1*b7da54b3SJerome Forissier // SPDX-License-Identifier: ISC
2*b7da54b3SJerome Forissier /*
3*b7da54b3SJerome Forissier * Copyright (C) 2019 Linaro Limited
4*b7da54b3SJerome Forissier * Copyright (C) 2004-2007, 2011, 2012 Internet Systems Consortium, Inc. ("ISC")
5*b7da54b3SJerome Forissier * Copyright (C) 1999-2001, 2003 Internet Software Consortium.
6*b7da54b3SJerome Forissier *
7*b7da54b3SJerome Forissier * Permission to use, copy, modify, and/or distribute this software for any
8*b7da54b3SJerome Forissier * purpose with or without fee is hereby granted, provided that the above
9*b7da54b3SJerome Forissier * copyright notice and this permission notice appear in all copies.
10*b7da54b3SJerome Forissier *
11*b7da54b3SJerome Forissier * THE SOFTWARE IS PROVIDED "AS IS" AND ISC DISCLAIMS ALL WARRANTIES WITH
12*b7da54b3SJerome Forissier * REGARD TO THIS SOFTWARE INCLUDING ALL IMPLIED WARRANTIES OF MERCHANTABILITY
13*b7da54b3SJerome Forissier * AND FITNESS. IN NO EVENT SHALL ISC BE LIABLE FOR ANY SPECIAL, DIRECT,
14*b7da54b3SJerome Forissier * INDIRECT, OR CONSEQUENTIAL DAMAGES OR ANY DAMAGES WHATSOEVER RESULTING FROM
15*b7da54b3SJerome Forissier * LOSS OF USE, DATA OR PROFITS, WHETHER IN AN ACTION OF CONTRACT, NEGLIGENCE
16*b7da54b3SJerome Forissier * OR OTHER TORTIOUS ACTION, ARISING OUT OF OR IN CONNECTION WITH THE USE OR
17*b7da54b3SJerome Forissier * PERFORMANCE OF THIS SOFTWARE.
18*b7da54b3SJerome Forissier */
19*b7da54b3SJerome Forissier
20*b7da54b3SJerome Forissier #include <limits.h>
21*b7da54b3SJerome Forissier #include <string_ext.h>
22*b7da54b3SJerome Forissier
23*b7da54b3SJerome Forissier /* Making a portable memcmp that has no internal branches and loops always
24*b7da54b3SJerome Forissier * once for every byte without early-out shortcut has a few challenges.
25*b7da54b3SJerome Forissier *
26*b7da54b3SJerome Forissier * Inspired by 'timingsafe_memcmp()' from the BSD system and
27*b7da54b3SJerome Forissier * https://github.com/libressl-portable/openbsd/blob/master/src/lib/libc/string/timingsafe_memcmp.c
28*b7da54b3SJerome Forissier *
29*b7da54b3SJerome Forissier * Sadly, that one is not portable C: It makes assumptions on the representation
30*b7da54b3SJerome Forissier * of negative integers and assumes sign-preserving right-shift of negative
31*b7da54b3SJerome Forissier * signed values. This is a rewrite from scratch that should not suffer from
32*b7da54b3SJerome Forissier * such issues.
33*b7da54b3SJerome Forissier *
34*b7da54b3SJerome Forissier * 2015-12-12, J. Perlinger (perlinger-at-ntp-dot-org)
35*b7da54b3SJerome Forissier */
consttime_memcmp(const void * p1,const void * p2,size_t nb)36*b7da54b3SJerome Forissier int consttime_memcmp(const void *p1, const void *p2, size_t nb) {
37*b7da54b3SJerome Forissier const unsigned char *ucp1 = p1;
38*b7da54b3SJerome Forissier const unsigned char *ucp2 = p2;
39*b7da54b3SJerome Forissier unsigned int isLT = 0u;
40*b7da54b3SJerome Forissier unsigned int isGT = 0u;
41*b7da54b3SJerome Forissier volatile unsigned int mask = (1u << CHAR_BIT);
42*b7da54b3SJerome Forissier
43*b7da54b3SJerome Forissier for (/*NOP*/; 0 != nb; --nb, ++ucp1, ++ucp2) {
44*b7da54b3SJerome Forissier isLT |= mask &
45*b7da54b3SJerome Forissier ((unsigned int)*ucp1 - (unsigned int)*ucp2);
46*b7da54b3SJerome Forissier isGT |= mask &
47*b7da54b3SJerome Forissier ((unsigned int)*ucp2 - (unsigned int)*ucp1);
48*b7da54b3SJerome Forissier mask &= ~(isLT | isGT);
49*b7da54b3SJerome Forissier }
50*b7da54b3SJerome Forissier return (int)(isGT >> CHAR_BIT) - (int)(isLT >> CHAR_BIT);
51*b7da54b3SJerome Forissier }
52