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annotate libtomcrypt/src/misc/mem_neq.c @ 1524:d35cf9a5e0b5

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rename default_options.h.in in docs too
author Matt Johnston <matt@ucc.asn.au>
date Tue, 20 Feb 2018 19:29:55 +0800
parents 6dba84798cd5
children
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1471
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1 /* LibTomCrypt, modular cryptographic library -- Tom St Denis
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2 *
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3 * LibTomCrypt is a library that provides various cryptographic
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4 * algorithms in a highly modular and flexible manner.
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5 *
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6 * The library is free for all purposes without any express
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7 * guarantee it works.
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8 */
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9 #include "tomcrypt.h"
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10
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11 /**
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12 @file mem_neq.c
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13 Compare two blocks of memory for inequality in constant time.
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14 Steffen Jaeckel
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15 */
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16
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17 /**
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18 Compare two blocks of memory for inequality in constant time.
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19
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20 The usage is similar to that of standard memcmp, but you can only test
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21 if the memory is equal or not - you can not determine by how much the
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22 first different byte differs.
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23
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24 This function shall be used to compare results of cryptographic
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25 operations where inequality means most likely usage of a wrong key.
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26 The execution time has therefore to be constant as otherwise
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27 timing attacks could be possible.
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28
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29 @param a The first memory region
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30 @param b The second memory region
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31 @param len The length of the area to compare (octets)
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32
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33 @return 0 when a and b are equal for len bytes, 1 they are not equal.
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34 */
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35 int mem_neq(const void *a, const void *b, size_t len)
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36 {
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37 unsigned char ret = 0;
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38 const unsigned char* pa;
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39 const unsigned char* pb;
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40
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41 LTC_ARGCHK(a != NULL);
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42 LTC_ARGCHK(b != NULL);
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43
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44 pa = a;
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45 pb = b;
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46
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47 while (len-- > 0) {
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48 ret |= *pa ^ *pb;
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49 ++pa;
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50 ++pb;
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51 }
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52
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53 ret |= ret >> 4;
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54 ret |= ret >> 2;
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55 ret |= ret >> 1;
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56 ret &= 1;
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57
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58 return ret;
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59 }
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60
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61 /* ref: $Format:%D$ */
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62 /* git commit: $Format:%H$ */
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63 /* commit time: $Format:%ai$ */