54 lines
2.0 KiB
C
54 lines
2.0 KiB
C
/* $OpenBSD: timingsafe_memcmp.c,v 1.1 2014/06/13 02:12:17 matthew Exp $ */
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/*
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* Copyright (c) 2014 Google Inc.
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*
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* Permission to use, copy, modify, and distribute this software for any
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* purpose with or without fee is hereby granted, provided that the above
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* 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 WARRANTIES
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* WITH REGARD TO THIS SOFTWARE INCLUDING ALL IMPLIED WARRANTIES OF
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* MERCHANTABILITY AND FITNESS. IN NO EVENT SHALL THE AUTHOR BE LIABLE FOR
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* ANY SPECIAL, DIRECT, INDIRECT, OR CONSEQUENTIAL DAMAGES OR ANY DAMAGES
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* WHATSOEVER RESULTING FROM LOSS OF USE, DATA OR PROFITS, WHETHER IN AN
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* ACTION OF CONTRACT, NEGLIGENCE OR OTHER TORTIOUS ACTION, ARISING OUT OF
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* OR IN CONNECTION WITH THE USE OR PERFORMANCE OF THIS SOFTWARE.
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*/
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#include <limits.h>
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#include <string.h>
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// TODO: I don't fully trust that this code is _always_ timing safe in the
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// presence of very smart compilers that can prove that this computes the
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// same value as memcmp and could be short-circuited. We should invent and
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// rely on compiler extensions that inform the compiler that sensitive
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// information is involved, which forbids the compiler code generation
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// from branching on the information (and such).
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// TODO: We should add testcases that verify this is actually timing safe.
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int timingsafe_memcmp(const void* a_ptr, const void* b_ptr, size_t size)
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{
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const unsigned char* a = (const unsigned char*) a_ptr;
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const unsigned char* b = (const unsigned char*) b_ptr;
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int result = 0;
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int done = 0;
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for ( size_t i = 0; i < size; i++ )
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{
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/* lt is -1 if a[i] < b[i]; else 0. */
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int lt = (a[i] - b[i]) >> CHAR_BIT;
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/* gt is -1 if a[i] > b[i]; else 0. */
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int gt = (b[i] - a[i]) >> CHAR_BIT;
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/* cmp is 1 if a[i] > b[i]; -1 if a[i] < b[i]; else 0. */
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int cmp = lt - gt;
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/* set result = cmp if !done. */
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result |= cmp & ~done;
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/* set done if a[i] != b[i]. */
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done |= lt | gt;
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}
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return result;
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}
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