mpi/mpi-mpow.c

   1 /* mpi-mpow.c  -  MPI functions
   2  * Copyright (C) 1998, 1999, 2000 Free Software Foundation, Inc.
   3  *
   4  * This file is part of GnuPG.
   5  *
   6  * GnuPG is free software; you can redistribute it and/or modify
   7  * it under the terms of the GNU General Public License as published by
   8  * the Free Software Foundation; either version 2 of the License, or
   9  * (at your option) any later version.
  10  *
  11  * GnuPG is distributed in the hope that it will be useful,
  12  * but WITHOUT ANY WARRANTY; without even the implied warranty of
  13  * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
  14  * GNU General Public License for more details.
  15  *
  16  * You should have received a copy of the GNU General Public License
  17  * along with this program; if not, write to the Free Software
  18  * Foundation, Inc., 51 Franklin Street, Fifth Floor, Boston, MA 02110-1301,
  19  * USA.
  20  */
  21
  22 #include <config.h>
  23 #include <stdio.h>
  24 #include <stdlib.h>
  25 #include "mpi-internal.h"
  26 #include "longlong.h"
  27 #include <assert.h>
  28
  29 static int
  30 build_index( MPI *exparray, int k, int i, int t )
  31 {
  32     int j, bitno;
  33     int idx = 0;
  34
  35     bitno = t-i;
  36     for(j=k-1; j >= 0; j-- ) {
  37         idx <<= 1;
  38         if( mpi_test_bit( exparray[j], bitno ) )
  39             idx |= 1;
  40     }
  41     return idx;
  42 }
  43
  44 /****************
  45  * RES = (BASE[0] ^ EXP[0]) *  (BASE[1] ^ EXP[1]) * ... * mod M
  46  */
  47 void
  48 mpi_mulpowm( MPI res, MPI *basearray, MPI *exparray, MPI m)
  49 {
  50     int k;      /* number of elements */
  51     int t;      /* bit size of largest exponent */
  52     int i, j, idx;
  53     MPI *G;     /* table with precomputed values of size 2^k */
  54     MPI tmp;
  55
  56     for(k=0; basearray[k]; k++ )
  57         ;
  58     assert(k);
  59     for(t=0, i=0; (tmp=exparray[i]); i++ ) {
  60         j = mpi_get_nbits(tmp);
  61         if( j > t )
  62             t = j;
  63     }
  64     assert(i==k);
  65     assert(t);
  66     assert( k < 10 );
  67
  68     G = xmalloc_clear( (1<<k) * sizeof *G );
  69     /* and calculate */
  70     tmp =  mpi_alloc( mpi_get_nlimbs(m)+1 );
  71     mpi_set_ui( res, 1 );
  72     for(i = 1; i <= t; i++ ) {
  73         mpi_mulm(tmp, res, res, m );
  74         idx = build_index( exparray, k, i, t );
  75         assert( idx >= 0 && idx < (1<<k) );
  76         if( !G[idx] ) {
  77             if( !idx )
  78                  G[0] = mpi_alloc_set_ui( 1 );
  79             else {
  80                 for(j=0; j < k; j++ ) {
  81                     if( (idx & (1<<j) ) ) {
  82                         if( !G[idx] )
  83                             G[idx] = mpi_copy( basearray[j] );
  84                         else
  85                             mpi_mulm( G[idx], G[idx], basearray[j], m );
  86                     }
  87                 }
  88                 if( !G[idx] )
  89                     G[idx] = mpi_alloc(0);
  90             }
  91         }
  92         mpi_mulm(res, tmp, G[idx], m );
  93     }
  94
  95     /* cleanup */
  96     mpi_free(tmp);
  97     for(i=0; i < (1<<k); i++ )
  98         mpi_free(G[i]);
  99     xfree(G);
 100 }