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 */ 33 uint32_t 34 arc4random_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