From eba5b622a3ad6c48a28a09d15ca32cdfac91f91b Mon Sep 17 00:00:00 2001
From: tb <>
Date: Fri, 4 Dec 2020 08:55:30 +0000
Subject: Move point-on-curve check to set_affine_coordinates

Bad API design makes it possible to set an EC_KEY public key to
a point not on the curve. As a consequence, it was possible to
have bogus ECDSA signatures validated. In practice, all software
uses either EC_POINT_oct2point*() to unmarshal public keys or
issues a call to EC_KEY_check_key() after setting it. This way,
a point on curve check is performed and the problem is mitigated.

In OpenSSL commit 1e2012b7ff4a5f12273446b281775faa5c8a1858, Emilia
Kasper moved the point-on-curve check from EC_POINT_oct2point to
EC_POINT_set_affine_coordinates_*, which results in more checking.
In addition to this commit, we also check in the currently unused
codepath of a user set callback for setting compressed coordinates,
just in case this will be used at some point in the future.

The documentation of EC_KEY_check_key() is very vague on what it
checks and when checks are needed.  It could certainly be improved
a lot. It's also strange that EC_KEY_set_key() performs no checks,
while EC_KEY_set_public_key_affine_coordinates() implicitly calls
EC_KEY_check_key().

It's a mess.

Issue found and reported by Guido Vranken who also tested an earlier
version of this fix.

ok jsing
---
 src/lib/libcrypto/ec/ecp_oct.c | 15 +++++++++------
 1 file changed, 9 insertions(+), 6 deletions(-)

(limited to 'src/lib/libcrypto/ec/ecp_oct.c')

diff --git a/src/lib/libcrypto/ec/ecp_oct.c b/src/lib/libcrypto/ec/ecp_oct.c
index 90c5ca2e4e..29d9990546 100644
--- a/src/lib/libcrypto/ec/ecp_oct.c
+++ b/src/lib/libcrypto/ec/ecp_oct.c
@@ -1,4 +1,4 @@
-/* $OpenBSD: ecp_oct.c,v 1.11 2018/07/15 16:27:39 tb Exp $ */
+/* $OpenBSD: ecp_oct.c,v 1.12 2020/12/04 08:55:30 tb Exp $ */
 /* Includes code written by Lenka Fibikova <fibikova@exp-math.uni-essen.de>
  * for the OpenSSL project.
  * Includes code written by Bodo Moeller for the OpenSSL project.
@@ -362,6 +362,10 @@ ec_GFp_simple_oct2point(const EC_GROUP * group, EC_POINT * point,
 		goto err;
 	}
 	if (form == POINT_CONVERSION_COMPRESSED) {
+		/*
+		 * EC_POINT_set_compressed_coordinates_GFp checks that the point
+		 * is on the curve as required by X9.62.
+		 */
 		if (!EC_POINT_set_compressed_coordinates_GFp(group, point, x, y_bit, ctx))
 			goto err;
 	} else {
@@ -377,15 +381,14 @@ ec_GFp_simple_oct2point(const EC_GROUP * group, EC_POINT * point,
 				goto err;
 			}
 		}
+		/*
+		 * EC_POINT_set_affine_coordinates_GFp checks that the point is
+		 * on the curve as required by X9.62.
+		 */
 		if (!EC_POINT_set_affine_coordinates_GFp(group, point, x, y, ctx))
 			goto err;
 	}
 
-	/* test required by X9.62 */
-	if (EC_POINT_is_on_curve(group, point, ctx) <= 0) {
-		ECerror(EC_R_POINT_IS_NOT_ON_CURVE);
-		goto err;
-	}
 	ret = 1;
 
  err:
-- 
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