git.distorted.org.uk Git - u/mdw/catacomb/blob - field-exp.c

   1 /* -*-c-*-
   2  *
   3  * $Id$
   4  *
   5  * Exponentiation in finite fields
   6  *
   7  * (c) 2004 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 /*----- 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 -------------------------------------------------*/