1 | /* $OpenBSD: arc4random_uniform.c,v 1.1 2014/07/12 13:24:54 deraadt Exp $ */ |
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2 | |
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3 | /* |
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4 | * Copyright (c) 2008, Damien Miller <djm@openbsd.org> |
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5 | * |
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6 | * Permission to use, copy, modify, and distribute this software for any |
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7 | * purpose with or without fee is hereby granted, provided that the above |
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8 | * copyright notice and this permission notice appear in all copies. |
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9 | * |
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10 | * THE SOFTWARE IS PROVIDED "AS IS" AND THE AUTHOR DISCLAIMS ALL WARRANTIES |
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11 | * WITH REGARD TO THIS SOFTWARE INCLUDING ALL IMPLIED WARRANTIES OF |
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12 | * MERCHANTABILITY AND FITNESS. IN NO EVENT SHALL THE AUTHOR BE LIABLE FOR |
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13 | * ANY SPECIAL, DIRECT, INDIRECT, OR CONSEQUENTIAL DAMAGES OR ANY DAMAGES |
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14 | * WHATSOEVER RESULTING FROM LOSS OF USE, DATA OR PROFITS, WHETHER IN AN |
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15 | * ACTION OF CONTRACT, NEGLIGENCE OR OTHER TORTIOUS ACTION, ARISING OUT OF |
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16 | * OR IN CONNECTION WITH THE USE OR PERFORMANCE OF THIS SOFTWARE. |
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17 | */ |
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18 | |
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19 | #include <sys/types.h> |
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20 | #include <stdlib.h> |
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21 | |
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22 | /* |
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23 | * Calculate a uniformly distributed random number less than upper_bound |
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24 | * avoiding "modulo bias". |
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25 | * |
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26 | * Uniformity is achieved by generating new random numbers until the one |
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27 | * returned is outside the range [0, 2**32 % upper_bound). This |
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28 | * guarantees the selected random number will be inside |
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29 | * [2**32 % upper_bound, 2**32) which maps back to [0, upper_bound) |
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30 | * after reduction modulo upper_bound. |
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31 | */ |
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32 | uint32_t |
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33 | arc4random_uniform(uint32_t upper_bound) |
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34 | { |
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35 | uint32_t r, min; |
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36 | |
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37 | if (upper_bound < 2) |
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38 | return 0; |
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39 | |
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40 | /* 2**32 % x == (2**32 - x) % x */ |
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41 | min = -upper_bound % upper_bound; |
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42 | |
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43 | /* |
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44 | * This could theoretically loop forever but each retry has |
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45 | * p > 0.5 (worst case, usually far better) of selecting a |
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46 | * number inside the range we need, so it should rarely need |
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47 | * to re-roll. |
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48 | */ |
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49 | for (;;) { |
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50 | r = arc4random(); |
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51 | if (r >= min) |
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52 | break; |
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53 | } |
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54 | |
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55 | return r % upper_bound; |
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56 | } |
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