txauthor: add taproot address type

wallet/txauthor/author.go

@@ -8,6 +8,7 @@ package txauthor
 import (
 	"errors"
 
+	"github.com/btcsuite/btcd/btcec/v2/schnorr"
 	"github.com/btcsuite/btcd/btcutil"
 	"github.com/btcsuite/btcd/chaincfg"
 	"github.com/btcsuite/btcd/txscript"
@@ -229,19 +230,32 @@ func AddAllInputScripts(tx *wire.MsgTx, prevPkScripts [][]byte,
 		// function which generates both the sigScript, and the witness
 		// script.
 		case txscript.IsPayToScriptHash(pkScript):
-			err := spendNestedWitnessPubKeyHash(inputs[i], pkScript,
-				int64(inputValues[i]), chainParams, secrets,
-				tx, hashCache, i)
+			err := spendNestedWitnessPubKeyHash(
+				inputs[i], pkScript, int64(inputValues[i]),
+				chainParams, secrets, tx, hashCache, i,
+			)
 			if err != nil {
 				return err
 			}
+
 		case txscript.IsPayToWitnessPubKeyHash(pkScript):
-			err := spendWitnessKeyHash(inputs[i], pkScript,
-				int64(inputValues[i]), chainParams, secrets,
-				tx, hashCache, i)
+			err := spendWitnessKeyHash(
+				inputs[i], pkScript, int64(inputValues[i]),
+				chainParams, secrets, tx, hashCache, i,
+			)
+			if err != nil {
+				return err
+			}
+
+		case txscript.IsPayToTaproot(pkScript):
+			err := spendTaprootKeyHash(
+				inputs[i], pkScript, int64(inputValues[i]),
+				chainParams, secrets, tx, hashCache, i,
+			)
 			if err != nil {
 				return err
 			}
+
 		default:
 			sigScript := inputs[i].SignatureScript
 			script, err := txscript.SignTxOutput(chainParams, tx, i,
@@ -309,6 +323,54 @@ func spendWitnessKeyHash(txIn *wire.TxIn, pkScript []byte,
 	return nil
 }
 
+// spendTaprootKey generates, and sets a valid witness for spending the passed
+// pkScript with the specified input amount. The input amount *must*
+// correspond to the output value of the previous pkScript, or else verification
+// will fail since the new sighash digest algorithm defined in BIP0341 includes
+// the input value in the sighash.
+func spendTaprootKey(txIn *wire.TxIn, pkScript []byte,
+	inputValue int64, chainParams *chaincfg.Params, secrets SecretsSource,
+	tx *wire.MsgTx, hashCache *txscript.TxSigHashes, idx int) error {
+
+	// First obtain the key pair associated with this p2tr address.
+	_, addrs, _, err := txscript.ExtractPkScriptAddrs(pkScript, chainParams)
+	if err != nil {
+		return err
+	}
+	privKey, _, err := secrets.GetKey(addrs[0])
+	if err != nil {
+		return err
+	}
+	pubKey := privKey.PubKey()
+
+	tapKey := txscript.ComputeTaprootKeyNoScript(pubKey)
+	p2trAddr, err := btcutil.NewAddressTaproot(
+		schnorr.SerializePubKey(tapKey), chainParams,
+	)
+	if err != nil {
+		return err
+	}
+
+	// With the concrete address type, we can now generate the
+	// corresponding witness program to be used to generate a valid witness
+	// which will allow us to spend this output.
+	witnessProgram, err := txscript.PayToAddrScript(p2trAddr)
+	if err != nil {
+		return err
+	}
+	witnessScript, err := txscript.TaprootWitnessSignature(
+		tx, hashCache, idx, inputValue, witnessProgram,
+		txscript.SigHashDefault, privKey,
+	)
+	if err != nil {
+		return err
+	}
+
+	txIn.Witness = witnessScript
+
+	return nil
+}
+
 // spendNestedWitnessPubKey generates both a sigScript, and valid witness for
 // spending the passed pkScript with the specified input amount. The generated
 // sigScript is the version 0 p2wkh witness program corresponding to the queried