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 */
33uint32_t
34arc4random_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