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1 | /* -*-c-*- |
2 | * |
3 | * $Id$ |
4 | * |
5 | * Exponentiation in finite fields |
6 | * |
7 | * (c) 2004 Straylight/Edgeware |
8 | */ |
9 | |
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10 | /*----- Licensing notice --------------------------------------------------* |
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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. |
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18 | * |
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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. |
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23 | * |
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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 | /*----- Header files ------------------------------------------------------*/ |
31 | |
32 | #include "field.h" |
33 | #include "field-exp.h" |
34 | |
35 | /*----- Main code ---------------------------------------------------------*/ |
36 | |
37 | /* --- @field_exp@ --- * |
38 | * |
39 | * Arguments: @field *f@ = pointer to field |
40 | * @mp *d@ = fake destination |
41 | * @mp *a@ = base |
42 | * @mp *e@ = exponent |
43 | * |
44 | * Returns: Result, %$a^e$%. |
45 | * |
46 | * Use: Exponentiation in a finite field. Note that all quantities |
47 | * are in internal format. |
48 | */ |
49 | |
50 | mp *field_exp(field *f, mp *d, mp *a, mp *e) |
51 | { |
52 | mp *x = MP_COPY(f->one); |
53 | mp *spare = (e->f & MP_BURN) ? MP_NEWSEC : MP_NEW; |
54 | |
55 | MP_COPY(a); |
56 | MP_SHRINK(e); |
57 | if (MP_ZEROP(e)) |
58 | ; |
59 | else { |
60 | if (MP_NEGP(e)) |
61 | a = F_INV(f, a, a); |
62 | if (MP_LEN(e) < EXP_THRESH) |
63 | EXP_SIMPLE(x, a, e); |
64 | else |
65 | EXP_WINDOW(x, a, e); |
66 | } |
67 | mp_drop(d); |
68 | mp_drop(spare); |
69 | mp_drop(a); |
70 | return (x); |
71 | } |
72 | |
73 | /*----- That's all, folks -------------------------------------------------*/ |