multi: add new tapscript root option to GenTaprootFundingScript

This'll allow us to create a funding output that uses musig2, but uses a tapscript tweak rather than a normal BIP 86 tweak.

contractcourt/chain_watcher.go

@@ -308,7 +308,7 @@ func (c *chainWatcher) Start() error {
 	)
 	if chanState.ChanType.IsTaproot() {
 		c.fundingPkScript, _, err = input.GenTaprootFundingScript(
-			localKey, remoteKey, 0,
+			localKey, remoteKey, 0, fn.None[chainhash.Hash](),
 		)
 		if err != nil {
 			return err

funding/manager.go

@@ -24,6 +24,7 @@ import (
 	"github.com/lightningnetwork/lnd/channeldb"
 	"github.com/lightningnetwork/lnd/channeldb/models"
 	"github.com/lightningnetwork/lnd/discovery"
+	"github.com/lightningnetwork/lnd/fn"
 	"github.com/lightningnetwork/lnd/graph"
 	"github.com/lightningnetwork/lnd/input"
 	"github.com/lightningnetwork/lnd/keychain"
@@ -2899,6 +2900,7 @@ func makeFundingScript(channel *channeldb.OpenChannel) ([]byte, error) {
 	if channel.ChanType.IsTaproot() {
 		pkScript, _, err := input.GenTaprootFundingScript(
 			localKey, remoteKey, int64(channel.Capacity),
+			fn.None[chainhash.Hash](),
 		)
 		if err != nil {
 			return nil, err

graph/builder.go

@@ -11,12 +11,14 @@ import (
 
 	"github.com/btcsuite/btcd/btcec/v2"
 	"github.com/btcsuite/btcd/btcutil"
+	"github.com/btcsuite/btcd/chaincfg/chainhash"
 	"github.com/btcsuite/btcd/wire"
 	"github.com/go-errors/errors"
 	"github.com/lightningnetwork/lnd/batch"
 	"github.com/lightningnetwork/lnd/chainntnfs"
 	"github.com/lightningnetwork/lnd/channeldb"
 	"github.com/lightningnetwork/lnd/channeldb/models"
+	"github.com/lightningnetwork/lnd/fn"
 	"github.com/lightningnetwork/lnd/input"
 	"github.com/lightningnetwork/lnd/kvdb"
 	"github.com/lightningnetwork/lnd/lnutils"
@@ -1138,7 +1140,7 @@ func makeFundingScript(bitcoinKey1, bitcoinKey2 []byte,
 		}
 
 		fundingScript, _, err := input.GenTaprootFundingScript(
-			pubKey1, pubKey2, 0,
+			pubKey1, pubKey2, 0, fn.None[chainhash.Hash](),
 		)
 		if err != nil {
 			return nil, err

input/script_utils.go

@@ -11,8 +11,10 @@ import (
 	"github.com/btcsuite/btcd/btcec/v2/schnorr"
 	"github.com/btcsuite/btcd/btcec/v2/schnorr/musig2"
 	"github.com/btcsuite/btcd/btcutil"
+	"github.com/btcsuite/btcd/chaincfg/chainhash"
 	"github.com/btcsuite/btcd/txscript"
 	"github.com/btcsuite/btcd/wire"
+	"github.com/lightningnetwork/lnd/fn"
 	"github.com/lightningnetwork/lnd/lntypes"
 	"github.com/lightningnetwork/lnd/lnutils"
 	"golang.org/x/crypto/ripemd160"
@@ -199,26 +201,30 @@ func GenFundingPkScript(aPub, bPub []byte, amt int64) ([]byte, *wire.TxOut, erro
 }
 
 // GenTaprootFundingScript constructs the taproot-native funding output that
-// uses musig2 to create a single aggregated key to anchor the channel.
+// uses MuSig2 to create a single aggregated key to anchor the channel.
 func GenTaprootFundingScript(aPub, bPub *btcec.PublicKey,
-	amt int64) ([]byte, *wire.TxOut, error) {
+	amt int64, tapscriptRoot fn.Option[chainhash.Hash]) ([]byte,
+	*wire.TxOut, error) {
+
+	muSig2Opt := musig2.WithBIP86KeyTweak()
+	tapscriptRoot.WhenSome(func(scriptRoot chainhash.Hash) {
+		muSig2Opt = musig2.WithTaprootKeyTweak(scriptRoot[:])
+	})
 
 	// Similar to the existing p2wsh funding script, we'll always make sure
 	// we sort the keys before any major operations. In order to ensure
 	// that there's no other way this output can be spent, we'll use a BIP
-	// 86 tweak here during aggregation.
-	//
-	// TODO(roasbeef): revisit if BIP 86 is needed here?
+	// 86 tweak here during aggregation, unless the user has explicitly
+	// specified a tapscript root.
 	combinedKey, _, _, err := musig2.AggregateKeys(
-		[]*btcec.PublicKey{aPub, bPub}, true,
-		musig2.WithBIP86KeyTweak(),
+		[]*btcec.PublicKey{aPub, bPub}, true, muSig2Opt,
 	)
 	if err != nil {
 		return nil, nil, fmt.Errorf("unable to combine keys: %w", err)
 	}
 
 	// Now that we have the combined key, we can create a taproot pkScript
-	// from this, and then make the txout given the amount.
+	// from this, and then make the txOut given the amount.
 	pkScript, err := PayToTaprootScript(combinedKey.FinalKey)
 	if err != nil {
 		return nil, nil, fmt.Errorf("unable to make taproot "+
@@ -228,7 +234,7 @@ func GenTaprootFundingScript(aPub, bPub *btcec.PublicKey,
 	txOut := wire.NewTxOut(amt, pkScript)
 
 	// For the "witness program" we just return the raw pkScript since the
-	// output we create can _only_ be spent with a musig2 signature.
+	// output we create can _only_ be spent with a MuSig2 signature.
 	return pkScript, txOut, nil
 }
 

itest/lnd_funding_test.go

@@ -12,6 +12,7 @@ import (
 	"github.com/btcsuite/btcd/txscript"
 	"github.com/btcsuite/btcd/wire"
 	"github.com/lightningnetwork/lnd/chainreg"
+	"github.com/lightningnetwork/lnd/fn"
 	"github.com/lightningnetwork/lnd/funding"
 	"github.com/lightningnetwork/lnd/input"
 	"github.com/lightningnetwork/lnd/labels"
@@ -1192,6 +1193,7 @@ func deriveFundingShim(ht *lntest.HarnessTest, carol, dave *node.HarnessNode,
 
 		_, fundingOutput, err = input.GenTaprootFundingScript(
 			carolKey, daveKey, int64(chanSize),
+			fn.None[chainhash.Hash](),
 		)
 		require.NoError(ht, err)
 

lnwallet/chanfunding/canned_assembler.go

@@ -5,7 +5,9 @@ import (
 
 	"github.com/btcsuite/btcd/btcec/v2"
 	"github.com/btcsuite/btcd/btcutil"
+	"github.com/btcsuite/btcd/chaincfg/chainhash"
 	"github.com/btcsuite/btcd/wire"
+	"github.com/lightningnetwork/lnd/fn"
 	"github.com/lightningnetwork/lnd/input"
 	"github.com/lightningnetwork/lnd/keychain"
 )
@@ -76,9 +78,8 @@ func (s *ShimIntent) FundingOutput() ([]byte, *wire.TxOut, error) {
 		// Similar to the existing p2wsh script, we'll always ensure
 		// the keys are sorted before use.
 		return input.GenTaprootFundingScript(
-			s.localKey.PubKey,
-			s.remoteKey,
-			int64(totalAmt),
+			s.localKey.PubKey, s.remoteKey, int64(totalAmt),
+			fn.None[chainhash.Hash](),
 		)
 	}
 

lnwallet/channel.go

@@ -1013,6 +1013,7 @@ func (lc *LightningChannel) createSignDesc() error {
 	if chanState.ChanType.IsTaproot() {
 		fundingPkScript, _, err = input.GenTaprootFundingScript(
 			localKey, remoteKey, int64(lc.channelState.Capacity),
+			fn.None[chainhash.Hash](),
 		)
 		if err != nil {
 			return err

lnwallet/wallet.go

@@ -23,6 +23,7 @@ import (
 	"github.com/btcsuite/btcwallet/wallet"
 	"github.com/davecgh/go-spew/spew"
 	"github.com/lightningnetwork/lnd/channeldb"
+	"github.com/lightningnetwork/lnd/fn"
 	"github.com/lightningnetwork/lnd/input"
 	"github.com/lightningnetwork/lnd/keychain"
 	"github.com/lightningnetwork/lnd/lntypes"
@@ -2102,6 +2103,7 @@ func (l *LightningWallet) verifyCommitSig(res *ChannelReservation,
 		if res.musigSessions == nil {
 			_, fundingOutput, err := input.GenTaprootFundingScript(
 				localKey, remoteKey, channelValue,
+				fn.None[chainhash.Hash](),
 			)
 			if err != nil {
 				return err
@@ -2341,11 +2343,14 @@ func (l *LightningWallet) handleSingleFunderSigs(req *addSingleFunderSigsMsg) {
 		fundingTxOut         *wire.TxOut
 	)
 	if chanType.IsTaproot() {
-		fundingWitnessScript, fundingTxOut, err = input.GenTaprootFundingScript( //nolint:lll
+		//nolint:lll
+		fundingWitnessScript, fundingTxOut, err = input.GenTaprootFundingScript(
 			ourKey.PubKey, theirKey.PubKey, channelValue,
+			fn.None[chainhash.Hash](),
 		)
 	} else {
-		fundingWitnessScript, fundingTxOut, err = input.GenFundingPkScript( //nolint:lll
+		//nolint:lll
+		fundingWitnessScript, fundingTxOut, err = input.GenFundingPkScript(
 			ourKey.PubKey.SerializeCompressed(),
 			theirKey.PubKey.SerializeCompressed(), channelValue,
 		)
@@ -2482,6 +2487,7 @@ func (l *LightningWallet) ValidateChannel(channelState *channeldb.OpenChannel,
 	if channelState.ChanType.IsTaproot() {
 		fundingScript, _, err = input.GenTaprootFundingScript(
 			localKey, remoteKey, int64(channel.Capacity),
+			fn.None[chainhash.Hash](),
 		)
 		if err != nil {
 			return err