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ed25519: fix ScalarMult and improve test to catch the bug (#1)

This commit is contained in:
George Tankersley 2019-01-28 09:32:56 -05:00
parent 982236110c
commit 968377f9c5
2 changed files with 18 additions and 25 deletions
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2
ed25519.go
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@ -125,7 +125,7 @@ func (curve ed25519Curve) ScalarMult(x1, y1 *big.Int, k []byte) (x, y *big.Int)
r0.Double(&r0)
} else {
r0.Add(&r0, &r1)
r1.Double(&r0)
r1.Double(&r1)
}
}

41
ed25519_test.go
View File

@ -104,35 +104,28 @@ func BenchmarkDouble(b *testing.B) {
func TestScalarMult(t *testing.T) {
ed := Ed25519()
x, y := ed.Params().Gx, ed.Params().Gy
Bx, By := ed.Params().Gx, ed.Params().Gy
var rX, rY, accX, accY = new(big.Int), new(big.Int), new(big.Int), new(big.Int)
twoX, twoY := ed.ScalarMult(x, y, big.NewInt(2).Bytes())
xPlusX, yPlusY := ed.Add(x, y, x, y)
for i := 1; i <= 1024; i++ {
rX, rY = ed.ScalarMult(Bx, By, big.NewInt(int64(i)).Bytes())
if i == 0 && (rX.Cmp(Bx) != 0 || rY.Cmp(By) != 0) {
t.Error("bad ScalarMul")
}
accX.Set(Bx)
accY.Set(By)
for j := 1; j < i; j++ {
accX, accY = ed.Add(accX, accY, Bx, By)
}
if !ed.IsOnCurve(twoX, twoY) {
t.Error("2*B is not on the curve")
}
if !ed.IsOnCurve(rX, rY) || !ed.IsOnCurve(accX, accY) {
t.Error("not on the curve")
}
if twoX.Cmp(xPlusX) != 0 || twoY.Cmp(yPlusY) != 0 {
t.Errorf("2*B != B+B")
}
// TODO: fuzz like it's going out of style
if !testing.Short() {
buf := make([]byte, 32)
for i := 0; i < 1000; i++ {
_, err := io.ReadFull(rand.Reader, buf)
if err != nil {
t.Fatal(err)
}
randX, randY := ed.ScalarMult(x, y, buf)
if !ed.IsOnCurve(randX, randY) {
t.Errorf("scalarMult return off-curve point for scalar %x", buf)
}
if rX.Cmp(accX) != 0 || rY.Cmp(accY) != 0 {
t.Errorf("inconsistent ScalarMult: %x", i)
}
}
}
func BenchmarkScalarMult(b *testing.B) {