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

   1 /* -*-c-*-
   2  *
   3  * $Id$
   4  *
   5  * Modular exponentiation with Montgomery 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 /*----- Header files ------------------------------------------------------*/
  31
  32 #include "mp.h"
  33 #include "mpmont.h"
  34 #include "mpmont-exp.h"
  35
  36 /*----- Exponentiation ----------------------------------------------------*/
  37
  38 /* --- @mpmont_expr@ --- *
  39  *
  40  * Arguments:   @mpmont *mm@ = pointer to Montgomery reduction context
  41  *              @mp *d@ = fake destination
  42  *              @mp *a@ = base
  43  *              @mp *e@ = exponent
  44  *
  45  * Returns:     Result, %$(a R^{-1})^e R \bmod m$%.
  46  */
  47
  48 mp *mpmont_expr(mpmont *mm, mp *d, mp *a, mp *e)
  49 {
  50   mp *x = MP_COPY(mm->r);
  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 = mpmont_reduce(mm, a, a);
  60       a = mp_modinv(a, a, mm->m);
  61       a = mpmont_mul(mm, a, a, mm->r2);
  62     }
  63     if (MP_LEN(e) < EXP_THRESH)
  64       EXP_SIMPLE(x, a, e);
  65     else
  66       EXP_WINDOW(x, a, e);
  67   }
  68   mp_drop(d);
  69   mp_drop(spare);
  70   mp_drop(a);
  71   return (x);
  72 }
  73
  74 /* --- @mpmont_exp@ --- *
  75  *
  76  * Arguments:   @mpmont *mm@ = pointer to Montgomery reduction context
  77  *              @mp *d@ = fake destination
  78  *              @mp *a@ = base
  79  *              @mp *e@ = exponent
  80  *
  81  * Returns:     Result, %$a^e \bmod m$%.
  82  */
  83
  84 mp *mpmont_exp(mpmont *mm, mp *d, mp *a, mp *e)
  85 {
  86   e = MP_COPY(e);
  87   d = mpmont_mul(mm, d, a, mm->r2);
  88   d = mpmont_expr(mm, d, d, e);
  89   d = mpmont_reduce(mm, d, d);
  90   MP_DROP(e);
  91   return (d);
  92 }
  93
  94 /*----- Test rig ----------------------------------------------------------*/
  95
  96 #ifdef TEST_RIG
  97
  98 static int texp(dstr *v)
  99 {
 100   mp *m = *(mp **)v[0].buf;
 101   mp *a = *(mp **)v[1].buf;
 102   mp *b = *(mp **)v[2].buf;
 103   mp *r = *(mp **)v[3].buf;
 104   mp *mr;
 105   int ok = 1;
 106
 107   mpmont mm;
 108   mpmont_create(&mm, m);
 109
 110   mr = mpmont_exp(&mm, MP_NEW, a, b);
 111
 112   if (!MP_EQ(mr, r)) {
 113     fputs("\n*** montgomery modexp failed", stderr);
 114     fputs("\n m = ", stderr); mp_writefile(m, stderr, 10);
 115     fputs("\n a = ", stderr); mp_writefile(a, stderr, 10);
 116     fputs("\n e = ", stderr); mp_writefile(b, stderr, 10);
 117     fputs("\n r = ", stderr); mp_writefile(r, stderr, 10);
 118     fputs("\nmr = ", stderr); mp_writefile(mr, stderr, 10);
 119     fputc('\n', stderr);
 120     ok = 0;
 121   }
 122
 123   MP_DROP(m);
 124   MP_DROP(a);
 125   MP_DROP(b);
 126   MP_DROP(r);
 127   MP_DROP(mr);
 128   mpmont_destroy(&mm);
 129   assert(mparena_count(MPARENA_GLOBAL) == 0);
 130   return ok;
 131 }
 132
 133 static test_chunk tests[] = {
 134   { "exp", texp, { &type_mp, &type_mp, &type_mp, &type_mp, 0 } },
 135   { 0, 0, { 0 } },
 136 };
 137
 138 int main(int argc, char *argv[])
 139 {
 140   sub_init();
 141   test_run(argc, argv, tests, SRCDIR "/tests/mpmont");
 142   return (0);
 143 }
 144
 145 #endif
 146
 147 /*----- That's all, folks -------------------------------------------------*/