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1 | /* -*-c-*- |
2 | * |
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3 | * Exponentiation in finite fields |
4 | * |
5 | * (c) 2004 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 | |
28 | /*----- Header files ------------------------------------------------------*/ |
29 | |
30 | #include "field.h" |
31 | #include "field-exp.h" |
32 | |
33 | /*----- Main code ---------------------------------------------------------*/ |
34 | |
35 | /* --- @field_exp@ --- * |
36 | * |
37 | * Arguments: @field *f@ = pointer to field |
38 | * @mp *d@ = fake destination |
39 | * @mp *a@ = base |
40 | * @mp *e@ = exponent |
41 | * |
42 | * Returns: Result, %$a^e$%. |
43 | * |
44 | * Use: Exponentiation in a finite field. Note that all quantities |
45 | * are in internal format. |
46 | */ |
47 | |
48 | mp *field_exp(field *f, mp *d, mp *a, mp *e) |
49 | { |
50 | mp *x = MP_COPY(f->one); |
51 | mp *spare = (e->f & MP_BURN) ? MP_NEWSEC : MP_NEW; |
52 | |
53 | MP_COPY(a); |
54 | MP_SHRINK(e); |
55 | if (MP_ZEROP(e)) |
56 | ; |
57 | else { |
58 | if (MP_NEGP(e)) |
59 | a = F_INV(f, a, a); |
60 | if (MP_LEN(e) < EXP_THRESH) |
61 | EXP_SIMPLE(x, a, e); |
62 | else |
63 | EXP_WINDOW(x, a, e); |
64 | } |
65 | mp_drop(d); |
66 | mp_drop(spare); |
67 | mp_drop(a); |
68 | return (x); |
69 | } |
70 | |
71 | /*----- That's all, folks -------------------------------------------------*/ |