common/lib/libc/string/consttime_memequal.c

   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 }