1 /* $OpenBSD: timingsafe_memcmp.c,v 1.1 2014/06/13 02:12:17 matthew Exp $ */
2 /*
3 * Copyright (c) 2014 Google Inc.
4 *
5 * Permission to use, copy, modify, and distribute this software for any
6 * purpose with or without fee is hereby granted, provided that the above
7 * copyright notice and this permission notice appear in all copies.
8 *
9 * THE SOFTWARE IS PROVIDED "AS IS" AND THE AUTHOR DISCLAIMS ALL WARRANTIES
10 * WITH REGARD TO THIS SOFTWARE INCLUDING ALL IMPLIED WARRANTIES OF
11 * MERCHANTABILITY AND FITNESS. IN NO EVENT SHALL THE AUTHOR BE LIABLE FOR
12 * ANY SPECIAL, DIRECT, INDIRECT, OR CONSEQUENTIAL DAMAGES OR ANY DAMAGES
13 * WHATSOEVER RESULTING FROM LOSS OF USE, DATA OR PROFITS, WHETHER IN AN
14 * ACTION OF CONTRACT, NEGLIGENCE OR OTHER TORTIOUS ACTION, ARISING OUT OF
15 * OR IN CONNECTION WITH THE USE OR PERFORMANCE OF THIS SOFTWARE.
16 */
17
18 #include <limits.h>
19 #include <string.h>
20
21 int
timingsafe_memcmp(const void * b1,const void * b2,size_t len)22 timingsafe_memcmp(const void *b1, const void *b2, size_t len)
23 {
24 const unsigned char *p1 = b1, *p2 = b2;
25 size_t i;
26 int res = 0, done = 0;
27
28 for (i = 0; i < len; i++) {
29 /* lt is -1 if p1[i] < p2[i]; else 0. */
30 int lt = (p1[i] - p2[i]) >> CHAR_BIT;
31
32 /* gt is -1 if p1[i] > p2[i]; else 0. */
33 int gt = (p2[i] - p1[i]) >> CHAR_BIT;
34
35 /* cmp is 1 if p1[i] > p2[i]; -1 if p1[i] < p2[i]; else 0. */
36 int cmp = lt - gt;
37
38 /* set res = cmp if !done. */
39 res |= cmp & ~done;
40
41 /* set done if p1[i] != p2[i]. */
42 done |= lt | gt;
43 }
44
45 return (res);
46 }
47
