| 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 | |
| 18 | int |
| 19 | consttime_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 | |