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