timingsafe_memcmp.c (1621B)
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 <dix-config.h> 18 #include <limits.h> 19 #include <string.h> 20 #include <X11/Xfuncproto.h> 21 #include "os.h" 22 23 int 24 timingsafe_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 }