From b709d39aa5989305f448f72ed25dc1fbce9a365a Mon Sep 17 00:00:00 2001
From: Moritz Lenz <moritz@faui2k3.org>
Date: Mon, 7 Oct 2013 17:35:26 -0500
Subject: [PATCH] libtommath: Fix mp_mod(a, b, c) if b < 0 and a = n * b, n
 integer

it used to return b, now it return 0.

From https://github.com/libtom/libtommath

Change-Id: I99f5d42c09a6fbddfb32b7dd0ed10ef04f1c08a5
---
 lib/hcrypto/libtommath/bn_mp_mod.c | 8 ++++----
 1 file changed, 4 insertions(+), 4 deletions(-)

diff --git a/lib/hcrypto/libtommath/bn_mp_mod.c b/lib/hcrypto/libtommath/bn_mp_mod.c
index f5cf8d09f..274e7dcd1 100644
--- a/lib/hcrypto/libtommath/bn_mp_mod.c
+++ b/lib/hcrypto/libtommath/bn_mp_mod.c
@@ -15,7 +15,7 @@
  * Tom St Denis, tomstdenis@gmail.com, http://libtom.org
  */
 
-/* c = a mod b, 0 <= c < b */
+/* c = a mod b, 0 <= c < b if b > 0, b < c <= 0 if b < 0 */
 int
 mp_mod (mp_int * a, mp_int * b, mp_int * c)
 {
@@ -31,11 +31,11 @@ mp_mod (mp_int * a, mp_int * b, mp_int * c)
     return res;
   }
 
-  if (t.sign != b->sign) {
-    res = mp_add (b, &t, c);
-  } else {
+  if (mp_iszero(&t) || t.sign == b.sign) {
     res = MP_OKAY;
     mp_exch (&t, c);
+  } else {
+    res = mp_add (b, &t, c);
   }
 
   mp_clear (&t);