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1 | /* -*-c-*- |
2 | * |
b817bfc6 |
3 | * $Id: mprand.c,v 1.5 2004/04/08 01:36:15 mdw Exp $ |
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4 | * |
5 | * Generate a random multiprecision integer |
6 | * |
7 | * (c) 1999 Straylight/Edgeware |
8 | */ |
9 | |
10 | /*----- Licensing notice --------------------------------------------------* |
11 | * |
12 | * This file is part of Catacomb. |
13 | * |
14 | * Catacomb is free software; you can redistribute it and/or modify |
15 | * it under the terms of the GNU Library General Public License as |
16 | * published by the Free Software Foundation; either version 2 of the |
17 | * License, or (at your option) any later version. |
18 | * |
19 | * Catacomb is distributed in the hope that it will be useful, |
20 | * but WITHOUT ANY WARRANTY; without even the implied warranty of |
21 | * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the |
22 | * GNU Library General Public License for more details. |
23 | * |
24 | * You should have received a copy of the GNU Library General Public |
25 | * License along with Catacomb; if not, write to the Free |
26 | * Software Foundation, Inc., 59 Temple Place - Suite 330, Boston, |
27 | * MA 02111-1307, USA. |
28 | */ |
29 | |
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30 | /*----- Header files ------------------------------------------------------*/ |
31 | |
32 | #include <mLib/alloc.h> |
33 | |
34 | #include "grand.h" |
35 | #include "mp.h" |
36 | #include "mprand.h" |
37 | |
38 | /*----- Main code ---------------------------------------------------------*/ |
39 | |
40 | /* --- @mprand@ --- * |
41 | * |
42 | * Arguments: @mp *d@ = destination integer |
43 | * @unsigned b@ = number of bits |
44 | * @grand *r@ = pointer to random number source |
45 | * @mpw or@ = mask to OR with low-order bits |
46 | * |
47 | * Returns: A random integer with the requested number of bits. |
48 | * |
49 | * Use: Constructs an arbitrarily large pseudorandom integer. |
50 | * Assuming that the generator @r@ is good, the result is |
51 | * uniformly distributed in the interval %$[2^{b - 1}, 2^b)$%. |
52 | * The result is then ORred with the given @or@ value. This |
53 | * will often be 1, to make the result odd. |
54 | */ |
55 | |
56 | mp *mprand(mp *d, unsigned b, grand *r, mpw or) |
57 | { |
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58 | size_t sz = (b + 7) >> 3; |
d34decd2 |
59 | arena *a = (d && (d->f & MP_BURN)) ? arena_secure : arena_global; |
60 | octet *v = x_alloc(a, sz); |
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61 | unsigned m; |
62 | |
63 | /* --- Fill buffer with random data --- */ |
64 | |
65 | r->ops->fill(r, v, sz); |
66 | |
67 | /* --- Force into the correct range --- * |
68 | * |
69 | * This is slightly tricky. Oh, well. |
70 | */ |
71 | |
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72 | b = (b - 1) & 7; |
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73 | m = (1 << b); |
74 | v[0] = (v[0] & (m - 1)) | m; |
75 | |
76 | /* --- Mask, load and return --- */ |
77 | |
78 | d = mp_loadb(d, v, sz); |
79 | d->v[0] |= or; |
d34decd2 |
80 | memset(v, 0, sz); |
81 | x_free(a, v); |
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82 | return (d); |
83 | } |
84 | |
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85 | /* --- @mprand_range@ --- * |
86 | * |
87 | * Arguments: @mp *d@ = destination integer |
88 | * @mp *l@ = limit for random number |
89 | * @grand *r@ = random number source |
90 | * @mpw or@ = mask for low-order bits |
91 | * |
92 | * Returns: A pseudorandom integer, unformly distributed over the |
93 | * interval %$[0, l)$%. |
94 | * |
95 | * Use: Generates a uniformly-distributed pseudorandom number in the |
96 | * appropriate range. |
97 | */ |
98 | |
99 | mp *mprand_range(mp *d, mp *l, grand *r, mpw or) |
100 | { |
101 | size_t b = mp_bits(l); |
102 | size_t sz = (b + 7) >> 3; |
d34decd2 |
103 | arena *a = (d && (d->f & MP_BURN)) ? arena_secure : arena_global; |
104 | octet *v = x_alloc(a, sz); |
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105 | unsigned m; |
106 | |
107 | /* --- The algorithm --- * |
108 | * |
109 | * Rather simpler than most. Find the number of bits in the number %$l$% |
110 | * (i.e., the integer %$b$% such that %$2^{b - 1} \le l < 2^b$%), and |
111 | * generate pseudorandom integers with %$n$% bits (but not, unlike in the |
112 | * function above, with the top bit forced to 1). If the integer is |
113 | * greater than or equal to %$l$%, try again. |
114 | * |
115 | * This is similar to the algorithms used in @lcrand_range@ and friends, |
116 | * except that I've forced the `raw' range of the random numbers such that |
117 | * %$l$% itself is the largest multiple of %$l$% in the range (since, by |
118 | * the inequality above, %$2^b \le 2l$%). This removes the need for costly |
119 | * division and remainder operations. |
120 | * |
121 | * As usual, the number of iterations expected is two. |
122 | */ |
123 | |
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124 | b = ((b - 1) & 7) + 1; |
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125 | m = (1 << b) - 1; |
126 | do { |
127 | r->ops->fill(r, v, sz); |
128 | v[0] &= m; |
129 | d = mp_loadb(d, v, sz); |
130 | d->v[0] |= or; |
131 | } while (MP_CMP(d, >=, l)); |
132 | |
133 | /* --- Done --- */ |
134 | |
d34decd2 |
135 | memset(v, 0, sz); |
136 | x_free(a, v); |
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137 | return (d); |
138 | } |
139 | |
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140 | /*----- That's all, folks -------------------------------------------------*/ |