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

   1 /* -*-c-*-
   2  *
   3  * $Id: mpbarrett-exp.c,v 1.1 2004/04/01 12:50:09 mdw Exp $
   4  *
   5  * Modular exponentiation using Barrett reduction
   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 /*----- Revision history --------------------------------------------------*
  31  *
  32  * $Log: mpbarrett-exp.c,v $
  33  * Revision 1.1  2004/04/01 12:50:09  mdw
  34  * Add cyclic group abstraction, with test code.  Separate off exponentation
  35  * functions for better static linking.  Fix a buttload of bugs on the way.
  36  * Generally ensure that negative exponents do inversion correctly.  Add
  37  * table of standard prime-field subgroups.  (Binary field subgroups are
  38  * currently unimplemented but easy to add if anyone ever finds a good one.)
  39  *
  40  */
  41
  42 /*----- Header files ------------------------------------------------------*/
  43
  44 #include "mp.h"
  45 #include "mpbarrett.h"
  46 #include "mpbarrett-exp.h"
  47
  48 /*----- Main code ---------------------------------------------------------*/
  49
  50 /* --- @mpbarrett_exp@ --- *
  51  *
  52  * Arguments:   @mpbarrett *mb@ = pointer to Barrett reduction context
  53  *              @mp *d@ = fake destination
  54  *              @mp *a@ = base
  55  *              @mp *e@ = exponent
  56  *
  57  * Returns:     Result, %$a^e \bmod m$%.
  58  */
  59
  60 mp *mpbarrett_exp(mpbarrett *mb, mp *d, mp *a, mp *e)
  61 {
  62   mp *x = MP_ONE;
  63   mp *spare = (e->f & MP_BURN) ? MP_NEWSEC : MP_NEW;
  64
  65   MP_COPY(a);
  66   MP_SHRINK(e);
  67   if (e->f & MP_NEG) {
  68     mp *g = MP_NEW;
  69     mp_gcd(&g, 0, &a, mb->m, a);
  70     assert(MP_EQ(g, MP_ONE));
  71     mp_drop(g);
  72   }
  73   if (!MP_LEN(e))
  74     ;
  75   else if (MP_LEN(e) < EXP_THRESH)
  76     EXP_SIMPLE(x, a, e);
  77   else
  78     EXP_WINDOW(x, a, e);
  79   mp_drop(d);
  80   mp_drop(spare);
  81   mp_drop(a);
  82   return (x);
  83 }
  84
  85 /*----- Test rig ----------------------------------------------------------*/
  86
  87 #ifdef TEST_RIG
  88
  89 static int vexp(dstr *v)
  90 {
  91   mp *m = *(mp **)v[0].buf;
  92   mp *a = *(mp **)v[1].buf;
  93   mp *b = *(mp **)v[2].buf;
  94   mp *r = *(mp **)v[3].buf;
  95   mp *mr;
  96   int ok = 1;
  97
  98   mpbarrett mb;
  99   mpbarrett_create(&mb, m);
 100
 101   mr = mpbarrett_exp(&mb, MP_NEW, a, b);
 102
 103   if (!MP_EQ(mr, r)) {
 104     fputs("\n*** barrett modexp failed", stderr);
 105     fputs("\n m = ", stderr); mp_writefile(m, stderr, 10);
 106     fputs("\n a = ", stderr); mp_writefile(a, stderr, 10);
 107     fputs("\n e = ", stderr); mp_writefile(b, stderr, 10);
 108     fputs("\n r = ", stderr); mp_writefile(r, stderr, 10);
 109     fputs("\nmr = ", stderr); mp_writefile(mr, stderr, 10);
 110     fputc('\n', stderr);
 111     ok = 0;
 112   }
 113
 114   mp_drop(m);
 115   mp_drop(a);
 116   mp_drop(b);
 117   mp_drop(r);
 118   mp_drop(mr);
 119   mpbarrett_destroy(&mb);
 120   assert(mparena_count(MPARENA_GLOBAL) == 0);
 121   return ok;
 122 }
 123
 124 static test_chunk tests[] = {
 125   { "mpbarrett-exp", vexp, { &type_mp, &type_mp, &type_mp, &type_mp, 0 } },
 126   { 0, 0, { 0 } }
 127 };
 128
 129 int main(int argc, char *argv[])
 130 {
 131   sub_init();
 132   test_run(argc, argv, tests, SRCDIR "/tests/mpbarrett");
 133   return (0);
 134 }
 135
 136 #endif
 137
 138 /*----- That's all, folks -------------------------------------------------*/