1 // SPDX-License-Identifier: ISC 2 /* 3 * Copyright (C) 2019 Linaro Limited 4 * Copyright (C) 2004-2007, 2011, 2012 Internet Systems Consortium, Inc. ("ISC") 5 * Copyright (C) 1999-2001, 2003 Internet Software Consortium. 6 * 7 * Permission to use, copy, modify, and/or distribute this software for any 8 * purpose with or without fee is hereby granted, provided that the above 9 * copyright notice and this permission notice appear in all copies. 10 * 11 * THE SOFTWARE IS PROVIDED "AS IS" AND ISC DISCLAIMS ALL WARRANTIES WITH 12 * REGARD TO THIS SOFTWARE INCLUDING ALL IMPLIED WARRANTIES OF MERCHANTABILITY 13 * AND FITNESS. IN NO EVENT SHALL ISC BE LIABLE FOR ANY SPECIAL, DIRECT, 14 * INDIRECT, OR CONSEQUENTIAL DAMAGES OR ANY DAMAGES WHATSOEVER RESULTING FROM 15 * LOSS OF USE, DATA OR PROFITS, WHETHER IN AN ACTION OF CONTRACT, NEGLIGENCE 16 * OR OTHER TORTIOUS ACTION, ARISING OUT OF OR IN CONNECTION WITH THE USE OR 17 * PERFORMANCE OF THIS SOFTWARE. 18 */ 19 20 #include <limits.h> 21 #include <string_ext.h> 22 23 /* Making a portable memcmp that has no internal branches and loops always 24 * once for every byte without early-out shortcut has a few challenges. 25 * 26 * Inspired by 'timingsafe_memcmp()' from the BSD system and 27 * https://github.com/libressl-portable/openbsd/blob/master/src/lib/libc/string/timingsafe_memcmp.c 28 * 29 * Sadly, that one is not portable C: It makes assumptions on the representation 30 * of negative integers and assumes sign-preserving right-shift of negative 31 * signed values. This is a rewrite from scratch that should not suffer from 32 * such issues. 33 * 34 * 2015-12-12, J. Perlinger (perlinger-at-ntp-dot-org) 35 */ 36 int consttime_memcmp(const void *p1, const void *p2, size_t nb) { 37 const unsigned char *ucp1 = p1; 38 const unsigned char *ucp2 = p2; 39 unsigned int isLT = 0u; 40 unsigned int isGT = 0u; 41 volatile unsigned int mask = (1u << CHAR_BIT); 42 43 for (/*NOP*/; 0 != nb; --nb, ++ucp1, ++ucp2) { 44 isLT |= mask & 45 ((unsigned int)*ucp1 - (unsigned int)*ucp2); 46 isGT |= mask & 47 ((unsigned int)*ucp2 - (unsigned int)*ucp1); 48 mask &= ~(isLT | isGT); 49 } 50 return (int)(isGT >> CHAR_BIT) - (int)(isLT >> CHAR_BIT); 51 } 52