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/* mpi-mpow.c - MPI functions
* Copyright (C) 1998, 1999, 2000 Free Software Foundation, Inc.
*
* This file is part of GnuPG.
*
* GnuPG is free software; you can redistribute it and/or modify
* it under the terms of the GNU General Public License as published by
* the Free Software Foundation; either version 2 of the License, or
* (at your option) any later version.
*
* GnuPG is distributed in the hope that it will be useful,
* but WITHOUT ANY WARRANTY; without even the implied warranty of
* MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
* GNU General Public License for more details.
*
* You should have received a copy of the GNU General Public License
* along with this program; if not, write to the Free Software
* Foundation, Inc., 51 Franklin Street, Fifth Floor, Boston, MA 02110-1301,
* USA.
*/
#include <config.h>
#include <stdio.h>
#include <stdlib.h>
#include "mpi-internal.h"
#include "longlong.h"
#include <assert.h>
static int
build_index( MPI *exparray, int k, int i, int t )
{
int j, bitno;
int idx = 0;
bitno = t-i;
for(j=k-1; j >= 0; j-- ) {
idx <<= 1;
if( mpi_test_bit( exparray[j], bitno ) )
idx |= 1;
}
return idx;
}
/****************
* RES = (BASE[0] ^ EXP[0]) * (BASE[1] ^ EXP[1]) * ... * mod M
*/
void
mpi_mulpowm( MPI res, MPI *basearray, MPI *exparray, MPI m)
{
int k; /* number of elements */
int t; /* bit size of largest exponent */
int i, j, idx;
MPI *G; /* table with precomputed values of size 2^k */
MPI tmp;
for(k=0; basearray[k]; k++ )
;
assert(k);
for(t=0, i=0; (tmp=exparray[i]); i++ ) {
j = mpi_get_nbits(tmp);
if( j > t )
t = j;
}
assert(i==k);
assert(t);
assert( k < 10 );
G = xmalloc_clear( (1<<k) * sizeof *G );
/* and calculate */
tmp = mpi_alloc( mpi_get_nlimbs(m)+1 );
mpi_set_ui( res, 1 );
for(i = 1; i <= t; i++ ) {
mpi_mulm(tmp, res, res, m );
idx = build_index( exparray, k, i, t );
assert( idx >= 0 && idx < (1<<k) );
if( !G[idx] ) {
if( !idx )
G[0] = mpi_alloc_set_ui( 1 );
else {
for(j=0; j < k; j++ ) {
if( (idx & (1<<j) ) ) {
if( !G[idx] )
G[idx] = mpi_copy( basearray[j] );
else
mpi_mulm( G[idx], G[idx], basearray[j], m );
}
}
if( !G[idx] )
G[idx] = mpi_alloc(0);
}
}
mpi_mulm(res, tmp, G[idx], m );
}
/* cleanup */
mpi_free(tmp);
for(i=0; i < (1<<k); i++ )
mpi_free(G[i]);
xfree(G);
}
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