/src/openssh/openbsd-compat/arc4random_uniform.c
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1 | | /* $OpenBSD: arc4random_uniform.c,v 1.3 2019/01/20 02:59:07 bcook Exp $ */ |
2 | | |
3 | | /* |
4 | | * Copyright (c) 2008, Damien Miller <djm@openbsd.org> |
5 | | * |
6 | | * Permission to use, copy, modify, and distribute this software for any |
7 | | * purpose with or without fee is hereby granted, provided that the above |
8 | | * copyright notice and this permission notice appear in all copies. |
9 | | * |
10 | | * THE SOFTWARE IS PROVIDED "AS IS" AND THE AUTHOR DISCLAIMS ALL WARRANTIES |
11 | | * WITH REGARD TO THIS SOFTWARE INCLUDING ALL IMPLIED WARRANTIES OF |
12 | | * MERCHANTABILITY AND FITNESS. IN NO EVENT SHALL THE AUTHOR BE LIABLE FOR |
13 | | * ANY SPECIAL, DIRECT, INDIRECT, OR CONSEQUENTIAL DAMAGES OR ANY DAMAGES |
14 | | * WHATSOEVER RESULTING FROM LOSS OF USE, DATA OR PROFITS, WHETHER IN AN |
15 | | * ACTION OF CONTRACT, NEGLIGENCE OR OTHER TORTIOUS ACTION, ARISING OUT OF |
16 | | * OR IN CONNECTION WITH THE USE OR PERFORMANCE OF THIS SOFTWARE. |
17 | | */ |
18 | | |
19 | | /* OPENBSD ORIGINAL: lib/libc/crypto/arc4random_uniform.c */ |
20 | | |
21 | | #include "includes.h" |
22 | | |
23 | | #include <stdint.h> |
24 | | #include <stdlib.h> |
25 | | |
26 | | #ifndef HAVE_ARC4RANDOM_UNIFORM |
27 | | /* |
28 | | * Calculate a uniformly distributed random number less than upper_bound |
29 | | * avoiding "modulo bias". |
30 | | * |
31 | | * Uniformity is achieved by generating new random numbers until the one |
32 | | * returned is outside the range [0, 2**32 % upper_bound). This |
33 | | * guarantees the selected random number will be inside |
34 | | * [2**32 % upper_bound, 2**32) which maps back to [0, upper_bound) |
35 | | * after reduction modulo upper_bound. |
36 | | */ |
37 | | uint32_t |
38 | | arc4random_uniform(uint32_t upper_bound) |
39 | 0 | { |
40 | 0 | uint32_t r, min; |
41 | |
|
42 | 0 | if (upper_bound < 2) |
43 | 0 | return 0; |
44 | | |
45 | | /* 2**32 % x == (2**32 - x) % x */ |
46 | 0 | min = -upper_bound % upper_bound; |
47 | | |
48 | | /* |
49 | | * This could theoretically loop forever but each retry has |
50 | | * p > 0.5 (worst case, usually far better) of selecting a |
51 | | * number inside the range we need, so it should rarely need |
52 | | * to re-roll. |
53 | | */ |
54 | 0 | for (;;) { |
55 | 0 | r = arc4random(); |
56 | 0 | if (r >= min) |
57 | 0 | break; |
58 | 0 | } |
59 | |
|
60 | 0 | return r % upper_bound; |
61 | 0 | } |
62 | | #endif /* !HAVE_ARC4RANDOM_UNIFORM */ |