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Add timingsafe_memcmp #13

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This commit is contained in:
Justine Tunney 2020-12-28 20:01:15 -08:00
parent 0e85b136ae
commit 1df136323b
2 changed files with 54 additions and 0 deletions
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1
libc/str/str.h
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@ -174,6 +174,7 @@ char16_t *memset16(char16_t *, char16_t, size_t) memcpyesque;
compatfn wchar_t *wmemcpy(wchar_t *, const wchar_t *, size_t) memcpyesque;
compatfn wchar_t *wmempcpy(wchar_t *, const wchar_t *, size_t) memcpyesque;
compatfn wchar_t *wmemmove(wchar_t *, const wchar_t *, size_t) memcpyesque;
int timingsafe_memcmp(const void *, const void *, size_t);
char *tinystrstr(const char *, const char *) strlenesque;
char16_t *tinystrstr16(const char16_t *, const char16_t *) strlenesque;

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