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1 | /* -*-c-*- |
2 | * |
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3 | * Generate a random multiprecision integer |
4 | * |
5 | * (c) 1999 Straylight/Edgeware |
6 | */ |
7 | |
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8 | /*----- Licensing notice --------------------------------------------------* |
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9 | * |
10 | * This file is part of Catacomb. |
11 | * |
12 | * Catacomb is free software; you can redistribute it and/or modify |
13 | * it under the terms of the GNU Library General Public License as |
14 | * published by the Free Software Foundation; either version 2 of the |
15 | * License, or (at your option) any later version. |
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16 | * |
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17 | * Catacomb is distributed in the hope that it will be useful, |
18 | * but WITHOUT ANY WARRANTY; without even the implied warranty of |
19 | * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the |
20 | * GNU Library General Public License for more details. |
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21 | * |
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22 | * You should have received a copy of the GNU Library General Public |
23 | * License along with Catacomb; if not, write to the Free |
24 | * Software Foundation, Inc., 59 Temple Place - Suite 330, Boston, |
25 | * MA 02111-1307, USA. |
26 | */ |
27 | |
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28 | /*----- Header files ------------------------------------------------------*/ |
29 | |
30 | #include <mLib/alloc.h> |
31 | |
32 | #include "grand.h" |
33 | #include "mp.h" |
34 | #include "mprand.h" |
35 | |
36 | /*----- Main code ---------------------------------------------------------*/ |
37 | |
38 | /* --- @mprand@ --- * |
39 | * |
40 | * Arguments: @mp *d@ = destination integer |
41 | * @unsigned b@ = number of bits |
42 | * @grand *r@ = pointer to random number source |
43 | * @mpw or@ = mask to OR with low-order bits |
44 | * |
45 | * Returns: A random integer with the requested number of bits. |
46 | * |
47 | * Use: Constructs an arbitrarily large pseudorandom integer. |
48 | * Assuming that the generator @r@ is good, the result is |
49 | * uniformly distributed in the interval %$[2^{b - 1}, 2^b)$%. |
50 | * The result is then ORred with the given @or@ value. This |
51 | * will often be 1, to make the result odd. |
52 | */ |
53 | |
54 | mp *mprand(mp *d, unsigned b, grand *r, mpw or) |
55 | { |
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56 | size_t sz = (b + 7) >> 3; |
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57 | arena *a = (d && (d->f & MP_BURN)) ? arena_secure : arena_global; |
58 | octet *v = x_alloc(a, sz); |
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59 | unsigned m; |
60 | |
61 | /* --- Fill buffer with random data --- */ |
62 | |
63 | r->ops->fill(r, v, sz); |
64 | |
65 | /* --- Force into the correct range --- * |
66 | * |
67 | * This is slightly tricky. Oh, well. |
68 | */ |
69 | |
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70 | b = (b - 1) & 7; |
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71 | m = (1 << b); |
72 | v[0] = (v[0] & (m - 1)) | m; |
73 | |
74 | /* --- Mask, load and return --- */ |
75 | |
76 | d = mp_loadb(d, v, sz); |
77 | d->v[0] |= or; |
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78 | memset(v, 0, sz); |
79 | x_free(a, v); |
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80 | return (d); |
81 | } |
82 | |
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83 | /* --- @mprand_range@ --- * |
84 | * |
85 | * Arguments: @mp *d@ = destination integer |
86 | * @mp *l@ = limit for random number |
87 | * @grand *r@ = random number source |
88 | * @mpw or@ = mask for low-order bits |
89 | * |
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90 | * Returns: A pseudorandom integer, unformly distributed over the |
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91 | * interval %$[0, l)$%. |
92 | * |
93 | * Use: Generates a uniformly-distributed pseudorandom number in the |
94 | * appropriate range. |
95 | */ |
96 | |
97 | mp *mprand_range(mp *d, mp *l, grand *r, mpw or) |
98 | { |
99 | size_t b = mp_bits(l); |
100 | size_t sz = (b + 7) >> 3; |
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101 | arena *a = (d && (d->f & MP_BURN)) ? arena_secure : arena_global; |
102 | octet *v = x_alloc(a, sz); |
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103 | unsigned m; |
104 | |
105 | /* --- The algorithm --- * |
106 | * |
107 | * Rather simpler than most. Find the number of bits in the number %$l$% |
108 | * (i.e., the integer %$b$% such that %$2^{b - 1} \le l < 2^b$%), and |
109 | * generate pseudorandom integers with %$n$% bits (but not, unlike in the |
110 | * function above, with the top bit forced to 1). If the integer is |
111 | * greater than or equal to %$l$%, try again. |
112 | * |
113 | * This is similar to the algorithms used in @lcrand_range@ and friends, |
114 | * except that I've forced the `raw' range of the random numbers such that |
115 | * %$l$% itself is the largest multiple of %$l$% in the range (since, by |
116 | * the inequality above, %$2^b \le 2l$%). This removes the need for costly |
117 | * division and remainder operations. |
118 | * |
119 | * As usual, the number of iterations expected is two. |
120 | */ |
121 | |
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122 | b = ((b - 1) & 7) + 1; |
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123 | m = (1 << b) - 1; |
124 | do { |
125 | r->ops->fill(r, v, sz); |
126 | v[0] &= m; |
127 | d = mp_loadb(d, v, sz); |
128 | d->v[0] |= or; |
129 | } while (MP_CMP(d, >=, l)); |
130 | |
131 | /* --- Done --- */ |
132 | |
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133 | memset(v, 0, sz); |
134 | x_free(a, v); |
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135 | return (d); |
136 | } |
137 | |
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138 | /*----- That's all, folks -------------------------------------------------*/ |