1 /* $OpenBSD: arc4random_uniform.c,v 1.3 2019/01/20 02:59:07 bcook 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 <sys/libkern.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 */ 32 uint32_t 33 arc4random_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