lnwallet: add support for taproot chans to createHtlcRetribution

As these are revoked HTLCs, we need to set the taptweak since the
revocation case is just a keyspend.

lnwallet/channel.go

@@ -2583,8 +2583,7 @@ func createHtlcRetribution(chanState *channeldb.OpenChannel,
 		return emptyRetribution, err
 	}
 
-	return HtlcRetribution{
+	signDesc := input.SignDescriptor{
-		SignDesc: input.SignDescriptor{
 		KeyDesc: chanState.LocalChanCfg.
 			RevocationBasePoint,
 		DoubleTweak:   commitmentSecret,
@@ -2594,7 +2593,20 @@ func createHtlcRetribution(chanState *channeldb.OpenChannel,
 			Value:    int64(htlc.Amt),
 		},
 		HashType: txscript.SigHashAll,
-		},
+	}
+
+	// For taproot HTLC outputs, we need to set the sign method to key
+	// spend, and also set the tap tweak root needed to derive the proper
+	// private key.
+	if chanState.ChanType.IsTaproot() {
+		signDesc.SignMethod = input.TaprootKeySpendSignMethod
+
+		tapscriptRoot := scriptInfo.ScriptTree.RootNode.TapHash()
+		signDesc.TapTweak = tapscriptRoot[:]
+	}
+
+	return HtlcRetribution{
+		SignDesc: signDesc,
 		OutPoint: wire.OutPoint{
 			Hash:  commitHash,
 			Index: uint32(htlc.OutputIndex),