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crypto/elliptic: document and test that IsOnCurve(∞) == false

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This also implies it can't be passed to Marshal.

Fixes #37294

Change-Id: I1e6b6abd87ff31f323486958d5cb34a5c8f76b5f
Reviewed-on: https://go-review.googlesource.com/c/go/+/239562
Run-TryBot: Filippo Valsorda <filippo@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Katie Hockman <katie@golang.org>
This commit is contained in:
Filippo Valsorda 2020-06-23 18:14:25 -04:00
parent 9f33108dfa
commit 320e4adc4b
2 changed files with 56 additions and 30 deletions
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11
src/crypto/elliptic/elliptic.go
View File

@ -20,7 +20,10 @@ import (
)
// A Curve represents a short-form Weierstrass curve with a=-3.
// See https://www.hyperelliptic.org/EFD/g1p/auto-shortw.html
//
// Note that the point at infinity (0, 0) is not considered on the curve, and
// although it can be returned by Add, Double, ScalarMult, or ScalarBaseMult, it
// can't be marshaled or unmarshaled, and IsOnCurve will return false for it.
type Curve interface {
// Params returns the parameters for the curve.
Params() *CurveParams
@ -307,7 +310,8 @@ func GenerateKey(curve Curve, rand io.Reader) (priv []byte, x, y *big.Int, err e
return
}
// Marshal converts a point into the uncompressed form specified in section 4.3.6 of ANSI X9.62.
// Marshal converts a point on the curve into the uncompressed form specified in
// section 4.3.6 of ANSI X9.62.
func Marshal(curve Curve, x, y *big.Int) []byte {
byteLen := (curve.Params().BitSize + 7) / 8
@ -320,7 +324,8 @@ func Marshal(curve Curve, x, y *big.Int) []byte {
return ret
}
// MarshalCompressed converts a point into the compressed form specified in section 4.3.6 of ANSI X9.62.
// MarshalCompressed converts a point on the curve into the compressed form
// specified in section 4.3.6 of ANSI X9.62.
func MarshalCompressed(curve Curve, x, y *big.Int) []byte {
byteLen := (curve.Params().BitSize + 7) / 8
compressed := make([]byte, 1+byteLen)

75
src/crypto/elliptic/elliptic_test.go
View File

@ -418,41 +418,62 @@ func TestP256Mult(t *testing.T) {
}
}
func testInfinity(t *testing.T, curve Curve) {
_, x, y, _ := GenerateKey(curve, rand.Reader)
x, y = curve.ScalarMult(x, y, curve.Params().N.Bytes())
if x.Sign() != 0 || y.Sign() != 0 {
t.Errorf("x^q != ∞")
}
x, y = curve.ScalarBaseMult([]byte{0})
if x.Sign() != 0 || y.Sign() != 0 {
t.Errorf("b^0 != ∞")
x.SetInt64(0)
y.SetInt64(0)
}
x2, y2 := curve.Double(x, y)
if x2.Sign() != 0 || y2.Sign() != 0 {
t.Errorf("2∞ != ∞")
}
baseX := curve.Params().Gx
baseY := curve.Params().Gy
x3, y3 := curve.Add(baseX, baseY, x, y)
if x3.Cmp(baseX) != 0 || y3.Cmp(baseY) != 0 {
t.Errorf("x+∞ != x")
}
x4, y4 := curve.Add(x, y, baseX, baseY)
if x4.Cmp(baseX) != 0 || y4.Cmp(baseY) != 0 {
t.Errorf("∞+x != x")
}
if curve.IsOnCurve(x, y) {
t.Errorf("IsOnCurve(∞) == true")
}
}
func TestInfinity(t *testing.T) {
tests := []struct {
name string
curve Curve
}{
{"p224", P224()},
{"p256", P256()},
{"P-224", P224()},
{"P-256", P256()},
{"P-256/Generic", P256().Params()},
{"P-384", P384()},
{"P-521", P521()},
}
if testing.Short() {
tests = tests[:1]
}
for _, test := range tests {
curve := test.curve
x, y := curve.ScalarBaseMult(nil)
if x.Sign() != 0 || y.Sign() != 0 {
t.Errorf("%s: x^0 != ∞", test.name)
}
x.SetInt64(0)
y.SetInt64(0)
x2, y2 := curve.Double(x, y)
if x2.Sign() != 0 || y2.Sign() != 0 {
t.Errorf("%s: 2∞ != ∞", test.name)
}
baseX := curve.Params().Gx
baseY := curve.Params().Gy
x3, y3 := curve.Add(baseX, baseY, x, y)
if x3.Cmp(baseX) != 0 || y3.Cmp(baseY) != 0 {
t.Errorf("%s: x+∞ != x", test.name)
}
x4, y4 := curve.Add(x, y, baseX, baseY)
if x4.Cmp(baseX) != 0 || y4.Cmp(baseY) != 0 {
t.Errorf("%s: ∞+x != x", test.name)
}
t.Run(test.name, func(t *testing.T) {
testInfinity(t, curve)
})
}
}