1 /* $OpenBSD: arc4random_uniform.c,v 1.3 2019/01/20 02:59:07 bcook 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 * $FreeBSD$
19 */
20
21 #include <stdint.h>
22 #include <stdlib.h>
23
24 /*
25 * Calculate a uniformly distributed random number less than upper_bound
26 * avoiding "modulo bias".
27 *
28 * Uniformity is achieved by generating new random numbers until the one
29 * returned is outside the range [0, 2**32 % upper_bound). This
30 * guarantees the selected random number will be inside
31 * [2**32 % upper_bound, 2**32) which maps back to [0, upper_bound)
32 * after reduction modulo upper_bound.
33 */
34 uint32_t
arc4random_uniform(uint32_t upper_bound)35 arc4random_uniform(uint32_t upper_bound)
36 {
37 uint32_t r, min;
38
39 if (upper_bound < 2)
40 return 0;
41
42 /* 2**32 % x == (2**32 - x) % x */
43 min = -upper_bound % upper_bound;
44
45 /*
46 * This could theoretically loop forever but each retry has
47 * p > 0.5 (worst case, usually far better) of selecting a
48 * number inside the range we need, so it should rarely need
49 * to re-roll.
50 */
51 for (;;) {
52 r = arc4random();
53 if (r >= min)
54 break;
55 }
56
57 return r % upper_bound;
58 }
59