lnwallet: add TaprootInternalKey method to ShimIntent

If this is a taproot channel, then we'll return the internal key which'll be useful to callers.

Author: Roasbeef

lnwallet/chanfunding/canned_assembler.go

@@ -4,6 +4,7 @@ import (
 	"fmt"
 
 	"github.com/btcsuite/btcd/btcec/v2"
+	"github.com/btcsuite/btcd/btcec/v2/schnorr/musig2"
 	"github.com/btcsuite/btcd/btcutil"
 	"github.com/btcsuite/btcd/chaincfg/chainhash"
 	"github.com/btcsuite/btcd/wire"
@@ -105,6 +106,26 @@ func (s *ShimIntent) FundingOutput() ([]byte, *wire.TxOut, error) {
 	)
 }
 
+// TaprootInternalKey may return the internal key for a MuSig2 funding output,
+// but only if this is actually a MuSig2 channel.
+func (s *ShimIntent) TaprootInternalKey() fn.Option[*btcec.PublicKey] {
+	if !s.musig2 {
+		return fn.None[*btcec.PublicKey]()
+	}
+
+	// Similar to the existing p2wsh script, we'll always ensure the keys
+	// are sorted before use. Since we're only interested in the internal
+	// key, we don't need to take into account any tapscript root.
+	//
+	// We ignore the error here as this is only called after FundingOutput
+	// is called.
+	combinedKey, _, _, _ := musig2.AggregateKeys(
+		[]*btcec.PublicKey{s.localKey.PubKey, s.remoteKey}, true,
+	)
+
+	return fn.Some(combinedKey.PreTweakedKey)
+}
+
 // Cancel allows the caller to cancel a funding Intent at any time.  This will
 // return any resources such as coins back to the eligible pool to be used in
 // order channel fundings.