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 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