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(-) (limited to 'src/Crypto') 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; -- cgit v1.2.3