/libfido2/fuzz/uniform_random.c
| Line | Count | Source | 
| 1 |  | /* | 
| 2 |  |  * Copyright (c) 2008, Damien Miller <djm@openbsd.org> | 
| 3 |  |  * | 
| 4 |  |  * Permission to use, copy, modify, and distribute this software for any | 
| 5 |  |  * purpose with or without fee is hereby granted, provided that the above | 
| 6 |  |  * copyright notice and this permission notice appear in all copies. | 
| 7 |  |  * | 
| 8 |  |  * THE SOFTWARE IS PROVIDED "AS IS" AND THE AUTHOR DISCLAIMS ALL WARRANTIES | 
| 9 |  |  * WITH REGARD TO THIS SOFTWARE INCLUDING ALL IMPLIED WARRANTIES OF | 
| 10 |  |  * MERCHANTABILITY AND FITNESS. IN NO EVENT SHALL THE AUTHOR BE LIABLE FOR | 
| 11 |  |  * ANY SPECIAL, DIRECT, INDIRECT, OR CONSEQUENTIAL DAMAGES OR ANY DAMAGES | 
| 12 |  |  * WHATSOEVER RESULTING FROM LOSS OF USE, DATA OR PROFITS, WHETHER IN AN | 
| 13 |  |  * ACTION OF CONTRACT, NEGLIGENCE OR OTHER TORTIOUS ACTION, ARISING OUT OF | 
| 14 |  |  * OR IN CONNECTION WITH THE USE OR PERFORMANCE OF THIS SOFTWARE. | 
| 15 |  |  */ | 
| 16 |  |  | 
| 17 |  | #include <stdint.h> | 
| 18 |  | #include <stdlib.h> | 
| 19 |  |  | 
| 20 |  | uint32_t uniform_random(uint32_t); | 
| 21 |  | unsigned long prng_uint32(void); | 
| 22 |  |  | 
| 23 |  | /* | 
| 24 |  |  * Calculate a uniformly distributed random number less than upper_bound | 
| 25 |  |  * avoiding "modulo bias". | 
| 26 |  |  * | 
| 27 |  |  * Uniformity is achieved by generating new random numbers until the one | 
| 28 |  |  * returned is outside the range [0, 2**32 % upper_bound).  This | 
| 29 |  |  * guarantees the selected random number will be inside | 
| 30 |  |  * [2**32 % upper_bound, 2**32) which maps back to [0, upper_bound) | 
| 31 |  |  * after reduction modulo upper_bound. | 
| 32 |  |  */ | 
| 33 |  | uint32_t | 
| 34 |  | uniform_random(uint32_t upper_bound) | 
| 35 | 109M | { | 
| 36 | 109M |         uint32_t r, min; | 
| 37 |  |  | 
| 38 | 109M |         if (upper_bound < 2) | 
| 39 | 0 |                 return 0; | 
| 40 |  |  | 
| 41 |  |         /* 2**32 % x == (2**32 - x) % x */ | 
| 42 | 109M |         min = -upper_bound % upper_bound; | 
| 43 |  |  | 
| 44 |  |         /* | 
| 45 |  |          * This could theoretically loop forever but each retry has | 
| 46 |  |          * p > 0.5 (worst case, usually far better) of selecting a | 
| 47 |  |          * number inside the range we need, so it should rarely need | 
| 48 |  |          * to re-roll. | 
| 49 |  |          */ | 
| 50 | 109M |         for (;;) { | 
| 51 | 109M |                 r = (uint32_t)prng_uint32(); | 
| 52 | 109M |                 if (r >= min) | 
| 53 | 109M |                         break; | 
| 54 | 109M |         } | 
| 55 |  |  | 
| 56 | 109M |         return r % upper_bound; | 
| 57 | 109M | } |