assets swap deposits [1/N]: add asset HTLC helpers

assets/htlc/script.go

@@ -0,0 +1,106 @@
+package htlc
+
+import (
+	"github.com/btcsuite/btcd/btcec/v2"
+	"github.com/btcsuite/btcd/btcec/v2/schnorr"
+	"github.com/btcsuite/btcd/txscript"
+	"github.com/decred/dcrd/dcrec/secp256k1/v4"
+	"github.com/lightninglabs/taproot-assets/asset"
+	"github.com/lightningnetwork/lnd/input"
+	"github.com/lightningnetwork/lnd/keychain"
+	"github.com/lightningnetwork/lnd/lntypes"
+)
+
+// GenSuccessPathScript constructs a script for the success path of the HTLC
+// payment. Optionally includes a CHECKSEQUENCEVERIFY (CSV) of 1 if `csv` is
+// true, to prevent potential pinning attacks when the HTLC is not part of a
+// package relay.
+func GenSuccessPathScript(receiverHtlcKey *btcec.PublicKey,
+	swapHash lntypes.Hash, csvOne bool) ([]byte, error) {
+
+	builder := txscript.NewScriptBuilder()
+
+	builder.AddData(schnorr.SerializePubKey(receiverHtlcKey))
+	builder.AddOp(txscript.OP_CHECKSIGVERIFY)
+	builder.AddOp(txscript.OP_SIZE)
+	builder.AddInt64(32)
+	builder.AddOp(txscript.OP_EQUALVERIFY)
+	builder.AddOp(txscript.OP_HASH160)
+	builder.AddData(input.Ripemd160H(swapHash[:]))
+	// OP_EQUAL will leave 0 or 1 on the stack depending on whether the hash
+	// matches.
+	// - If it matches and CSV is not used, the script will
+	// evaulate to true.
+	// - If it matches and CSV is used, we'll have 1 on the stack which is
+	// used to verify the CSV condition.
+	// - If it does not match, we'll have 0 on the stack which will cause
+	// the script to fail even if CSV is used.
+	builder.AddOp(txscript.OP_EQUAL)
+
+	if csvOne {
+		// If csvOne is true, we add a CHECKSEQUENCEVERIFY to ensure
+		// that the HTLC can only be claimed after at least one
+		// confirmation.
+		builder.AddOp(txscript.OP_CHECKSEQUENCEVERIFY)
+	}
+
+	return builder.Script()
+}
+
+// GenTimeoutPathScript constructs an HtlcScript for the timeout payment path.
+func GenTimeoutPathScript(senderHtlcKey *btcec.PublicKey, csvExpiry int64) (
+	[]byte, error) {
+
+	builder := txscript.NewScriptBuilder()
+	builder.AddData(schnorr.SerializePubKey(senderHtlcKey))
+	builder.AddOp(txscript.OP_CHECKSIGVERIFY)
+	builder.AddInt64(csvExpiry)
+	builder.AddOp(txscript.OP_CHECKSEQUENCEVERIFY)
+	return builder.Script()
+}
+
+// GetOpTrueScript returns a script that always evaluates to true.
+func GetOpTrueScript() ([]byte, error) {
+	return txscript.NewScriptBuilder().AddOp(txscript.OP_TRUE).Script()
+}
+
+// CreateOpTrueLeaf creates a taproot leaf that always evaluates to true.
+func CreateOpTrueLeaf() (asset.ScriptKey, txscript.TapLeaf,
+	*txscript.IndexedTapScriptTree, *txscript.ControlBlock, error) {
+
+	// Create the taproot OP_TRUE script.
+	tapScript, err := GetOpTrueScript()
+	if err != nil {
+		return asset.ScriptKey{}, txscript.TapLeaf{}, nil, nil, err
+	}
+
+	tapLeaf := txscript.NewBaseTapLeaf(tapScript)
+	tree := txscript.AssembleTaprootScriptTree(tapLeaf)
+	rootHash := tree.RootNode.TapHash()
+	tapKey := txscript.ComputeTaprootOutputKey(
+		asset.NUMSPubKey, rootHash[:],
+	)
+
+	merkleRootHash := tree.RootNode.TapHash()
+
+	controlBlock := &txscript.ControlBlock{
+		LeafVersion: txscript.BaseLeafVersion,
+		InternalKey: asset.NUMSPubKey,
+	}
+	tapScriptKey := asset.ScriptKey{
+		PubKey: tapKey,
+		TweakedScriptKey: &asset.TweakedScriptKey{
+			RawKey: keychain.KeyDescriptor{
+				PubKey: asset.NUMSPubKey,
+			},
+			Tweak: merkleRootHash[:],
+		},
+	}
+	if tapKey.SerializeCompressed()[0] ==
+		secp256k1.PubKeyFormatCompressedOdd {
+
+		controlBlock.OutputKeyYIsOdd = true
+	}
+
+	return tapScriptKey, tapLeaf, tree, controlBlock, nil
+}