rng.c - rpass - Strong password generator for humans

rng.c (2180B)
      1 /*
      2  * Copyright (c) 2008, Damien Miller <djm@openbsd.org>
      3  * Copyright (c) 2020, Stephen Gregoratto <dev@sgregoratto.me>
      4  *
      5  * Permission to use, copy, modify, and/or distribute this software for any
      6  * purpose with or without fee is hereby granted, provided that the above
      7  * copyright notice and this permission notice appear in all copies.
      8  *
      9  * THE SOFTWARE IS PROVIDED "AS IS" AND THE AUTHOR DISCLAIMS ALL WARRANTIES
     10  * WITH REGARD TO THIS SOFTWARE INCLUDING ALL IMPLIED WARRANTIES OF
     11  * MERCHANTABILITY AND FITNESS. IN NO EVENT SHALL THE AUTHOR BE LIABLE FOR
     12  * ANY SPECIAL, DIRECT, INDIRECT, OR CONSEQUENTIAL DAMAGES OR ANY DAMAGES
     13  * WHATSOEVER RESULTING FROM LOSS OF USE, DATA OR PROFITS, WHETHER IN AN
     14  * ACTION OF CONTRACT, NEGLIGENCE OR OTHER TORTIOUS ACTION, ARISING OUT OF
     15  * OR IN CONNECTION WITH THE USE OR PERFORMANCE OF THIS SOFTWARE.
     16  */
     17 #include "config.h"
     18 
     19 #include <stdint.h>
     20 
     21 #include "rng.h"
     22 
     23 #if HAVE_ARC4RANDOM
     24 #include <stdlib.h>
     25 
     26 uint32_t
     27 rng(void)
     28 {
     29 
     30 	return arc4random();
     31 }
     32 #else
     33 #include <fcntl.h>
     34 #include <stdlib.h>
     35 #include <time.h>
     36 #include <unistd.h>
     37 
     38 uint32_t
     39 rng(void)
     40 {
     41 	uint32_t num = 0;
     42 	int fd;
     43 
     44 	if ((fd = open("/dev/urandom", O_RDONLY)) != -1) {
     45 		(void)read(fd, &num, 4);
     46 		close(fd);
     47 	} else {
     48 		srandom(time(NULL));
     49 		num = random();
     50 	}
     51 
     52 	return num;
     53 }
     54 #endif
     55 
     56 /*
     57  * Calculate a uniformly distributed random number less than upper_bound
     58  * avoiding "modulo bias".
     59  *
     60  * Uniformity is achieved by generating new random numbers until the one
     61  * returned is outside the range [0, 2**32 % upper_bound).  This
     62  * guarantees the selected random number will be inside
     63  * [2**32 % upper_bound, 2**32) which maps back to [0, upper_bound)
     64  * after reduction modulo upper_bound.
     65  */
     66 uint32_t
     67 rng_uniform(uint32_t upper_bound)
     68 {
     69 	uint32_t r, min;
     70 
     71 	if (upper_bound < 2)
     72 		return 0;
     73 
     74 	/* 2**32 % x == (2**32 - x) % x */
     75 	min = -upper_bound % upper_bound;
     76 
     77 	/*
     78 	 * This could theoretically loop forever but each retry has
     79 	 * p > 0.5 (worst case, usually far better) of selecting a
     80 	 * number inside the range we need, so it should rarely need
     81 	 * to re-roll.
     82 	 */
     83 	for (;;) {
     84 		r = rng();
     85 		if (r >= min)
     86 			break;
     87 	}
     88 
     89 	return r % upper_bound;
     90 }