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 /* OPENBSD ORIGINAL: lib/libc/crypto/arc4random_uniform.c */ 20 21 #include "includes.h" 22 23 #ifdef HAVE_STDINT_H 24 # include <stdint.h> 25 #endif 26 #include <stdlib.h> 27 28 #ifndef HAVE_ARC4RANDOM_UNIFORM 29 /* 30 * Calculate a uniformly distributed random number less than upper_bound 31 * avoiding "modulo bias". 32 * 33 * Uniformity is achieved by generating new random numbers until the one 34 * returned is outside the range [0, 2**32 % upper_bound). This 35 * guarantees the selected random number will be inside 36 * [2**32 % upper_bound, 2**32) which maps back to [0, upper_bound) 37 * after reduction modulo upper_bound. 38 */ 39 uint32_t 40 arc4random_uniform(uint32_t upper_bound) 41 { 42 uint32_t r, min; 43 44 if (upper_bound < 2) 45 return 0; 46 47 /* 2**32 % x == (2**32 - x) % x */ 48 min = -upper_bound % upper_bound; 49 50 /* 51 * This could theoretically loop forever but each retry has 52 * p > 0.5 (worst case, usually far better) of selecting a 53 * number inside the range we need, so it should rarely need 54 * to re-roll. 55 */ 56 for (;;) { 57 r = arc4random(); 58 if (r >= min) 59 break; 60 } 61 62 return r % upper_bound; 63 } 64 #endif /* !HAVE_ARC4RANDOM_UNIFORM */ 65