lnwallet: update GenTaprootFundingScript to also return the taproot internal key

Author: Roasbeef

contractcourt/chain_watcher.go

@@ -308,7 +308,7 @@ func (c *chainWatcher) Start() error {
 		fundingOpts := fn.MapOptionZ(
 			chanState.TapscriptRoot, lnwallet.TapscriptRootToOpt,
 		)
-		c.fundingPkScript, _, err = input.GenTaprootFundingScript(
+		c.fundingPkScript, _, _, err = input.GenTaprootFundingScript(
 			localKey, remoteKey, 0, fundingOpts...,
 		)
 		if err != nil {

funding/manager.go

@@ -2881,7 +2881,7 @@ func makeFundingScript(channel *channeldb.OpenChannel) ([]byte, error) {
 		fundingOpts := fn.MapOptionZ(
 			channel.TapscriptRoot, lnwallet.TapscriptRootToOpt,
 		)
-		pkScript, _, err := input.GenTaprootFundingScript(
+		pkScript, _, _, err := input.GenTaprootFundingScript(
 			localKey, remoteKey, int64(channel.Capacity),
 			fundingOpts...,
 		)

input/script_utils.go

@@ -224,9 +224,12 @@ func WithTapscriptRoot(root chainhash.Hash) FundingScriptOpt {
 }
 
 // GenTaprootFundingScript constructs the taproot-native funding output that
-// uses musig2 to create a single aggregated key to anchor the channel.
-func GenTaprootFundingScript(aPub, bPub *btcec.PublicKey,
-	amt int64, opts ...FundingScriptOpt) ([]byte, *wire.TxOut, error) {
+// uses musig2 to create a single aggregated key to anchor the channel. This
+// also returns the musig2 aggregated key to allow the callers to examine the
+// pre tweaked key as well as the final combined key.
+func GenTaprootFundingScript(aPub, bPub *btcec.PublicKey, amt int64,
+	opts ...FundingScriptOpt) ([]byte, *wire.TxOut,
+	*musig2.AggregateKey, error) {
 
 	options := defaultFundingScriptOpts()
 	for _, optFunc := range opts {
@@ -248,22 +251,23 @@ func GenTaprootFundingScript(aPub, bPub *btcec.PublicKey,
 		[]*btcec.PublicKey{aPub, bPub}, true, musig2Opts,
 	)
 	if err != nil {
-		return nil, nil, fmt.Errorf("unable to combine keys: %w", err)
+		return nil, 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.
 	pkScript, err := PayToTaprootScript(combinedKey.FinalKey)
 	if err != nil {
-		return nil, nil, fmt.Errorf("unable to make taproot "+
+		return nil, nil, nil, fmt.Errorf("unable to make taproot "+
 			"pkscript: %w", err)
 	}
 
 	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.
-	return pkScript, txOut, nil
+	return pkScript, txOut, combinedKey, nil
 }
 
 // SpendMultiSig generates the witness stack required to redeem the 2-of-2 p2wsh

itest/lnd_funding_test.go

@@ -1163,7 +1163,7 @@ func deriveFundingShim(ht *lntest.HarnessTest, carol, dave *node.HarnessNode,
 		daveKey, err = btcec.ParsePubKey(daveFundingKey.RawKeyBytes)
 		require.NoError(ht, err)
 
-		_, fundingOutput, err = input.GenTaprootFundingScript(
+		_, fundingOutput, _, err = input.GenTaprootFundingScript(
 			carolKey, daveKey, int64(chanSize),
 		)
 		require.NoError(ht, err)

lnwallet/chanfunding/canned_assembler.go

@@ -92,10 +92,12 @@ 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(
+		pkScript, txOut, _, err := input.GenTaprootFundingScript(
 			s.localKey.PubKey, s.remoteKey, int64(totalAmt),
 			scriptOpts...,
 		)
+
+		return pkScript, txOut, err
 	}
 
 	return input.GenFundingPkScript(

lnwallet/channel.go

@@ -1537,7 +1537,7 @@ func (lc *LightningChannel) createSignDesc() error {
 			chanState.TapscriptRoot, TapscriptRootToOpt,
 		)
 
-		fundingPkScript, _, err = input.GenTaprootFundingScript(
+		fundingPkScript, _, _, err = input.GenTaprootFundingScript(
 			localKey, remoteKey, int64(lc.channelState.Capacity),
 			fundingOpts...,
 		)

lnwallet/wallet.go

@@ -2151,7 +2151,7 @@ func (l *LightningWallet) verifyCommitSig(res *ChannelReservation,
 				TapscriptRootToOpt,
 			)
 
-			_, fundingOutput, err := input.GenTaprootFundingScript(
+			_, fundingOutput, _, err := input.GenTaprootFundingScript(
 				localKey, remoteKey, channelValue,
 				fundingOpts...,
 			)
@@ -2398,7 +2398,7 @@ func (l *LightningWallet) handleSingleFunderSigs(req *addSingleFunderSigsMsg) {
 			pendingReservation.partialState.TapscriptRoot,
 			TapscriptRootToOpt,
 		)
-		fundingWitnessScript, fundingTxOut, err = input.GenTaprootFundingScript( //nolint:lll
+		fundingWitnessScript, fundingTxOut, _, err = input.GenTaprootFundingScript( //nolint:lll
 			ourKey.PubKey, theirKey.PubKey, channelValue,
 			fundingOpts...,
 		)
@@ -2556,7 +2556,7 @@ func (l *LightningWallet) ValidateChannel(channelState *channeldb.OpenChannel,
 			channelState.TapscriptRoot, TapscriptRootToOpt,
 		)
 
-		fundingScript, _, err = input.GenTaprootFundingScript(
+		fundingScript, _, _, err = input.GenTaprootFundingScript(
 			localKey, remoteKey, int64(channel.Capacity),
 			fundingOpts...,
 		)

routing/router.go

@@ -1542,7 +1542,7 @@ func makeFundingScript(bitcoinKey1, bitcoinKey2 []byte,
 			return nil, err
 		}
 
-		fundingScript, _, err := input.GenTaprootFundingScript(
+		fundingScript, _, _, err := input.GenTaprootFundingScript(
 			pubKey1, pubKey2, 0,
 		)
 		if err != nil {