From e2abcdca396661cbe0ae2ddb13d5c2b85682c13a Mon Sep 17 00:00:00 2001 From: Cem Keylan Date: Fri, 16 Oct 2020 17:41:25 +0300 Subject: initial commit --- lib/libc/crypt/arc4random_uniform.c | 57 +++++++++++++++++++++++++++++++++++++ 1 file changed, 57 insertions(+) create mode 100644 lib/libc/crypt/arc4random_uniform.c (limited to 'lib/libc/crypt/arc4random_uniform.c') diff --git a/lib/libc/crypt/arc4random_uniform.c b/lib/libc/crypt/arc4random_uniform.c new file mode 100644 index 0000000..a18b5b1 --- /dev/null +++ b/lib/libc/crypt/arc4random_uniform.c @@ -0,0 +1,57 @@ +/* $OpenBSD: arc4random_uniform.c,v 1.3 2019/01/20 02:59:07 bcook Exp $ */ + +/* + * Copyright (c) 2008, Damien Miller + * + * Permission to use, copy, modify, and distribute this software for any + * purpose with or without fee is hereby granted, provided that the above + * copyright notice and this permission notice appear in all copies. + * + * THE SOFTWARE IS PROVIDED "AS IS" AND THE AUTHOR DISCLAIMS ALL WARRANTIES + * WITH REGARD TO THIS SOFTWARE INCLUDING ALL IMPLIED WARRANTIES OF + * MERCHANTABILITY AND FITNESS. IN NO EVENT SHALL THE AUTHOR BE LIABLE FOR + * ANY SPECIAL, DIRECT, INDIRECT, OR CONSEQUENTIAL DAMAGES OR ANY DAMAGES + * WHATSOEVER RESULTING FROM LOSS OF USE, DATA OR PROFITS, WHETHER IN AN + * ACTION OF CONTRACT, NEGLIGENCE OR OTHER TORTIOUS ACTION, ARISING OUT OF + * OR IN CONNECTION WITH THE USE OR PERFORMANCE OF THIS SOFTWARE. + */ + +#include +#include + +/* + * Calculate a uniformly distributed random number less than upper_bound + * avoiding "modulo bias". + * + * Uniformity is achieved by generating new random numbers until the one + * returned is outside the range [0, 2**32 % upper_bound). This + * guarantees the selected random number will be inside + * [2**32 % upper_bound, 2**32) which maps back to [0, upper_bound) + * after reduction modulo upper_bound. + */ +uint32_t +arc4random_uniform(uint32_t upper_bound) +{ + uint32_t r, min; + + if (upper_bound < 2) + return 0; + + /* 2**32 % x == (2**32 - x) % x */ + min = -upper_bound % upper_bound; + + /* + * This could theoretically loop forever but each retry has + * p > 0.5 (worst case, usually far better) of selecting a + * number inside the range we need, so it should rarely need + * to re-roll. + */ + for (;;) { + r = arc4random(); + if (r >= min) + break; + } + + return r % upper_bound; +} +DEF_WEAK(arc4random_uniform); -- cgit v1.2.3