1/* $NetBSD: consttime_memequal.c,v 1.6 2015/03/18 20:11:35 riastradh Exp $ */
2
3/*
4 * Written by Matthias Drochner <drochner@NetBSD.org>.
5 * Public domain.
6 */
7
8#if !defined(_KERNEL) && !defined(_STANDALONE)
9#include "namespace.h"
10#include <string.h>
11#ifdef __weak_alias
12__weak_alias(consttime_memequal,_consttime_memequal)
13#endif
14#else
15#include <lib/libkern/libkern.h>
16#endif
17
18int
19consttime_memequal(const void *b1, const void *b2, size_t len)
20{
21 const unsigned char *c1 = b1, *c2 = b2;
22 unsigned int res = 0;
23
24 while (len--)
25 res |= *c1++ ^ *c2++;
26
27 /*
28 * Map 0 to 1 and [1, 256) to 0 using only constant-time
29 * arithmetic.
30 *
31 * This is not simply `!res' because although many CPUs support
32 * branchless conditional moves and many compilers will take
33 * advantage of them, certain compilers generate branches on
34 * certain CPUs for `!res'.
35 */
36 return (1 & ((res - 1) >> 8));
37}
38