From: tb <tb@openbsd.org>
Date: Sun, 26 Mar 2023 18:49:48 +0000 (+0000)
Subject: Correctly reduce negative inpot to BN_mod_exp2_mont()
X-Git-Url: http://artulab.com/gitweb/?a=commitdiff_plain;h=416d67b76dc50ff6a462f904cb1e1d436a5be76b;p=openbsd

Correctly reduce negative inpot to BN_mod_exp2_mont()

Negative bases could result in a negative modulus being returned. This is
not strictly speaking incorrect but slightly surprising. This is all a
consequence of the shortcut of defining BN_mod() as a macro using BN_div().

Fixes ossfuzz #55997

ok jsing
---

diff --git a/lib/libcrypto/bn/bn_exp.c b/lib/libcrypto/bn/bn_exp.c
index 9abf574b576..ba9b2700f19 100644
--- a/lib/libcrypto/bn/bn_exp.c
+++ b/lib/libcrypto/bn/bn_exp.c
@@ -1,4 +1,4 @@
-/* $OpenBSD: bn_exp.c,v 1.38 2023/03/15 04:30:20 jsing Exp $ */
+/* $OpenBSD: bn_exp.c,v 1.39 2023/03/26 18:49:48 tb Exp $ */
 /* Copyright (C) 1995-1998 Eric Young (eay@cryptsoft.com)
  * All rights reserved.
  *
@@ -1206,7 +1206,7 @@ BN_mod_exp2_mont(BIGNUM *rr, const BIGNUM *a1, const BIGNUM *p1,
 	 * Build table for a1:   val1[i] := a1^(2*i + 1) mod m  for i = 0 .. 2^(window1-1)
 	 */
 	if (a1->neg || BN_ucmp(a1, m) >= 0) {
-		if (!BN_mod_ct(val1[0], a1, m, ctx))
+		if (!BN_nnmod(val1[0], a1, m, ctx))
 			goto err;
 		a_mod_m = val1[0];
 	} else
@@ -1237,7 +1237,7 @@ BN_mod_exp2_mont(BIGNUM *rr, const BIGNUM *a1, const BIGNUM *p1,
 	 * Build table for a2:   val2[i] := a2^(2*i + 1) mod m  for i = 0 .. 2^(window2-1)
 	 */
 	if (a2->neg || BN_ucmp(a2, m) >= 0) {
-		if (!BN_mod_ct(val2[0], a2, m, ctx))
+		if (!BN_nnmod(val2[0], a2, m, ctx))
 			goto err;
 		a_mod_m = val2[0];
 	} else