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Werner Koch 1997-11-18 14:06:00 +00:00
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cipher/primegen.c Normal file
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/* primegen.c - prime number generator
* Copyright (c) 1997 by Werner Koch (dd9jn)
*
* This file is part of G10.
*
* G10 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.
*
* G10 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., 59 Temple Place - Suite 330, Boston, MA 02111-1307, USA
*/
#include <config.h>
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <assert.h>
#include "util.h"
#include "mpi.h"
#include "cipher.h"
static int no_of_small_prime_numbers;
static int rabin_miller( MPI n );
/****************
* Generate a prime number (stored in secure memory)
*/
MPI
generate_random_prime( unsigned nbits )
{
unsigned nlimbs;
MPI prime, val_2, val_3, result;
int i;
unsigned x, step;
unsigned count1, count2;
int *mods;
if( DBG_CIPHER )
log_debug("generate a prime of %u bits ", nbits );
if( !no_of_small_prime_numbers ) {
for(i=0; small_prime_numbers[i]; i++ )
no_of_small_prime_numbers++;
}
mods = m_alloc( no_of_small_prime_numbers * sizeof *mods );
/* make nbits fit into MPI implementation */
nlimbs = (nbits + BITS_PER_MPI_LIMB - 1) / BITS_PER_MPI_LIMB;
assert( nlimbs );
val_2 = mpi_alloc( nlimbs );
mpi_set_ui(val_2, 2);
val_3 = mpi_alloc( nlimbs );
mpi_set_ui(val_3, 3);
result = mpi_alloc( nlimbs );
prime = mpi_alloc_secure( nlimbs );
count1 = count2 = 0;
/* enter (endless) loop */
for(;;) {
/* generate a random number */
mpi_set_bytes( prime, nbits, get_random_byte, 2 );
/* set high order bit to 1, set low order bit to 1 */
mpi_set_bit( prime, nbits-1 );
mpi_set_bit( prime, 0 );
/* calculate all remainders */
for(i=0; (x = small_prime_numbers[i]); i++ )
mods[i] = mpi_fdiv_r_ui(NULL, prime, x);
for(step=0; step < 20000; step += 2 ) {
/* check against all the small primes we have in mods */
count1++;
for(i=0; (x = small_prime_numbers[i]); i++ ) {
while( mods[i] + step >= x )
mods[i] -= x;
if( !(mods[i] + step) )
break;
}
if( x )
continue; /* found a multiple of a already known prime */
if( DBG_CIPHER )
fputc('.', stderr);
mpi_add_ui( prime, prime, step );
/* do a Fermat test */
count2++;
mpi_powm( result, val_2, prime, prime );
if( mpi_cmp_ui(result, 2) )
continue; /* stepping (fermat test failed) */
if( DBG_CIPHER )
fputc('+', stderr);
/* and a second one */
count2++;
mpi_powm( result, val_3, prime, prime );
if( mpi_cmp_ui(result, 3) )
continue; /* stepping (fermat test failed) */
if( DBG_CIPHER )
fputc('+', stderr);
/* perform Rabin-Miller tests */
for(i=5; i > 0; i-- ) {
if( DBG_CIPHER )
fputc('+', stderr);
if( rabin_miller(prime) )
break;
}
if( !i ) {
if( !mpi_test_bit( prime, nbits-1 ) ) {
if( DBG_CIPHER ) {
fputc('\n', stderr);
log_debug("overflow in prime generation\n");
break; /* step loop, cont with a new prime */
}
}
if( DBG_CIPHER ) {
fputc('\n', stderr);
log_debug("performed %u simple and %u Fermat/Rabin-Miller tests\n",
count1, count2 );
log_mpidump("found prime: ", prime );
}
mpi_free(val_2);
mpi_free(val_3);
mpi_free(result);
m_free(mods);
return prime;
}
}
if( DBG_CIPHER )
fputc(':', stderr); /* restart with a new random value */
}
}
/****************
* Return 1 if n is not a prime
*/
static int
rabin_miller( MPI n )
{
return 0;
}