1 /* $OpenBSD: arc4random_uniform.c,v 1.2 2015/09/13 08:31:47 guenther 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 #define _DEFAULT_SOURCE
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
arc4random_uniform(uint32_t upper_bound)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 }
58
