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