unbound/compat/arc4random_uniform.c

   1 /*      $OpenBSD: arc4random_uniform.c,v 1.1 2014/07/12 13:24:54 deraadt Exp $  */
   2
   3 /*
   4  * Copyright (c) 2008, Damien Miller <djm@openbsd.org>
   5  *
   6  * Permission to use, copy, modify, and distribute this software for any
   7  * purpose with or without fee is hereby granted, provided that the above
   8  * copyright notice and this permission notice appear in all copies.
   9  *
  10  * THE SOFTWARE IS PROVIDED "AS IS" AND THE AUTHOR DISCLAIMS ALL WARRANTIES
  11  * WITH REGARD TO THIS SOFTWARE INCLUDING ALL IMPLIED WARRANTIES OF
  12  * MERCHANTABILITY AND FITNESS. IN NO EVENT SHALL THE AUTHOR BE LIABLE FOR
  13  * ANY SPECIAL, DIRECT, INDIRECT, OR CONSEQUENTIAL DAMAGES OR ANY DAMAGES
  14  * WHATSOEVER RESULTING FROM LOSS OF USE, DATA OR PROFITS, WHETHER IN AN
  15  * ACTION OF CONTRACT, NEGLIGENCE OR OTHER TORTIOUS ACTION, ARISING OUT OF
  16  * OR IN CONNECTION WITH THE USE OR PERFORMANCE OF THIS SOFTWARE.
  17  */
  18
  19 #include "config.h"
  20 #include <sys/types.h>
  21 #include <stdlib.h>
  22
  23 /*
  24  * Calculate a uniformly distributed random number less than upper_bound
  25  * avoiding "modulo bias".
  26  *
  27  * Uniformity is achieved by generating new random numbers until the one
  28  * returned is outside the range [0, 2**32 % upper_bound).  This
  29  * guarantees the selected random number will be inside
  30  * [2**32 % upper_bound, 2**32) which maps back to [0, upper_bound)
  31  * after reduction modulo upper_bound.
  32  */
  33 uint32_t
  34 arc4random_uniform(uint32_t upper_bound)
  35 {
  36         uint32_t r, min;
  37
  38         if (upper_bound < 2)
  39                 return 0;
  40
  41         /* 2**32 % x == (2**32 - x) % x */
  42         min = -upper_bound % upper_bound;
  43
  44         /*
  45          * This could theoretically loop forever but each retry has
  46          * p > 0.5 (worst case, usually far better) of selecting a
  47          * number inside the range we need, so it should rarely need
  48          * to re-roll.
  49          */
  50         for (;;) {
  51                 r = arc4random();
  52                 if (r >= min)
  53                         break;
  54         }
  55
  56         return r % upper_bound;
  57 }