1d25a1430SXin LI /* $OpenBSD: arc4random_uniform.c,v 1.3 2019/01/20 02:59:07 bcook Exp $ */ 26b97c2e3SConrad Meyer 36b97c2e3SConrad Meyer /* 46b97c2e3SConrad Meyer * Copyright (c) 2008, Damien Miller <djm@openbsd.org> 56b97c2e3SConrad Meyer * 66b97c2e3SConrad Meyer * Permission to use, copy, modify, and distribute this software for any 76b97c2e3SConrad Meyer * purpose with or without fee is hereby granted, provided that the above 86b97c2e3SConrad Meyer * copyright notice and this permission notice appear in all copies. 96b97c2e3SConrad Meyer * 106b97c2e3SConrad Meyer * THE SOFTWARE IS PROVIDED "AS IS" AND THE AUTHOR DISCLAIMS ALL WARRANTIES 116b97c2e3SConrad Meyer * WITH REGARD TO THIS SOFTWARE INCLUDING ALL IMPLIED WARRANTIES OF 126b97c2e3SConrad Meyer * MERCHANTABILITY AND FITNESS. IN NO EVENT SHALL THE AUTHOR BE LIABLE FOR 136b97c2e3SConrad Meyer * ANY SPECIAL, DIRECT, INDIRECT, OR CONSEQUENTIAL DAMAGES OR ANY DAMAGES 146b97c2e3SConrad Meyer * WHATSOEVER RESULTING FROM LOSS OF USE, DATA OR PROFITS, WHETHER IN AN 156b97c2e3SConrad Meyer * ACTION OF CONTRACT, NEGLIGENCE OR OTHER TORTIOUS ACTION, ARISING OUT OF 166b97c2e3SConrad Meyer * OR IN CONNECTION WITH THE USE OR PERFORMANCE OF THIS SOFTWARE. 176b97c2e3SConrad Meyer * 186b97c2e3SConrad Meyer * $FreeBSD$ 196b97c2e3SConrad Meyer */ 206b97c2e3SConrad Meyer 21d25a1430SXin LI #include <stdint.h> 226b97c2e3SConrad Meyer #include <stdlib.h> 236b97c2e3SConrad Meyer 246b97c2e3SConrad Meyer /* 256b97c2e3SConrad Meyer * Calculate a uniformly distributed random number less than upper_bound 266b97c2e3SConrad Meyer * avoiding "modulo bias". 276b97c2e3SConrad Meyer * 286b97c2e3SConrad Meyer * Uniformity is achieved by generating new random numbers until the one 296b97c2e3SConrad Meyer * returned is outside the range [0, 2**32 % upper_bound). This 306b97c2e3SConrad Meyer * guarantees the selected random number will be inside 316b97c2e3SConrad Meyer * [2**32 % upper_bound, 2**32) which maps back to [0, upper_bound) 326b97c2e3SConrad Meyer * after reduction modulo upper_bound. 336b97c2e3SConrad Meyer */ 346b97c2e3SConrad Meyer uint32_t 356b97c2e3SConrad Meyer arc4random_uniform(uint32_t upper_bound) 366b97c2e3SConrad Meyer { 376b97c2e3SConrad Meyer uint32_t r, min; 386b97c2e3SConrad Meyer 396b97c2e3SConrad Meyer if (upper_bound < 2) 406b97c2e3SConrad Meyer return 0; 416b97c2e3SConrad Meyer 426b97c2e3SConrad Meyer /* 2**32 % x == (2**32 - x) % x */ 436b97c2e3SConrad Meyer min = -upper_bound % upper_bound; 446b97c2e3SConrad Meyer 456b97c2e3SConrad Meyer /* 466b97c2e3SConrad Meyer * This could theoretically loop forever but each retry has 476b97c2e3SConrad Meyer * p > 0.5 (worst case, usually far better) of selecting a 486b97c2e3SConrad Meyer * number inside the range we need, so it should rarely need 496b97c2e3SConrad Meyer * to re-roll. 506b97c2e3SConrad Meyer */ 516b97c2e3SConrad Meyer for (;;) { 526b97c2e3SConrad Meyer r = arc4random(); 536b97c2e3SConrad Meyer if (r >= min) 546b97c2e3SConrad Meyer break; 556b97c2e3SConrad Meyer } 566b97c2e3SConrad Meyer 576b97c2e3SConrad Meyer return r % upper_bound; 586b97c2e3SConrad Meyer } 59