From 0a0b5f89c95e6b1382122322316710bc62a6403f Mon Sep 17 00:00:00 2001
From: "Johan T. Halseth" <johanth@gmail.com>
Date: Mon, 26 Apr 2021 15:29:59 +0200
Subject: [PATCH] input: create IsHtlcSpendRevoke

---
 input/script_utils.go | 60 ++++++++++++++++++++++++++++---------------
 1 file changed, 40 insertions(+), 20 deletions(-)

diff --git a/input/script_utils.go b/input/script_utils.go
index dc3c4ccf..7ce6fff5 100644
--- a/input/script_utils.go
+++ b/input/script_utils.go
@@ -313,21 +313,32 @@ func SenderHtlcSpendRevokeWithKey(signer Signer, signDesc *SignDescriptor,
 func SenderHtlcSpendRevoke(signer Signer, signDesc *SignDescriptor,
 	sweepTx *wire.MsgTx) (wire.TxWitness, error) {
 
-	if signDesc.KeyDesc.PubKey == nil {
-		return nil, fmt.Errorf("cannot generate witness with nil " +
-			"KeyDesc pubkey")
+	revokeKey, err := deriveRevokePubKey(signDesc)
+	if err != nil {
+		return nil, err
 	}
 
-	// Derive the revocation key using the local revocation base point and
-	// commitment point.
-	revokeKey := DeriveRevocationPubkey(
-		signDesc.KeyDesc.PubKey,
-		signDesc.DoubleTweak.PubKey(),
-	)
-
 	return SenderHtlcSpendRevokeWithKey(signer, signDesc, revokeKey, sweepTx)
 }
 
+// IsHtlcSpendRevoke is used to determine if the passed spend is spending a
+// HTLC output using the revocation key.
+func IsHtlcSpendRevoke(txIn *wire.TxIn, signDesc *SignDescriptor) (
+	bool, error) {
+
+	revokeKey, err := deriveRevokePubKey(signDesc)
+	if err != nil {
+		return false, err
+	}
+
+	if len(txIn.Witness) == 3 &&
+		bytes.Equal(txIn.Witness[1], revokeKey.SerializeCompressed()) {
+		return true, nil
+	}
+
+	return false, nil
+}
+
 // SenderHtlcSpendRedeem constructs a valid witness allowing the receiver of an
 // HTLC to redeem the pending output in the scenario that the sender broadcasts
 // their version of the commitment transaction. A valid spend requires
@@ -575,16 +586,7 @@ func ReceiverHtlcSpendRevokeWithKey(signer Signer, signDesc *SignDescriptor,
 	return witnessStack, nil
 }
 
-// ReceiverHtlcSpendRevoke constructs a valid witness allowing the sender of an
-// HTLC within a previously revoked commitment transaction to re-claim the
-// pending funds in the case that the receiver broadcasts this revoked
-// commitment transaction. This method first derives the appropriate revocation
-// key, and requires that the provided SignDescriptor has a local revocation
-// basepoint and commitment secret in the PubKey and DoubleTweak fields,
-// respectively.
-func ReceiverHtlcSpendRevoke(signer Signer, signDesc *SignDescriptor,
-	sweepTx *wire.MsgTx) (wire.TxWitness, error) {
-
+func deriveRevokePubKey(signDesc *SignDescriptor) (*btcec.PublicKey, error) {
 	if signDesc.KeyDesc.PubKey == nil {
 		return nil, fmt.Errorf("cannot generate witness with nil " +
 			"KeyDesc pubkey")
@@ -597,6 +599,24 @@ func ReceiverHtlcSpendRevoke(signer Signer, signDesc *SignDescriptor,
 		signDesc.DoubleTweak.PubKey(),
 	)
 
+	return revokeKey, nil
+}
+
+// ReceiverHtlcSpendRevoke constructs a valid witness allowing the sender of an
+// HTLC within a previously revoked commitment transaction to re-claim the
+// pending funds in the case that the receiver broadcasts this revoked
+// commitment transaction. This method first derives the appropriate revocation
+// key, and requires that the provided SignDescriptor has a local revocation
+// basepoint and commitment secret in the PubKey and DoubleTweak fields,
+// respectively.
+func ReceiverHtlcSpendRevoke(signer Signer, signDesc *SignDescriptor,
+	sweepTx *wire.MsgTx) (wire.TxWitness, error) {
+
+	revokeKey, err := deriveRevokePubKey(signDesc)
+	if err != nil {
+		return nil, err
+	}
+
 	return ReceiverHtlcSpendRevokeWithKey(signer, signDesc, revokeKey, sweepTx)
 }