1*9d12795fSRobert Mustacchi /* $OpenBSD: arc4random_uniform.c,v 1.1 2014/07/12 13:24:54 deraadt Exp $ */
2*9d12795fSRobert Mustacchi
3*9d12795fSRobert Mustacchi /*
4*9d12795fSRobert Mustacchi * Copyright (c) 2008, Damien Miller <djm@openbsd.org>
5*9d12795fSRobert Mustacchi *
6*9d12795fSRobert Mustacchi * Permission to use, copy, modify, and distribute this software for any
7*9d12795fSRobert Mustacchi * purpose with or without fee is hereby granted, provided that the above
8*9d12795fSRobert Mustacchi * copyright notice and this permission notice appear in all copies.
9*9d12795fSRobert Mustacchi *
10*9d12795fSRobert Mustacchi * THE SOFTWARE IS PROVIDED "AS IS" AND THE AUTHOR DISCLAIMS ALL WARRANTIES
11*9d12795fSRobert Mustacchi * WITH REGARD TO THIS SOFTWARE INCLUDING ALL IMPLIED WARRANTIES OF
12*9d12795fSRobert Mustacchi * MERCHANTABILITY AND FITNESS. IN NO EVENT SHALL THE AUTHOR BE LIABLE FOR
13*9d12795fSRobert Mustacchi * ANY SPECIAL, DIRECT, INDIRECT, OR CONSEQUENTIAL DAMAGES OR ANY DAMAGES
14*9d12795fSRobert Mustacchi * WHATSOEVER RESULTING FROM LOSS OF USE, DATA OR PROFITS, WHETHER IN AN
15*9d12795fSRobert Mustacchi * ACTION OF CONTRACT, NEGLIGENCE OR OTHER TORTIOUS ACTION, ARISING OUT OF
16*9d12795fSRobert Mustacchi * OR IN CONNECTION WITH THE USE OR PERFORMANCE OF THIS SOFTWARE.
17*9d12795fSRobert Mustacchi */
18*9d12795fSRobert Mustacchi
19*9d12795fSRobert Mustacchi #include <sys/types.h>
20*9d12795fSRobert Mustacchi #include <stdlib.h>
21*9d12795fSRobert Mustacchi
22*9d12795fSRobert Mustacchi /*
23*9d12795fSRobert Mustacchi * Calculate a uniformly distributed random number less than upper_bound
24*9d12795fSRobert Mustacchi * avoiding "modulo bias".
25*9d12795fSRobert Mustacchi *
26*9d12795fSRobert Mustacchi * Uniformity is achieved by generating new random numbers until the one
27*9d12795fSRobert Mustacchi * returned is outside the range [0, 2**32 % upper_bound). This
28*9d12795fSRobert Mustacchi * guarantees the selected random number will be inside
29*9d12795fSRobert Mustacchi * [2**32 % upper_bound, 2**32) which maps back to [0, upper_bound)
30*9d12795fSRobert Mustacchi * after reduction modulo upper_bound.
31*9d12795fSRobert Mustacchi */
32*9d12795fSRobert Mustacchi uint32_t
arc4random_uniform(uint32_t upper_bound)33*9d12795fSRobert Mustacchi arc4random_uniform(uint32_t upper_bound)
34*9d12795fSRobert Mustacchi {
35*9d12795fSRobert Mustacchi uint32_t r, min;
36*9d12795fSRobert Mustacchi
37*9d12795fSRobert Mustacchi if (upper_bound < 2)
38*9d12795fSRobert Mustacchi return 0;
39*9d12795fSRobert Mustacchi
40*9d12795fSRobert Mustacchi /* 2**32 % x == (2**32 - x) % x */
41*9d12795fSRobert Mustacchi min = -upper_bound % upper_bound;
42*9d12795fSRobert Mustacchi
43*9d12795fSRobert Mustacchi /*
44*9d12795fSRobert Mustacchi * This could theoretically loop forever but each retry has
45*9d12795fSRobert Mustacchi * p > 0.5 (worst case, usually far better) of selecting a
46*9d12795fSRobert Mustacchi * number inside the range we need, so it should rarely need
47*9d12795fSRobert Mustacchi * to re-roll.
48*9d12795fSRobert Mustacchi */
49*9d12795fSRobert Mustacchi for (;;) {
50*9d12795fSRobert Mustacchi r = arc4random();
51*9d12795fSRobert Mustacchi if (r >= min)
52*9d12795fSRobert Mustacchi break;
53*9d12795fSRobert Mustacchi }
54*9d12795fSRobert Mustacchi
55*9d12795fSRobert Mustacchi return r % upper_bound;
56*9d12795fSRobert Mustacchi }
57