1 /*
2 * Written by Matthias Drochner <drochner@NetBSD.org>.
3 * Public domain.
4 */
5
6 #ifndef CONSTTIME_MEMEQUAL_H
7 #define CONSTTIME_MEMEQUAL_H
8 inline static int
consttime_memequal(const void * b1,const void * b2,size_t len)9 consttime_memequal(const void *b1, const void *b2, size_t len)
10 {
11 const unsigned char *c1 = b1, *c2 = b2;
12 unsigned int res = 0;
13
14 while (len--)
15 res |= *c1++ ^ *c2++;
16
17 /*
18 * Map 0 to 1 and [1, 256) to 0 using only constant-time
19 * arithmetic.
20 *
21 * This is not simply `!res' because although many CPUs support
22 * branchless conditional moves and many compilers will take
23 * advantage of them, certain compilers generate branches on
24 * certain CPUs for `!res'.
25 */
26 return (1 & ((res - 1) >> 8));
27 }
28 #endif /* CONSTTIME_MEMEQUAL_H */
29