implement adapt method for adaptor signature

Author: keithsue

crypto/adaptor/adapt.go

@@ -0,0 +1,41 @@
+package adaptor
+
+import (
+	"github.com/btcsuite/btcd/btcec/v2/schnorr"
+	"github.com/decred/dcrd/dcrec/secp256k1/v4"
+)
+
+// Adapt adapts the given adaptor signature with the specified secret
+// Asume that the given adaptor signature is valid
+func Adapt(sigBytes []byte, secretBytes []byte) []byte {
+	rPoint, _ := schnorr.ParsePubKey(sigBytes[0:32])
+	var R secp256k1.JacobianPoint
+	rPoint.AsJacobian(&R)
+
+	var s secp256k1.ModNScalar
+	s.SetByteSlice(sigBytes[32:64])
+
+	var secret secp256k1.ModNScalar
+	secret.SetByteSlice(secretBytes)
+
+	var adaptorPoint secp256k1.JacobianPoint
+	secp256k1.ScalarBaseMultNonConst(&secret, &adaptorPoint)
+
+	var adaptedR secp256k1.JacobianPoint
+	secp256k1.AddNonConst(&R, &adaptorPoint, &adaptedR)
+	adaptedR.ToAffine()
+
+	var adaptedS secp256k1.ModNScalar
+	if !adaptedR.Y.IsOdd() {
+		adaptedS = *s.Add(&secret)
+	} else {
+		adaptedS = *s.Add(secret.Negate())
+	}
+
+	adaptedSig := Signature{
+		r: adaptedR.X,
+		s: adaptedS,
+	}
+
+	return adaptedSig.Serialize()
+}

crypto/adaptor/types.go

@@ -7,7 +7,7 @@ import (
 // scalarSize is the size of an encoded big endian scalar
 const scalarSize = 32
 
-// Signature is same with schnorr.Signature
+// Signature represents the signature
 type Signature struct {
 	r btcec.FieldVal
 	s btcec.ModNScalar
@@ -28,16 +28,40 @@ func NewSignature(sigBytes []byte) *Signature {
 	}
 }
 
-// NegatePoint negates the given point
-func NegatePoint(point *btcec.JacobianPoint) *btcec.JacobianPoint {
-	result := *point
-	result.Y.Negate(1).Normalize()
+// Serialize serializes the signature
+func (s *Signature) Serialize() []byte {
+	sig := make([]byte, 64)
 
-	return &result
+	rBytes := *s.r.Bytes()
+	sBytes := s.s.Bytes()
+
+	copy(sig[0:32], rBytes[:])
+	copy(sig[32:64], sBytes[:])
+
+	return sig
 }
 
 // SerializeScalar serializes the given scalar
 func SerializeScalar(scalar *btcec.ModNScalar) []byte {
 	bz := scalar.Bytes()
 	return bz[:]
 }
+
+// SecretToPubKey gets the serialized public key of the given secret on the secp256k1 curve
+func SecretToPubKey(secretBytes []byte) []byte {
+	var secret btcec.ModNScalar
+	secret.SetByteSlice(secretBytes)
+
+	var result btcec.JacobianPoint
+	btcec.ScalarBaseMultNonConst(&secret, &result)
+
+	return btcec.JacobianToByteSlice(result)
+}
+
+// NegatePoint negates the given point
+func NegatePoint(point *btcec.JacobianPoint) *btcec.JacobianPoint {
+	result := *point
+	result.Y.Negate(1).Normalize()
+
+	return &result
+}