From 0a42f7750216ff7ca3aa1c0fb3fcf6a1f431cca0 Mon Sep 17 00:00:00 2001
From: Filippo Valsorda <filippo@golang.org>
Date: Thu, 31 Mar 2022 12:31:58 -0400
Subject: [Backport] [release-branch.go1.17] crypto/elliptic: tolerate
 zero-padded scalars in generic P-256

Updates #52075
Fixes #52076
Fixes CVE-2022-28327

Change-Id: I595a7514c9a0aa1b9c76aedfc2307e1124271f27
Reviewed-on: https://go-review.googlesource.com/c/go/+/397136
Trust: Filippo Valsorda <filippo@golang.org>
Reviewed-by: Julie Qiu <julie@golang.org>

Conflict: NA
Reference: https://go-review.googlesource.com/c/go/+/397136
---
 src/crypto/elliptic/elliptic_test.go | 14 ++++++++++++++
 src/crypto/elliptic/p256.go          |  2 +-
 2 files changed, 15 insertions(+), 1 deletion(-)

diff --git a/src/crypto/elliptic/elliptic_test.go b/src/crypto/elliptic/elliptic_test.go
index bb16b0d163..498f617198 100644
--- a/src/crypto/elliptic/elliptic_test.go
+++ b/src/crypto/elliptic/elliptic_test.go
@@ -802,3 +802,17 @@ func testInvalidCoordinates(t *testing.T, curve Curve) {
 		checkIsOnCurveFalse("P, y", p, yy)
 	}
 }
+
+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)
+	}
+}
diff --git a/src/crypto/elliptic/p256.go b/src/crypto/elliptic/p256.go
index c23e414156..787e3e7444 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 {
-- 
2.30.0

