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