mirror of
https://github.com/jart/cosmopolitan.git
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53 lines
2.9 KiB
C
53 lines
2.9 KiB
C
/*-*- mode:c;indent-tabs-mode:nil;c-basic-offset:2;tab-width:8;coding:utf-8 -*-│
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│vi: set net ft=c ts=2 sts=2 sw=2 fenc=utf-8 :vi│
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╞══════════════════════════════════════════════════════════════════════════════╡
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│ Copyright 2014 Google Inc. │
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│ │
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│ Permission to use, copy, modify, and/or distribute this software for │
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│ any purpose with or without fee is hereby granted, provided that the │
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│ above copyright notice and this permission notice appear in all copies. │
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│ │
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│ THE SOFTWARE IS PROVIDED "AS IS" AND THE AUTHOR DISCLAIMS ALL │
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│ WARRANTIES WITH REGARD TO THIS SOFTWARE INCLUDING ALL IMPLIED │
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│ WARRANTIES OF MERCHANTABILITY AND FITNESS. IN NO EVENT SHALL THE │
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│ AUTHOR BE LIABLE FOR ANY SPECIAL, DIRECT, INDIRECT, OR CONSEQUENTIAL │
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│ DAMAGES OR ANY DAMAGES WHATSOEVER RESULTING FROM LOSS OF USE, DATA OR │
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│ PROFITS, WHETHER IN AN ACTION OF CONTRACT, NEGLIGENCE OR OTHER │
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│ TORTIOUS ACTION, ARISING OUT OF OR IN CONNECTION WITH THE USE OR │
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│ PERFORMANCE OF THIS SOFTWARE. │
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╚─────────────────────────────────────────────────────────────────────────────*/
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#include "libc/str/str.h"
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asm(".ident\t\"\\n\\n\
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timingsafe_memcmp (ISC License)\\n\
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Copyright 2014 Google Inc.\"");
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asm(".include \"libc/disclaimer.inc\"");
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/**
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* Lexicographically compares the first len bytes in b1 and b2.
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*
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* Running time is independent of the byte sequences compared, making
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* this safe to use for comparing secret values such as cryptographic
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* MACs. In contrast, memcmp() may short-circuit afterw finding the
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* first differing byte.
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*
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* @note each byte is interpreted as unsigned char
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*/
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int timingsafe_memcmp(const void *b1, const void *b2, size_t len) {
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const unsigned char *p1 = b1, *p2 = b2;
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size_t i;
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int res = 0, done = 0;
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for (i = 0; i < len; i++) {
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/* lt is -1 if p1[i] < p2[i]; else 0. */
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int lt = (p1[i] - p2[i]) >> CHAR_BIT;
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/* gt is -1 if p1[i] > p2[i]; else 0. */
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int gt = (p2[i] - p1[i]) >> CHAR_BIT;
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/* cmp is 1 if p1[i] > p2[i]; -1 if p1[i] < p2[i]; else 0. */
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int cmp = lt - gt;
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/* set res = cmp if !done. */
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res |= cmp & ~done;
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/* set done if p1[i] != p2[i]. */
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done |= lt | gt;
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}
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return res;
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}
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