From e16331902340c02bdf1831b5508df2307b871ef6 Mon Sep 17 00:00:00 2001 From: Filippo Valsorda Date: Wed, 2 Feb 2022 09:14:57 -0800 Subject: [release-branch.go1.17] crypto/elliptic: make IsOnCurve return false for invalid field elements Updates #50974 Fixes #50978 Fixes CVE-2022-23806 Change-Id: I0201c2c88f13dd82910985a495973f1683af9259 Reviewed-on: https://go-review.googlesource.com/c/go/+/382854 Trust: Filippo Valsorda Run-TryBot: Filippo Valsorda Reviewed-by: Katie Hockman Trust: Katie Hockman TryBot-Result: Gopher Robot --- src/crypto/elliptic/elliptic.go | 5 ++++ src/crypto/elliptic/elliptic_test.go | 55 ++++++++++++++++++++++++++++++++++++ src/crypto/elliptic/p224.go | 5 ++++ src/crypto/elliptic/p521.go | 5 ++++ 4 files changed, 70 insertions(+) diff --git a/src/crypto/elliptic/elliptic.go b/src/crypto/elliptic/elliptic.go index f072960bfed..b84339ec1c2 100644 --- a/src/crypto/elliptic/elliptic.go +++ b/src/crypto/elliptic/elliptic.go @@ -86,6 +86,11 @@ func (curve *CurveParams) IsOnCurve(x, y *big.Int) bool { return specific.IsOnCurve(x, y) } + if x.Sign() < 0 || x.Cmp(curve.P) >= 0 || + y.Sign() < 0 || y.Cmp(curve.P) >= 0 { + return false + } + // y² = x³ - 3x + b y2 := new(big.Int).Mul(y, y) y2.Mod(y2, curve.P) diff --git a/src/crypto/elliptic/elliptic_test.go b/src/crypto/elliptic/elliptic_test.go index 183861a54b5..3fe53c5f332 100644 --- a/src/crypto/elliptic/elliptic_test.go +++ b/src/crypto/elliptic/elliptic_test.go @@ -174,6 +174,61 @@ func testUnmarshalToLargeCoordinates(t *testing.T, curve Curve) { } } +// TestInvalidCoordinates tests big.Int values that are not valid field elements +// (negative or bigger than P). They are expected to return false from +// IsOnCurve, all other behavior is undefined. +func TestInvalidCoordinates(t *testing.T) { + testAllCurves(t, testInvalidCoordinates) +} + +func testInvalidCoordinates(t *testing.T, curve Curve) { + checkIsOnCurveFalse := func(name string, x, y *big.Int) { + if curve.IsOnCurve(x, y) { + t.Errorf("IsOnCurve(%s) unexpectedly returned true", name) + } + } + + p := curve.Params().P + _, x, y, _ := GenerateKey(curve, rand.Reader) + xx, yy := new(big.Int), new(big.Int) + + // Check if the sign is getting dropped. + xx.Neg(x) + checkIsOnCurveFalse("-x, y", xx, y) + yy.Neg(y) + checkIsOnCurveFalse("x, -y", x, yy) + + // Check if negative values are reduced modulo P. + xx.Sub(x, p) + checkIsOnCurveFalse("x-P, y", xx, y) + yy.Sub(y, p) + checkIsOnCurveFalse("x, y-P", x, yy) + + // Check if positive values are reduced modulo P. + xx.Add(x, p) + checkIsOnCurveFalse("x+P, y", xx, y) + yy.Add(y, p) + checkIsOnCurveFalse("x, y+P", x, yy) + + // Check if the overflow is dropped. + xx.Add(x, new(big.Int).Lsh(big.NewInt(1), 535)) + checkIsOnCurveFalse("x+2⁵³⁵, y", xx, y) + yy.Add(y, new(big.Int).Lsh(big.NewInt(1), 535)) + checkIsOnCurveFalse("x, y+2⁵³⁵", x, yy) + + // Check if P is treated like zero (if possible). + // y^2 = x^3 - 3x + B + // y = mod_sqrt(x^3 - 3x + B) + // y = mod_sqrt(B) if x = 0 + // If there is no modsqrt, there is no point with x = 0, can't test x = P. + if yy := new(big.Int).ModSqrt(curve.Params().B, p); yy != nil { + if !curve.IsOnCurve(big.NewInt(0), yy) { + t.Fatal("(0, mod_sqrt(B)) is not on the curve?") + } + checkIsOnCurveFalse("P, y", p, yy) + } +} + func TestMarshalCompressed(t *testing.T) { t.Run("P-256/03", func(t *testing.T) { data, _ := hex.DecodeString("031e3987d9f9ea9d7dd7155a56a86b2009e1e0ab332f962d10d8beb6406ab1ad79") diff --git a/src/crypto/elliptic/p224.go b/src/crypto/elliptic/p224.go index 8c760214642..ff5c8344522 100644 --- a/src/crypto/elliptic/p224.go +++ b/src/crypto/elliptic/p224.go @@ -48,6 +48,11 @@ func (curve p224Curve) Params() *CurveParams { } func (curve p224Curve) IsOnCurve(bigX, bigY *big.Int) bool { + if bigX.Sign() < 0 || bigX.Cmp(curve.P) >= 0 || + bigY.Sign() < 0 || bigY.Cmp(curve.P) >= 0 { + return false + } + var x, y p224FieldElement p224FromBig(&x, bigX) p224FromBig(&y, bigY) diff --git a/src/crypto/elliptic/p521.go b/src/crypto/elliptic/p521.go index 3d355943ec7..587991e31bf 100644 --- a/src/crypto/elliptic/p521.go +++ b/src/crypto/elliptic/p521.go @@ -32,6 +32,11 @@ func (curve p521Curve) Params() *CurveParams { } func (curve p521Curve) IsOnCurve(x, y *big.Int) bool { + if x.Sign() < 0 || x.Cmp(curve.P) >= 0 || + y.Sign() < 0 || y.Cmp(curve.P) >= 0 { + return false + } + x1 := bigIntToFiatP521(x) y1 := bigIntToFiatP521(y) b := bigIntToFiatP521(curve.B) // TODO: precompute this value. -- cgit v1.2.3-54-g00ecf