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

   1 /* -*-c-*-
   2  *
   3  * Modular exponentiation with Montgomery reduction
   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 "mp.h"
  31 #include "mpmont.h"
  32 #include "mpmont-exp.h"
  33
  34 /*----- Exponentiation ----------------------------------------------------*/
  35
  36 /* --- @mpmont_expr@ --- *
  37  *
  38  * Arguments:   @mpmont *mm@ = pointer to Montgomery reduction context
  39  *              @mp *d@ = fake destination
  40  *              @mp *a@ = base
  41  *              @mp *e@ = exponent
  42  *
  43  * Returns:     Result, %$(a R^{-1})^e R \bmod m$%.
  44  */
  45
  46 mp *mpmont_expr(mpmont *mm, mp *d, mp *a, mp *e)
  47 {
  48   mp *x = MP_COPY(mm->r);
  49   mp *spare = (e->f & MP_BURN) ? MP_NEWSEC : MP_NEW;
  50
  51   MP_COPY(a);
  52   MP_SHRINK(e);
  53   if (MP_ZEROP(e))
  54     ;
  55   else {
  56     if (MP_NEGP(e)) {
  57       a = mpmont_reduce(mm, a, a);
  58       a = mp_modinv(a, a, mm->m);
  59       a = mpmont_mul(mm, a, a, mm->r2);
  60     }
  61     if (MP_LEN(e) < EXP_THRESH)
  62       EXP_SIMPLE(x, a, e);
  63     else
  64       EXP_WINDOW(x, a, e);
  65   }
  66   mp_drop(d);
  67   mp_drop(spare);
  68   mp_drop(a);
  69   return (x);
  70 }
  71
  72 /* --- @mpmont_exp@ --- *
  73  *
  74  * Arguments:   @mpmont *mm@ = pointer to Montgomery reduction context
  75  *              @mp *d@ = fake destination
  76  *              @mp *a@ = base
  77  *              @mp *e@ = exponent
  78  *
  79  * Returns:     Result, %$a^e \bmod m$%.
  80  */
  81
  82 mp *mpmont_exp(mpmont *mm, mp *d, mp *a, mp *e)
  83 {
  84   e = MP_COPY(e);
  85   d = mpmont_mul(mm, d, a, mm->r2);
  86   d = mpmont_expr(mm, d, d, e);
  87   d = mpmont_reduce(mm, d, d);
  88   MP_DROP(e);
  89   return (d);
  90 }
  91
  92 /*----- Test rig ----------------------------------------------------------*/
  93
  94 #ifdef TEST_RIG
  95
  96 static int texp(dstr *v)
  97 {
  98   mp *m = *(mp **)v[0].buf;
  99   mp *a = *(mp **)v[1].buf;
 100   mp *b = *(mp **)v[2].buf;
 101   mp *r = *(mp **)v[3].buf;
 102   mp *mr;
 103   int ok = 1;
 104
 105   mpmont mm;
 106   mpmont_create(&mm, m);
 107
 108   mr = mpmont_exp(&mm, MP_NEW, a, b);
 109
 110   if (!MP_EQ(mr, r)) {
 111     fputs("\n*** montgomery modexp failed", stderr);
 112     fputs("\n m = ", stderr); mp_writefile(m, stderr, 10);
 113     fputs("\n a = ", stderr); mp_writefile(a, stderr, 10);
 114     fputs("\n e = ", stderr); mp_writefile(b, stderr, 10);
 115     fputs("\n r = ", stderr); mp_writefile(r, stderr, 10);
 116     fputs("\nmr = ", stderr); mp_writefile(mr, stderr, 10);
 117     fputc('\n', stderr);
 118     ok = 0;
 119   }
 120
 121   MP_DROP(m);
 122   MP_DROP(a);
 123   MP_DROP(b);
 124   MP_DROP(r);
 125   MP_DROP(mr);
 126   mpmont_destroy(&mm);
 127   assert(mparena_count(MPARENA_GLOBAL) == 0);
 128   return ok;
 129 }
 130
 131 static test_chunk tests[] = {
 132   { "exp", texp, { &type_mp, &type_mp, &type_mp, &type_mp, 0 } },
 133   { 0, 0, { 0 } },
 134 };
 135
 136 int main(int argc, char *argv[])
 137 {
 138   sub_init();
 139   test_run(argc, argv, tests, SRCDIR "/t/mpmont");
 140   return (0);
 141 }
 142
 143 #endif
 144
 145 /*----- That's all, folks -------------------------------------------------*/