IABSD.fr/src/lib/libc/crypt/arc4random_uniform.c

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• Author : bcook
  Date : 2019-01-20 02:59:07
  Hash : 9ed5ae46
  Message : include stdint.h over sys/types.h ok deraadt@ tedu@

• lib/libc/crypt/arc4random_uniform.c

• /*	$OpenBSD: arc4random_uniform.c,v 1.3 2019/01/20 02:59:07 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 <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;
  }
  DEF_WEAK(arc4random_uniform);