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246 lines
6.0 KiB
C
246 lines
6.0 KiB
C
/* dsa.c - DSA signature scheme
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* Copyright (c) 1997 by Werner Koch (dd9jn)
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*
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* This file is part of G10.
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*
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* G10 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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* G10 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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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->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->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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static void
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test_keys( DSA_public_key *pk, DSA_secret_key *sk, unsigned nbits )
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{
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MPI test = mpi_alloc( nbits / BITS_PER_MPI_LIMB );
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MPI out1_a = mpi_alloc( nbits / BITS_PER_MPI_LIMB );
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MPI out1_b = mpi_alloc( nbits / BITS_PER_MPI_LIMB );
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MPI out2 = mpi_alloc( nbits / BITS_PER_MPI_LIMB );
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mpi_set_bytes( test, nbits, get_random_byte, 0 );
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dsa_sign( out1_a, out1_b, test, sk );
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if( !dsa_verify( out1_a, out1_b, test, pk ) )
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log_fatal("DSA operation: sign, verify failed\n");
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mpi_free( test );
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mpi_free( out1_a );
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mpi_free( out1_b );
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mpi_free( out2 );
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}
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/****************
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* generate a random secret exponent k from prime p, so
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* that k is relatively prime to p-1
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*/
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static MPI
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gen_k( MPI p )
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{
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MPI k = mpi_alloc_secure( mpi_get_nlimbs(p) );
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MPI temp = mpi_alloc( mpi_get_nlimbs(p) );
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MPI p_1 = mpi_copy(p);
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unsigned nbits = mpi_get_nbits(p);
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if( DBG_CIPHER )
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log_debug("choosing a random k ");
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mpi_sub_ui( p_1, p, 1);
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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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mpi_set_highbit( k, nbits-1 ); /* make sure it's high (really needed?) */
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if( mpi_cmp( k, p_1 ) >= 0 )
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continue; /* is not smaller than (p-1) */
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if( mpi_gcd( temp, k, p_1 ) )
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break; /* okay, k is relatively prime to (p-1) */
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}
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if( DBG_CIPHER )
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fputc('\n', stderr);
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mpi_free(p_1);
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mpi_free(temp);
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return k;
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}
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/****************
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* Generate a key pair with a key of size NBITS
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* Returns: 2 structures filles with all needed values
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*/
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void
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dsa_generate( DSA_public_key *pk, DSA_secret_key *sk, unsigned nbits )
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{
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MPI p; /* the prime */
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MPI g;
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MPI x; /* the secret exponent */
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MPI y;
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p = generate_public_prime( nbits );
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/* FIXME: check wether we shall assert that (p-1)/2 is also prime
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* Schneier votes against it
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*/
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g = mpi_alloc_set_ui(3);
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/* select a random number */
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x = mpi_alloc_secure( nbits/BITS_PER_MPI_LIMB );
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if( DBG_CIPHER )
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log_debug("choosing a random x ");
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do {
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if( DBG_CIPHER )
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fputc('.', stderr);
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mpi_set_bytes( x, nbits, get_random_byte, 1 ); /* fixme: should be 2 */
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mpi_set_highbit( x, nbits-1 ); /* make sure it's high (needed?) */
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} while( mpi_cmp( x, p ) >= 0 ); /* x must be smaller than p */
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y = mpi_alloc(nbits/BITS_PER_MPI_LIMB);
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mpi_powm( y, g, x, p );
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if( DBG_CIPHER ) {
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fputc('\n', stderr);
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log_mpidump("dsa p= ", p );
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log_mpidump("dsa g= ", g );
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log_mpidump("dsa y= ", y );
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log_mpidump("dsa x= ", x );
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}
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/* copy the stuff to the key structures */
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pk->p = mpi_copy(p);
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pk->g = mpi_copy(g);
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pk->y = mpi_copy(y);
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sk->p = p;
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sk->g = g;
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sk->y = y;
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sk->x = x;
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/* now we can test our keys (this should never fail!) */
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test_keys( pk, sk, nbits - 64 );
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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 an Elgamal signature out of INPUT
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*/
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void
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dsa_sign(MPI a, MPI b, MPI input, DSA_secret_key *skey )
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{
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MPI k;
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MPI t = mpi_alloc( mpi_get_nlimbs(a) );
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MPI inv = mpi_alloc( mpi_get_nlimbs(a) );
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MPI p_1 = mpi_copy(skey->p);
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/*
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* b = (t * inv) mod (p-1)
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* b = (t * inv(k,(p-1),(p-1)) mod (p-1)
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* b = (((M-x*a) mod (p-1)) * inv(k,(p-1),(p-1))) mod (p-1)
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*
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*/
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mpi_sub_ui(p_1, p_1, 1);
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k = gen_k( skey->p );
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mpi_powm( a, skey->g, k, skey->p );
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mpi_mul(t, skey->x, a );
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mpi_subm(t, input, t, p_1 );
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while( mpi_is_neg(t) )
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mpi_add(t, t, p_1);
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mpi_invm(inv, k, p_1 );
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mpi_mulm(b, t, inv, p_1 );
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#if 0
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if( DBG_CIPHER ) {
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log_mpidump("dsa sign p= ", skey->p);
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log_mpidump("dsa sign g= ", skey->g);
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log_mpidump("dsa sign y= ", skey->y);
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log_mpidump("dsa sign x= ", skey->x);
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log_mpidump("dsa sign k= ", k);
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log_mpidump("dsa sign M= ", input);
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log_mpidump("dsa sign a= ", a);
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log_mpidump("dsa sign b= ", b);
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}
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#endif
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mpi_free(k);
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mpi_free(t);
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mpi_free(inv);
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mpi_free(p_1);
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}
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/****************
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* Returns true if the signature composed from A and B is valid.
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*/
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int
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dsa_verify(MPI a, MPI b, MPI input, DSA_public_key *pkey )
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{
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int rc;
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MPI t1 = mpi_alloc( mpi_get_nlimbs(a) );
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MPI t2 = mpi_alloc( mpi_get_nlimbs(a) );
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mpi_powm( t1, pkey->y, a, pkey->p );
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mpi_powm( t2, a, b, pkey->p );
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mpi_mulm( t1, t1, t2, pkey->p );
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mpi_powm( t2, pkey->g, input, pkey->p );
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rc = !mpi_cmp( t1, t2 );
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mpi_free(t1);
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mpi_free(t2);
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return rc;
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}
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