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99 lines
2.4 KiB
C
99 lines
2.4 KiB
C
/* mpi-mpow.c - MPI functions
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* Copyright (C) 1998, 1999, 2000 Free Software Foundation, Inc.
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*
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* This file is part of GnuPG.
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*
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* GnuPG is free software; you can redistribute it and/or modify
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* it under the terms of the GNU General Public License as published by
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* the Free Software Foundation; either version 3 of the License, or
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* (at your option) any later version.
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*
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* GnuPG is distributed in the hope that it will be useful,
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* but WITHOUT ANY WARRANTY; without even the implied warranty of
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* MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
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* GNU General Public License for more details.
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*
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* You should have received a copy of the GNU General Public License
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* along with this program; if not, see <http://www.gnu.org/licenses/>.
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*/
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#include <config.h>
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#include <stdio.h>
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#include <stdlib.h>
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#include "mpi-internal.h"
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#include "longlong.h"
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#include <assert.h>
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static int
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build_index( MPI *exparray, int k, int i, int t )
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{
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int j, bitno;
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int idx = 0;
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bitno = t-i;
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for(j=k-1; j >= 0; j-- ) {
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idx <<= 1;
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if( mpi_test_bit( exparray[j], bitno ) )
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idx |= 1;
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}
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return idx;
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}
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/****************
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* RES = (BASE[0] ^ EXP[0]) * (BASE[1] ^ EXP[1]) * ... * mod M
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*/
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void
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mpi_mulpowm( MPI res, MPI *basearray, MPI *exparray, MPI m)
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{
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int k; /* number of elements */
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int t; /* bit size of largest exponent */
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int i, j, idx;
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MPI *G; /* table with precomputed values of size 2^k */
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MPI tmp;
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for(k=0; basearray[k]; k++ )
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;
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assert(k);
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for(t=0, i=0; (tmp=exparray[i]); i++ ) {
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j = mpi_get_nbits(tmp);
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if( j > t )
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t = j;
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}
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assert(i==k);
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assert(t);
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assert( k < 10 );
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G = xmalloc_clear( (1<<k) * sizeof *G );
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/* and calculate */
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tmp = mpi_alloc( mpi_get_nlimbs(m)+1 );
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mpi_set_ui( res, 1 );
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for(i = 1; i <= t; i++ ) {
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mpi_mulm(tmp, res, res, m );
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idx = build_index( exparray, k, i, t );
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assert( idx >= 0 && idx < (1<<k) );
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if( !G[idx] ) {
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if( !idx )
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G[0] = mpi_alloc_set_ui( 1 );
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else {
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for(j=0; j < k; j++ ) {
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if( (idx & (1<<j) ) ) {
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if( !G[idx] )
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G[idx] = mpi_copy( basearray[j] );
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else
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mpi_mulm( G[idx], G[idx], basearray[j], m );
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}
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}
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if( !G[idx] )
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G[idx] = mpi_alloc(0);
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}
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}
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mpi_mulm(res, tmp, G[idx], m );
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}
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/* cleanup */
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mpi_free(tmp);
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for(i=0; i < (1<<k); i++ )
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mpi_free(G[i]);
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xfree(G);
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}
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