add extract method for adaptor signature

Author: keithsue

crypto/adaptor/extract.go

@@ -0,0 +1,44 @@
+package adaptor
+
+import (
+	"github.com/btcsuite/btcd/btcec/v2/schnorr"
+	"github.com/decred/dcrd/dcrec/secp256k1/v4"
+)
+
+// Extract extracts the secret from the given adapted signature and adaptor signature
+func Extract(adaptedSigBytes []byte, adaptorSigBytes []byte) []byte {
+	adaptedR, err := schnorr.ParsePubKey(adaptedSigBytes[0:32])
+	if err != nil {
+		return nil
+	}
+
+	adaptorR, err := schnorr.ParsePubKey(adaptorSigBytes[0:32])
+	if err != nil {
+		return nil
+	}
+
+	var adaptedRPoint, adaptorRPoint secp256k1.JacobianPoint
+	adaptedR.AsJacobian(&adaptedRPoint)
+	adaptorR.AsJacobian(&adaptorRPoint)
+
+	var rPointSub, rPointAdd secp256k1.JacobianPoint
+	secp256k1.AddNonConst(&adaptedRPoint, NegatePoint(&adaptorRPoint), &rPointSub)
+	secp256k1.AddNonConst(&adaptedRPoint, &adaptorRPoint, &rPointAdd)
+
+	adaptedSig := NewSignature(adaptedSigBytes)
+	adaptorSig := NewSignature(adaptorSigBytes)
+
+	t := adaptedSig.s.Add(adaptorSig.s.Negate())
+
+	var T secp256k1.JacobianPoint
+	secp256k1.ScalarBaseMultNonConst(t, &T)
+
+	switch T {
+	case rPointSub:
+		return SerializeScalar(t)
+	case rPointAdd:
+		return SerializeScalar(t.Negate())
+	default:
+		return nil
+	}
+}

crypto/adaptor/types.go

@@ -14,16 +14,30 @@ type Signature struct {
 }
 
 // NewSignature creates a new Signature from bytes
-// Assume that the given byte slice is valid
+// Assume that the given signature is valid
 func NewSignature(sigBytes []byte) *Signature {
 	var r btcec.FieldVal
-	_ = r.SetByteSlice(sigBytes[0:32])
+	r.SetByteSlice(sigBytes[0:32])
 
 	var s btcec.ModNScalar
-	_ = s.SetByteSlice(sigBytes[32:])
+	s.SetByteSlice(sigBytes[32:])
 
 	return &Signature{
 		r,
 		s,
 	}
 }
+
+// NegatePoint negates the given point
+func NegatePoint(point *btcec.JacobianPoint) *btcec.JacobianPoint {
+	result := *point
+	result.Y.Negate(1).Normalize()
+
+	return &result
+}
+
+// SerializeScalar serializes the given scalar
+func SerializeScalar(scalar *btcec.ModNScalar) []byte {
+	bz := scalar.Bytes()
+	return bz[:]
+}