From a11cada73596ce5b67e460ae5259d227f349e83c Mon Sep 17 00:00:00 2001
From: Mounir IDRASSI
Date: Sun, 5 Aug 2018 16:18:23 +0200
Subject: crypto: cleaner code for Streebog carry bit handling and add comment
about missing handling of overflow caused by carry bit.
---
src/Crypto/Streebog.c | 29 ++++++++++++++++++++++++-----
1 file changed, 24 insertions(+), 5 deletions(-)
diff --git a/src/Crypto/Streebog.c b/src/Crypto/Streebog.c
index 6c52ce75..e443ecae 100644
--- a/src/Crypto/Streebog.c
+++ b/src/Crypto/Streebog.c
@@ -1845,21 +1845,40 @@ add512(const unsigned long long *x, const unsigned long long *y, unsigned long l
{
#ifndef __GOST3411_BIG_ENDIAN__
unsigned int CF, OF;
+ unsigned long long tmp;
unsigned int i;
CF = 0;
for (i = 0; i < 8; i++)
{
- r[i] = x[i] + y[i];
- if ( (r[i] < y[i]) ||
- (r[i] < x[i]) )
+ /* Detecting integer overflow condition for three numbers
+ * in a portable way is tricky a little. */
+
+ /* Step 1: numbers cause overflow */
+ tmp = x[i] + y[i];
+
+ /* Compare with any of two summands, no need to check both */
+ if (tmp < x[i])
OF = 1;
else
OF = 0;
- r[i] += CF;
+ /* Step 2: carry bit causes overflow */
+ tmp += CF;
+
+ /*
+ * We don't include the carry bit overflow since it can break
+ * mounting for some containers eventhough the probability of
+ * such case is very low
+ */
+ /*
+ if (CF > 0 && tmp == 0)
+ OF = 1;
+ */
CF = OF;
- }
+
+ r[i] = tmp;
+ }
#else
const unsigned char *xp, *yp;
unsigned char *rp;
--
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