1 /* $OpenBSD: arc4random_uniform.c,v 1.2 2015/09/13 08:31:47 guenther Exp $ */
4 * SPDX-License-Identifier: ISC
6 * Copyright (c) 2008, Damien Miller <djm@openbsd.org>
8 * Permission to use, copy, modify, and distribute this software for any
9 * purpose with or without fee is hereby granted, provided that the above
10 * copyright notice and this permission notice appear in all copies.
12 * THE SOFTWARE IS PROVIDED "AS IS" AND THE AUTHOR DISCLAIMS ALL WARRANTIES
13 * WITH REGARD TO THIS SOFTWARE INCLUDING ALL IMPLIED WARRANTIES OF
14 * MERCHANTABILITY AND FITNESS. IN NO EVENT SHALL THE AUTHOR BE LIABLE FOR
15 * ANY SPECIAL, DIRECT, INDIRECT, OR CONSEQUENTIAL DAMAGES OR ANY DAMAGES
16 * WHATSOEVER RESULTING FROM LOSS OF USE, DATA OR PROFITS, WHETHER IN AN
17 * ACTION OF CONTRACT, NEGLIGENCE OR OTHER TORTIOUS ACTION, ARISING OUT OF
18 * OR IN CONNECTION WITH THE USE OR PERFORMANCE OF THIS SOFTWARE.
22 * This is an open source non-commercial project. Dear PVS-Studio, please check it.
23 * PVS-Studio Static Code Analyzer for C, C++ and C#: http://www.viva64.com
28 #ifndef HAVE_ARC4RANDOM_UNIFORM
30 #include <sys/types.h>
32 #if defined(HAVE_STDINT_H)
34 #elif defined(HAVE_INTTYPES_H)
35 # include <inttypes.h>
38 #include "sudo_compat.h"
39 #include "sudo_rand.h"
42 * Calculate a uniformly distributed random number less than upper_bound
43 * avoiding "modulo bias".
45 * Uniformity is achieved by generating new random numbers until the one
46 * returned is outside the range [0, 2**32 % upper_bound). This
47 * guarantees the selected random number will be inside
48 * [2**32 % upper_bound, 2**32) which maps back to [0, upper_bound)
49 * after reduction modulo upper_bound.
52 sudo_arc4random_uniform(uint32_t upper_bound)
59 /* 2**32 % x == (2**32 - x) % x */
60 min = -upper_bound % upper_bound;
63 * This could theoretically loop forever but each retry has
64 * p > 0.5 (worst case, usually far better) of selecting a
65 * number inside the range we need, so it should rarely need
74 return r % upper_bound;
77 #endif /* HAVE_ARC4RANDOM_UNIFORM */