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   1 /* -*-c-*-
   2  *
   3  * $Id: mpbarrett.h,v 1.2 2000/10/08 12:03:44 mdw Exp $
   4  *
   5  * Barrett modular reduction
   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: mpbarrett.h,v $
  33  * Revision 1.2  2000/10/08 12:03:44  mdw
  34  * (mpbarrett_reduce): Cope with negative numbers.
  35  *
  36  * Revision 1.1  1999/12/10 23:22:00  mdw
  37  * Barrett reduction support: works with even moduli.
  38  *
  39  */
  40
  41 /*----- Notes on Barrett reduction ----------------------------------------*
  42  *
  43  * Barrett reduction is a technique for computing modular residues.  Unlike
  44  * Montgomery reduction, it doesn't have restrictions on the modulus (except
  45  * that it be positive) and doesn't confuse matters by putting an extra
  46  * factor all the way through your computation.
  47  *
  48  * It's useful for slightly less heavy-duty work than Montgomery reduction
  49  * because the precomputation phase is rather simpler, involving a single
  50  * division operation.
  51  *
  52  * Sometimes it's useful to exponentiate modulo an even number, so there's a
  53  * modexp routine provided which uses Barrett reduction rather than
  54  * Montgomery reduction.  This is handy when you're working on indices in an
  55  * even-order cyclic group or something.
  56  */
  57
  58 #ifndef CATACOMB_MPBARRETT_H
  59 #define CATACOMB_MPBARRETT_H
  60
  61 #ifdef __cplusplus
  62   extern "C" {
  63 #endif
  64
  65 /*----- Header files ------------------------------------------------------*/
  66
  67 #ifndef CATACOMB_MP_H
  68 #  include "mp.h"
  69 #endif
  70
  71 /*----- Data structures ---------------------------------------------------*/
  72
  73 typedef struct mpbarrett {
  74   mp *m;
  75   mp *mu;
  76   size_t k;
  77 } mpbarrett;
  78
  79 /*----- Functions provided ------------------------------------------------*/
  80
  81 /* --- @mpbarrett_create@ --- *
  82  *
  83  * Arguments:   @mpbarrett *mb@ = pointer to Barrett reduction context
  84  *              @mp *m@ = modulus to work to
  85  *
  86  *
  87  * Returns:     ---
  88  *
  89  * Use:         Initializes a Barrett reduction context ready for use.
  90  */
  91
  92 extern void mpbarrett_create(mpbarrett */*mb*/, mp */*m*/);
  93
  94 /* --- @mpbarrett_destroy@ --- *
  95  *
  96  * Arguments:   @mpbarrett *mb@ = pointer to Barrett reduction context
  97  *
  98  * Returns:     ---
  99  *
 100  * Use:         Destroys a Barrett reduction context releasing any resources
 101  *              claimed.
 102  */
 103
 104 extern void mpbarrett_destroy(mpbarrett */*mb*/);
 105
 106 /* --- @mpbarrett_reduce@ --- *
 107  *
 108  * Arguments:   @mpbarrett *mb@ = pointer to Barrett reduction context
 109  *              @mp *d@ = destination for result
 110  *              @mp *m@ = number to reduce
 111  *
 112  * Returns:     The residue of @m@ modulo the number in the reduction
 113  *              context.
 114  *
 115  * Use:         Performs an efficient modular reduction.
 116  */
 117
 118 extern mp *mpbarrett_reduce(mpbarrett */*mb*/, mp */*d*/, mp */*m*/);
 119
 120 /* --- @mpbarrett_exp@ --- *
 121  *
 122  * Arguments:   @mpbarrett *mb@ = pointer to Barrett reduction context
 123  *              @mp *d@ = fake destination
 124  *              @mp *a@ = base
 125  *              @mp *e@ = exponent
 126  *
 127  * Returns:     Result, %$a^e \bmod m$%.
 128  */
 129
 130 extern mp *mpbarrett_exp(mpbarrett */*mb*/, mp */*d*/, mp */*a*/, mp */*e*/);
 131
 132 /*----- That's all, folks -------------------------------------------------*/
 133
 134 #ifdef __cplusplus
 135   }
 136 #endif
 137
 138 #endif