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/* dsa.c - DSA signature scheme
* Copyright (C) 1998 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., 59 Temple Place - Suite 330, Boston, MA 02111-1307, USA
*/
#include <config.h>
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include "util.h"
#include "mpi.h"
#include "cipher.h"
#include "dsa.h"
/****************
* Generate a random secret exponent k less than q
*/
static MPI
gen_k( MPI q )
{
MPI k = mpi_alloc_secure( mpi_get_nlimbs(q) );
unsigned nbits = mpi_get_nbits(q);
if( DBG_CIPHER )
log_debug("choosing a random k ");
for(;;) {
if( DBG_CIPHER )
fputc('.', stderr);
mpi_set_bytes( k, nbits , get_random_byte, 1 );
if( !(mpi_cmp( k, q ) < 0) ) /* check: k < q */
continue; /* no */
if( !(mpi_cmp_ui( k, 0 ) > 0) ) /* check: k > 0 */
continue; /* no */
break; /* okay */
}
if( DBG_CIPHER )
fputc('\n', stderr);
return k;
}
void
dsa_free_public_key( DSA_public_key *pk )
{
mpi_free( pk->p ); pk->p = NULL;
mpi_free( pk->q ); pk->q = NULL;
mpi_free( pk->g ); pk->g = NULL;
mpi_free( pk->y ); pk->y = NULL;
}
void
dsa_free_secret_key( DSA_secret_key *sk )
{
mpi_free( sk->p ); sk->p = NULL;
mpi_free( sk->q ); sk->q = NULL;
mpi_free( sk->g ); sk->g = NULL;
mpi_free( sk->y ); sk->y = NULL;
mpi_free( sk->x ); sk->x = NULL;
}
/****************
* Test whether the secret key is valid.
* Returns: if this is a valid key.
*/
int
dsa_check_secret_key( DSA_secret_key *sk )
{
int rc;
MPI y = mpi_alloc( mpi_get_nlimbs(sk->y) );
mpi_powm( y, sk->g, sk->x, sk->p );
rc = !mpi_cmp( y, sk->y );
mpi_free( y );
return rc;
}
/****************
* Make a DSA signature from HASH and put it into r and s.
*/
void
dsa_sign(MPI r, MPI s, MPI hash, DSA_secret_key *skey )
{
MPI k;
MPI kinv;
MPI tmp;
/* select a random k with 0 < k < q */
k = gen_k( skey->q );
/* r = (a^k mod p) mod q */
mpi_powm( r, skey->g, k, skey->p );
mpi_fdiv_r( r, r, skey->q );
/* kinv = k^(-1) mod q */
kinv = mpi_alloc( mpi_get_nlimbs(k) );
mpi_invm(kinv, k, skey->q );
/* s = (kinv * ( hash + x * r)) mod q */
tmp = mpi_alloc( mpi_get_nlimbs(skey->p) );
mpi_mul( tmp, skey->x, r );
mpi_add( tmp, tmp, hash );
mpi_mulm( s , kinv, tmp, skey->q );
mpi_free(k);
mpi_free(kinv);
mpi_free(tmp);
}
/****************
* Returns true if the signature composed from R and S is valid.
*/
int
dsa_verify(MPI r, MPI s, MPI hash, DSA_public_key *pkey )
{
int rc;
MPI w, u1, u2, v;
MPI base[3];
MPI exp[3];
if( !(mpi_cmp_ui( r, 0 ) > 0 && mpi_cmp( r, pkey->q ) < 0) )
return 0; /* assertion 0 < r < q failed */
if( !(mpi_cmp_ui( s, 0 ) > 0 && mpi_cmp( s, pkey->q ) < 0) )
return 0; /* assertion 0 < s < q failed */
w = mpi_alloc( mpi_get_nlimbs(pkey->q) );
u1 = mpi_alloc( mpi_get_nlimbs(pkey->q) );
u2 = mpi_alloc( mpi_get_nlimbs(pkey->q) );
v = mpi_alloc( mpi_get_nlimbs(pkey->p) );
/* w = s^(-1) mod q */
mpi_invm( w, s, pkey->q );
/* u1 = (hash * w) mod q */
mpi_mulm( u1, hash, w, pkey->q );
/* u2 = r * w mod q */
mpi_mulm( u2, r, w, pkey->q );
/* v = g^u1 * y^u2 mod p mod q */
base[0] = pkey->g; exp[0] = u1;
base[1] = pkey->y; exp[1] = u2;
base[2] = NULL; exp[2] = NULL;
mpi_mulpowm( v, base, exp, pkey->p );
mpi_fdiv_r( v, v, pkey->q );
rc = !mpi_cmp( v, r );
mpi_free(w);
mpi_free(u1);
mpi_free(u2);
mpi_free(v);
return rc;
}
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