| 1 | /* -*-c-*- |
| 2 | * |
| 3 | * Exponentiation in finite fields |
| 4 | * |
| 5 | * (c) 2004 Straylight/Edgeware |
| 6 | */ |
| 7 | |
| 8 | /*----- Licensing notice --------------------------------------------------* |
| 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. |
| 16 | * |
| 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. |
| 21 | * |
| 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 -------------------------------------------------*/ |