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author | Christian Neukirchen <chneukirchen@gmail.com> | 2014-07-31 16:09:53 +0200 |
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committer | Christian Neukirchen <chneukirchen@gmail.com> | 2014-07-31 16:09:53 +0200 |
commit | 879d51525932a2ba2841744ed302c170b5dc8270 (patch) | |
tree | 003d31fb6c5e7e8e1654ef07247215be93dfa960 /src/liboutils/arc4random_uniform.c | |
parent | 92f91930172eec04f302898d2957e32f1d773348 (diff) | |
download | outils-879d51525932a2ba2841744ed302c170b5dc8270.tar.gz outils-879d51525932a2ba2841744ed302c170b5dc8270.tar.xz outils-879d51525932a2ba2841744ed302c170b5dc8270.zip |
add liboutils (from OpenBSD, libressl-portable, obase)
Diffstat (limited to 'src/liboutils/arc4random_uniform.c')
-rw-r--r-- | src/liboutils/arc4random_uniform.c | 57 |
1 files changed, 57 insertions, 0 deletions
diff --git a/src/liboutils/arc4random_uniform.c b/src/liboutils/arc4random_uniform.c new file mode 100644 index 0000000..e961166 --- /dev/null +++ b/src/liboutils/arc4random_uniform.c @@ -0,0 +1,57 @@ +/* $OpenBSD: arc4random.c,v 1.40 2014/07/09 16:52:09 bcook Exp $ */ + +/* + * Copyright (c) 2008, Damien Miller <djm@openbsd.org> + * + * 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 <sys/types.h> +#include <stdint.h> +#include <stdlib.h> + +/* + * 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; +} |