1247738Sbapt/* $OpenBSD: arc4random_uniform.c,v 1.1 2014/07/12 13:24:54 deraadt Exp $ */ 2247738Sbapt 3247738Sbapt/* 4247738Sbapt * Copyright (c) 2008, Damien Miller <djm@openbsd.org> 5247738Sbapt * 6247738Sbapt * Permission to use, copy, modify, and distribute this software for any 7247738Sbapt * purpose with or without fee is hereby granted, provided that the above 8247738Sbapt * copyright notice and this permission notice appear in all copies. 9247738Sbapt * 10247738Sbapt * THE SOFTWARE IS PROVIDED "AS IS" AND THE AUTHOR DISCLAIMS ALL WARRANTIES 11247738Sbapt * WITH REGARD TO THIS SOFTWARE INCLUDING ALL IMPLIED WARRANTIES OF 12247738Sbapt * MERCHANTABILITY AND FITNESS. IN NO EVENT SHALL THE AUTHOR BE LIABLE FOR 13247738Sbapt * ANY SPECIAL, DIRECT, INDIRECT, OR CONSEQUENTIAL DAMAGES OR ANY DAMAGES 14247738Sbapt * WHATSOEVER RESULTING FROM LOSS OF USE, DATA OR PROFITS, WHETHER IN AN 15247738Sbapt * ACTION OF CONTRACT, NEGLIGENCE OR OTHER TORTIOUS ACTION, ARISING OUT OF 16247738Sbapt * OR IN CONNECTION WITH THE USE OR PERFORMANCE OF THIS SOFTWARE. 17247738Sbapt */ 18247738Sbapt 19247738Sbapt#include "config.h" 20247738Sbapt#include <sys/types.h> 21247738Sbapt#include <stdlib.h> 22247738Sbapt 23247738Sbapt/* 24247738Sbapt * Calculate a uniformly distributed random number less than upper_bound 25247738Sbapt * avoiding "modulo bias". 26247738Sbapt * 27247738Sbapt * Uniformity is achieved by generating new random numbers until the one 28247738Sbapt * returned is outside the range [0, 2**32 % upper_bound). This 29247738Sbapt * guarantees the selected random number will be inside 30247738Sbapt * [2**32 % upper_bound, 2**32) which maps back to [0, upper_bound) 31247738Sbapt * after reduction modulo upper_bound. 32247738Sbapt */ 33247738Sbaptuint32_t 34247738Sbaptarc4random_uniform(uint32_t upper_bound) 35247738Sbapt{ 36247738Sbapt uint32_t r, min; 37247738Sbapt 38247738Sbapt if (upper_bound < 2) 39247738Sbapt return 0; 40247738Sbapt 41247738Sbapt /* 2**32 % x == (2**32 - x) % x */ 42247738Sbapt min = -upper_bound % upper_bound; 43247738Sbapt 44247738Sbapt /* 45247738Sbapt * This could theoretically loop forever but each retry has 46247738Sbapt * p > 0.5 (worst case, usually far better) of selecting a 47247738Sbapt * number inside the range we need, so it should rarely need 48247738Sbapt * to re-roll. 49247738Sbapt */ 50247738Sbapt for (;;) { 51247738Sbapt r = arc4random(); 52247738Sbapt if (r >= min) 53247738Sbapt break; 54247738Sbapt } 55247738Sbapt 56247738Sbapt return r % upper_bound; 57247738Sbapt} 58247738Sbapt