newlib/libc/string/timingsafe_memcmp.c

   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
  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 }