hyperloop: add hyperloop script

Author: sputn1ck

hyperloop/script.go

@@ -0,0 +1,119 @@
+package hyperloop
+
+import (
+	"fmt"
+
+	"github.com/btcsuite/btcd/btcec/v2"
+	"github.com/btcsuite/btcd/btcec/v2/schnorr"
+	"github.com/btcsuite/btcd/txscript"
+	"github.com/lightningnetwork/lnd/input"
+)
+
+const (
+
+	// TaprootMultiSigWitnessSize evaluates to 66 bytes:
+	//	- num_witness_elements: 1 byte
+	//	- sig_varint_len: 1 byte
+	//	- <sig>: 64 bytes
+	TaprootMultiSigWitnessSize = 1 + 1 + 64
+
+	// TaprootExpiryScriptSize evaluates to 39 bytes:
+	//	- OP_DATA: 1 byte (trader_key length)
+	//	- <trader_key>: 32 bytes
+	//	- OP_CHECKSIGVERIFY: 1 byte
+	//	- <reservation_expiry>: 4 bytes
+	//	- OP_CHECKLOCKTIMEVERIFY: 1 byte
+	TaprootExpiryScriptSize = 1 + 32 + 1 + 4 + 1
+
+	// TaprootExpiryWitnessSize evaluates to 140 bytes:
+	//	- num_witness_elements: 1 byte
+	//	- trader_sig_varint_len: 1 byte (trader_sig length)
+	//	- <trader_sig>: 64 bytes
+	//	- witness_script_varint_len: 1 byte (script length)
+	//	- <witness_script>: 39 bytes
+	//	- control_block_varint_len: 1 byte (control block length)
+	//	- <control_block>: 33 bytes
+	TaprootExpiryWitnessSize = 1 + 1 + 64 + 1 + TaprootExpiryScriptSize + 1 + 33
+)
+
+func HyperLoopScript(expiry uint32, serverKey *btcec.PublicKey,
+	clientKeys []*btcec.PublicKey) ([]byte, error) {
+
+	taprootKey, _, _, err := TaprootKey(expiry, serverKey, clientKeys)
+	if err != nil {
+		return nil, err
+	}
+	return PayToWitnessTaprootScript(taprootKey)
+}
+
+// TaprootKey returns the aggregated MuSig2 combined internal key and the
+// tweaked Taproot key of an hyperloop output, as well as the expiry script tap
+// leaf.
+func TaprootKey(expiry uint32, serverKey *btcec.PublicKey,
+	clientKeys []*btcec.PublicKey) (*btcec.PublicKey, *btcec.PublicKey,
+	*txscript.TapLeaf, error) {
+
+	expiryLeaf, err := TaprootExpiryScript(expiry, serverKey)
+	if err != nil {
+		return nil, nil, nil, err
+	}
+
+	rootHash := expiryLeaf.TapHash()
+
+	aggregateKey, err := input.MuSig2CombineKeys(
+		input.MuSig2Version100RC2,
+		clientKeys,
+		true,
+		&input.MuSig2Tweaks{
+			TaprootTweak: rootHash[:],
+		},
+	)
+	if err != nil {
+		return nil, nil, nil, fmt.Errorf("error combining keys: %v", err)
+	}
+
+	return aggregateKey.FinalKey, aggregateKey.PreTweakedKey, expiryLeaf, nil
+}
+
+// PayToWitnessTaprootScript creates a new script to pay to a version 1
+// (taproot) witness program. The passed hash is expected to be valid.
+func PayToWitnessTaprootScript(taprootKey *btcec.PublicKey) ([]byte, error) {
+	builder := txscript.NewScriptBuilder()
+
+	builder.AddOp(txscript.OP_1)
+	builder.AddData(schnorr.SerializePubKey(taprootKey))
+
+	return builder.Script()
+}
+
+// TaprootExpiryScript returns the leaf script of the expiry script path.
+//
+// <server_key> OP_CHECKSIGVERIFY <hyperloop_expiry> OP_CHECKSEQUENCEVERIFY.
+func TaprootExpiryScript(expiry uint32,
+	serverKey *btcec.PublicKey) (*txscript.TapLeaf, error) {
+
+	builder := txscript.NewScriptBuilder()
+
+	builder.AddData(schnorr.SerializePubKey(serverKey))
+	builder.AddOp(txscript.OP_CHECKSIGVERIFY)
+
+	builder.AddInt64(int64(expiry))
+	builder.AddOp(txscript.OP_CHECKSEQUENCEVERIFY)
+
+	script, err := builder.Script()
+	if err != nil {
+		return nil, err
+	}
+
+	leaf := txscript.NewBaseTapLeaf(script)
+	return &leaf, nil
+}
+
+func ExpirySpendWeight() int64 {
+	var weightEstimator input.TxWeightEstimator
+	weightEstimator.AddWitnessInput(TaprootExpiryWitnessSize)
+
+	weightEstimator.AddP2TROutput()
+
+	return int64(weightEstimator.Weight())
+}