the
// controlBlock for a.
func (h *HtlcScriptV3) genControlBlock(leafScript []byte) ([]byte, error) {
	var outputKeyYIsOdd bool

	// Check for odd bit.
	if h.TaprootKey.SerializeCompressed()[0] == secp.PubKeyFormatCompressedOdd {
		outputKeyYIsOdd = true
	}

	// Generate proof with unused script path.
	leaf := txscript.NewBaseTapLeaf(leafScript)
	proof := leaf.TapHash()

	controlBlock := txscript.ControlBlock{
		InternalKey:     h.InternalPubKey,
		OutputKeyYIsOdd: outputKeyYIsOdd,
		LeafVersion:     txscript.BaseLeafVersion,
		InclusionProof:  proof[:],
	}

	return controlBlock.ToBytes()
}

// genSuccessWitness returns the success script to spend this htlc with
// the preimage.
func (h *HtlcScriptV3) genSuccessWitness(
	receiverSig []byte, preimage lntypes.Preimage) (wire.TxWitness, error) {

	controlBlockBytes, err := h.genControlBlock(h.TimeoutScript)
	if err != nil {
		return nil, err
	}

	return wire.TxWitness{
		preimage[:],
		receiverSig,
		h.SuccessScript,
		controlBlockBytes,
	}, nil
}

// GenTimeoutWitness returns the timeout script to spend this htlc after
// timeout.
func (h *HtlcScriptV3) GenTimeoutWitness(
	senderSig []byte) (wire.TxWitness, error) {

	controlBlockBytes, err := h.genControlBlock(h.SuccessScript)
	if err != nil {
		return nil, err
	}

	return wire.TxWitness{
		senderSig,
		h.TimeoutScript,
		controlBlockBytes,
	}, nil
}

// IsSuccessWitness checks whether the given stack is valid for
// redeeming the htlc.
func (h *HtlcScriptV3) IsSuccessWitness(witness wire.TxWitness) bool {
	return len(witness) == 4
}

// Script is not implemented, but necessary to conform to interface.
func (h *HtlcScriptV3) Script() []byte {
	return nil
}

// MaxSuccessWitnessSize returns the maximum witness size for the
// success case witness.
func (h *HtlcScriptV3) MaxSuccessWitnessSize() int {
	// Calculate maximum success witness size
	//
	// - number_of_witness_elements: 1 byte
	// - sigLength: 1 byte
	// - sig: 64 bytes
	// - preimage_length: 1 byte
	// - preimage: 32 bytes
	// - witness_script_length: 1 byte
	// - witness_script: 60 bytes
	// - control_block_length: 1 byte
	// - control_block: 65 bytes
	//	- leafVersionAndParity: 1
	//	- internalPubkey: 32
	//	- proof: 32
	return 1 + 1 + 64 + 1 + 32 + 1 + 60 + 1 + 65
}

// MaxTimeoutWitnessSize returns the maximum witness size for the
// timeout case witness.
func (h *HtlcScriptV3) MaxTimeoutWitnessSize() int {
	// Calculate maximum timeout witness size
	//
	// - number_of_witness_elements: 1 byte
	// - sigLength: 1 byte
	// - sig: 64 bytes
	// - witness_script_length: 1 byte
	// - witness_script: 36 bytes
	// - control_block_length: 1 byte
	// - control_block: 65 bytes
	//	- leafVersionAndParity: 1
	//	- internalPubkey: 32
	//	- proof: 32
	return 1 + 1 + 64 + 1 + 36 + 1 + 65
}

// SuccessSequence returns the sequence to spend this htlc in the
// success case.
func (h *HtlcScriptV3) SuccessSequence() uint32 {
	return 1
}