timingsafe_memcmp.c revision 030cabe0 
1/*
2 * Copyright (c) 2014 Google Inc.
3 *
4 * Permission to use, copy, modify, and distribute this software for any
5 * purpose with or without fee is hereby granted, provided that the above
6 * copyright notice and this permission notice appear in all copies.
7 *
8 * THE SOFTWARE IS PROVIDED "AS IS" AND THE AUTHOR DISCLAIMS ALL WARRANTIES
9 * WITH REGARD TO THIS SOFTWARE INCLUDING ALL IMPLIED WARRANTIES OF
10 * MERCHANTABILITY AND FITNESS. IN NO EVENT SHALL THE AUTHOR BE LIABLE FOR
11 * ANY SPECIAL, DIRECT, INDIRECT, OR CONSEQUENTIAL DAMAGES OR ANY DAMAGES
12 * WHATSOEVER RESULTING FROM LOSS OF USE, DATA OR PROFITS, WHETHER IN AN
13 * ACTION OF CONTRACT, NEGLIGENCE OR OTHER TORTIOUS ACTION, ARISING OUT OF
14 * OR IN CONNECTION WITH THE USE OR PERFORMANCE OF THIS SOFTWARE.
15 */
16
17#include <limits.h>
18#include <string.h>
19#include <X11/Xfuncproto.h>
20#include <dix-config.h>
21#include "os.h"
22
23int
24timingsafe_memcmp(const void *b1, const void *b2, size_t len)
25{
26        const unsigned char *p1 = b1, *p2 = b2;
27        size_t i;
28        int res = 0, done = 0;
29
30        for (i = 0; i < len; i++) {
31                /* lt is -1 if p1[i] < p2[i]; else 0. */
32                int lt = (p1[i] - p2[i]) >> CHAR_BIT;
33
34                /* gt is -1 if p1[i] > p2[i]; else 0. */
35                int gt = (p2[i] - p1[i]) >> CHAR_BIT;
36
37                /* cmp is 1 if p1[i] > p2[i]; -1 if p1[i] < p2[i]; else 0. */
38                int cmp = lt - gt;
39
40                /* set res = cmp if !done. */
41                res |= cmp & ~done;
42
43                /* set done if p1[i] != p2[i]. */
44                done |= lt | gt;
45        }
46
47        return (res);
48}
49