From 37065847d87df92b5eb246c88ba2085efcf0b331 Mon Sep 17 00:00:00 2001 From: Filippo Valsorda Date: Thu, 31 Mar 2022 12:31:58 -0400 Subject: [PATCH] crypto/elliptic: tolerate zero-padded scalars in generic P-256 Fixes #52075 Change-Id: I595a7514c9a0aa1b9c76aedfc2307e1124271f27 Reviewed-on: https://go-review.googlesource.com/c/go/+/397135 Trust: Filippo Valsorda Run-TryBot: Filippo Valsorda TryBot-Result: Gopher Robot Reviewed-by: Roland Shoemaker --- src/crypto/elliptic/p256.go | 2 +- src/crypto/elliptic/p256_test.go | 14 ++++++++++++++ 2 files changed, 15 insertions(+), 1 deletion(-) diff --git a/src/crypto/elliptic/p256.go b/src/crypto/elliptic/p256.go index e1c6ff4f87..99c39ea9d5 100644 --- a/src/crypto/elliptic/p256.go +++ b/src/crypto/elliptic/p256.go @@ -51,7 +51,7 @@ func p256GetScalar(out *[32]byte, in []byte) { n := new(big.Int).SetBytes(in) var scalarBytes []byte - if n.Cmp(p256Params.N) >= 0 { + if n.Cmp(p256Params.N) >= 0 || len(in) > len(out) { n.Mod(n, p256Params.N) scalarBytes = n.Bytes() } else { diff --git a/src/crypto/elliptic/p256_test.go b/src/crypto/elliptic/p256_test.go index c6862d9547..a607766bc6 100644 --- a/src/crypto/elliptic/p256_test.go +++ b/src/crypto/elliptic/p256_test.go @@ -136,3 +136,17 @@ func TestP256CombinedMult(t *testing.T) { t.Errorf("1×G + (-1)×G = (%d, %d), should be ∞", x, y) } } + +func TestIssue52075(t *testing.T) { + Gx, Gy := P256().Params().Gx, P256().Params().Gy + scalar := make([]byte, 33) + scalar[32] = 1 + x, y := P256().ScalarBaseMult(scalar) + if x.Cmp(Gx) != 0 || y.Cmp(Gy) != 0 { + t.Errorf("unexpected output (%v,%v)", x, y) + } + x, y = P256().ScalarMult(Gx, Gy, scalar) + if x.Cmp(Gx) != 0 || y.Cmp(Gy) != 0 { + t.Errorf("unexpected output (%v,%v)", x, y) + } +}