FourQ ecc_mul_double implementation

ecc/fourq/curve.go

@@ -81,6 +81,65 @@ func (P *Point) ScalarBaseMult(k *[Size]byte) {
 	P.fromR1(&_P)
 }
 
+// ScalarMultCofactor calculates P = k*Q, where Q is an N-torsion point. Allows for not clearing the cofactor.
+func (P *Point) ScalarMultCofactor(k *[Size]byte, Q *Point, clearCofactor bool) {
+	var _P, _Q pointR1
+	Q.toR1(&_Q)
+
+	if clearCofactor {
+		_Q.ClearCofactor()
+	}
+	_P.ScalarMult(k, &_Q)
+	P.fromR1(&_P)
+}
+
+// DoubleScalarMult calculates P = k*G + l*Q, where the G is the generator.
+func (P *Point) DoubleScalarMult(k *[Size]byte, Q *Point, l *[Size]byte) {
+
+	//Based on FourQLib's no endomorphism ecc_mul_double:
+
+	/*
+		point_t A;
+		point_extproj_t T;
+		point_extproj_precomp_t S;
+
+		if (ecc_mul(Q, l, A, false) == false) {
+			return false;
+		}
+		point_setup(A, T);
+		R1_to_R2(T, S);
+
+		ecc_mul_fixed(k, A);
+		point_setup(A, T);
+		eccadd(S, T);
+		eccnorm(T, R);
+	*/
+
+	var _A Point
+	var _T pointR1
+	var _S pointR2
+
+	// _A = q * l, don't clear cofactor
+	_A.ScalarMultCofactor(l, Q, false)
+
+	// _A -> _T
+	_A.toR1(&_T)
+
+	// _T -> _S
+	_S.FromR1(&_T)
+
+	// _A = k * G
+	_A.ScalarBaseMult(k)
+
+	// _A -> _T
+	_A.toR1(&_T)
+
+	// _T = _T * _S
+	_T.add(&_S)
+
+	P.fromR1(&_T)
+}
+
 func (P *Point) fromR1(Q *pointR1) {
 	Q.ToAffine()
 	P.X = Q.X

ecc/fourq/point_test.go

@@ -2,6 +2,7 @@ package fourq
 
 import (
 	"crypto/rand"
+	"encoding/hex"
 	"math/big"
 	"testing"
 
@@ -123,6 +124,41 @@ func TestScalarMult(t *testing.T) {
 	})
 }
 
+func TestDoubleScalarMult(t *testing.T) {
+
+	want := "8cd1b92a5ee686eed483b2fc691a202ba09566e416a8774a2fca098075f2e774"
+
+	var k = [32]byte{
+		0x09, 0x58, 0x2b, 0xb4, 0x34, 0x9a, 0xfd, 0x1c,
+		0x23, 0xa8, 0xba, 0x06, 0x19, 0x62, 0xfb, 0x34,
+		0x35, 0x77, 0xe6, 0x77, 0x1e, 0x31, 0x5c, 0x21,
+		0x81, 0x98, 0x87, 0x01, 0x6d, 0xdb, 0x84, 0x27,
+	}
+
+	var l = [32]byte{
+		0xd1, 0x68, 0x56, 0xdc, 0x3d, 0xad, 0x5d, 0x45,
+		0x6c, 0x7b, 0x5e, 0x51, 0x17, 0x22, 0xb7, 0xa6,
+		0x90, 0x4b, 0x55, 0x76, 0x54, 0xbb, 0x2c, 0xca,
+		0x33, 0xe8, 0x3e, 0x77, 0xc5, 0x34, 0xb4, 0xff,
+	}
+
+	P := Point{}
+	P.SetGenerator()
+
+	P.DoubleScalarMult(&k, &P, &l)
+
+	encoded := [32]byte{}
+
+	P.Marshal(&encoded)
+
+	got := hex.EncodeToString(encoded[:])
+
+	if got != want {
+		test.ReportError(t, got, want, P)
+	}
+
+}
+
 func TestScalar(t *testing.T) {
 	const testTimes = 1 << 12
 	var xx [5]uint64