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1 | /* -*-c-*- |
2 | * |
3 | * $Id: mprand.c,v 1.1 1999/12/10 23:23:05 mdw Exp $ |
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 | |
30 | /*----- Revision history --------------------------------------------------* |
31 | * |
32 | * $Log: mprand.c,v $ |
33 | * Revision 1.1 1999/12/10 23:23:05 mdw |
34 | * Support for generating random large integers. |
35 | * |
36 | */ |
37 | |
38 | /*----- Header files ------------------------------------------------------*/ |
39 | |
40 | #include <mLib/alloc.h> |
41 | |
42 | #include "grand.h" |
43 | #include "mp.h" |
44 | #include "mprand.h" |
45 | |
46 | /*----- Main code ---------------------------------------------------------*/ |
47 | |
48 | /* --- @mprand@ --- * |
49 | * |
50 | * Arguments: @mp *d@ = destination integer |
51 | * @unsigned b@ = number of bits |
52 | * @grand *r@ = pointer to random number source |
53 | * @mpw or@ = mask to OR with low-order bits |
54 | * |
55 | * Returns: A random integer with the requested number of bits. |
56 | * |
57 | * Use: Constructs an arbitrarily large pseudorandom integer. |
58 | * Assuming that the generator @r@ is good, the result is |
59 | * uniformly distributed in the interval %$[2^{b - 1}, 2^b)$%. |
60 | * The result is then ORred with the given @or@ value. This |
61 | * will often be 1, to make the result odd. |
62 | */ |
63 | |
64 | mp *mprand(mp *d, unsigned b, grand *r, mpw or) |
65 | { |
66 | size_t sz = (b + 7) / 8; |
67 | octet *v = xmalloc(sz); |
68 | unsigned m; |
69 | |
70 | /* --- Fill buffer with random data --- */ |
71 | |
72 | r->ops->fill(r, v, sz); |
73 | |
74 | /* --- Force into the correct range --- * |
75 | * |
76 | * This is slightly tricky. Oh, well. |
77 | */ |
78 | |
79 | b = (b - 1) % 8; |
80 | m = (1 << b); |
81 | v[0] = (v[0] & (m - 1)) | m; |
82 | |
83 | /* --- Mask, load and return --- */ |
84 | |
85 | d = mp_loadb(d, v, sz); |
86 | d->v[0] |= or; |
87 | free(v); |
88 | return (d); |
89 | } |
90 | |
91 | /*----- That's all, folks -------------------------------------------------*/ |