gnupg/g10/seskey.c

/* seskey.c -  make sesssion keys etc.
 *	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 <assert.h>

#include <gcrypt.h>
#include "util.h"
#include "main.h"
#include "i18n.h"


/****************
 * Make a session key and put it into DEK
 */
void
make_session_key( DEK *dek )
{
    GCRY_CIPHER_HD chd;
    int i, rc;

    dek->keylen = gcry_cipher_get_algo_keylen( dek->algo );

    if( !(chd = gcry_cipher_open( dek->algo, GCRY_CIPHER_MODE_CFB,
				       GCRY_CIPHER_SECURE
				       | (dek->algo >= 100 ?
					   0 : GCRY_CIPHER_ENABLE_SYNC) ))
			  ) {
	BUG();
    }

    gcry_randomize( dek->key, dek->keylen, GCRY_STRONG_RANDOM );
    for(i=0; i < 16; i++ ) {
	rc = gcry_cipher_setkey( chd, dek->key, dek->keylen );
	if( !rc ) {
	    gcry_cipher_close( chd );
	    return;
	}
	if( rc != GCRYERR_WEAK_KEY )
	    BUG();
	log_info(_("weak key created - retrying\n") );
	/* Renew the session key until we get a non-weak key. */
	gcry_randomize( dek->key, dek->keylen, GCRY_STRONG_RANDOM );
    }
    log_fatal(_(
	    "cannot avoid weak key for symmetric cipher; tried %d times!\n"),
		  i);
}


/****************
 * Encode the session key. NBITS is the number of bits which should be used
 * for packing the session key.
 * returns: A mpi with the session key (caller must free)
 */
MPI
encode_session_key( DEK *dek, unsigned nbits )
{
    int nframe = (nbits+7) / 8;
    byte *p;
    byte *frame;
    int i,n;
    u16 csum;
    MPI a;

    /* the current limitation is that we can only use a session key
     * whose length is a multiple of BITS_PER_MPI_LIMB
     * I think we can live with that.
     */
    if( dek->keylen + 7 > nframe || !nframe )
	log_bug("can't encode a %d bit key in a %d bits frame\n",
		    dek->keylen*8, nbits );

    /* We encode the session key in this way:
     *
     *	   0  2  RND(n bytes)  0  A  DEK(k bytes)  CSUM(2 bytes)
     *
     * (But how can we store the leading 0 - the external representaion
     *	of MPIs doesn't allow leading zeroes =:-)
     *
     * RND are non-zero random bytes.
     * A   is the cipher algorithm
     * DEK is the encryption key (session key) length k depends on the
     *	   cipher algorithm (20 is used with blowfish160).
     * CSUM is the 16 bit checksum over the DEK
     */
    csum = 0;
    for( p = dek->key, i=0; i < dek->keylen; i++ )
	csum += *p++;

    frame = gcry_xmalloc_secure( nframe );
    n = 0;
    frame[n++] = 0;
    frame[n++] = 2;
    i = nframe - 6 - dek->keylen;
    assert( i > 0 );
    p = gcry_random_bytes_secure( i, GCRY_STRONG_RANDOM );
    /* replace zero bytes by new values */
    for(;;) {
	int j, k;
	byte *pp;

	/* count the zero bytes */
	for(j=k=0; j < i; j++ )
	    if( !p[j] )
		k++;
	if( !k )
	    break; /* okay: no zero bytes */
	k += k/128; /* better get some more */
	pp = gcry_random_bytes_secure( k, GCRY_STRONG_RANDOM);
	for(j=0; j < i && k ; j++ )
	    if( !p[j] )
		p[j] = pp[--k];
	gcry_free(pp);
    }
    memcpy( frame+n, p, i );
    gcry_free(p);
    n += i;
    frame[n++] = 0;
    frame[n++] = dek->algo;
    memcpy( frame+n, dek->key, dek->keylen ); n += dek->keylen;
    frame[n++] = csum >>8;
    frame[n++] = csum;
    assert( n == nframe );
    if( gcry_mpi_scan( &a, GCRYMPI_FMT_USG, frame, &nframe ) )
	BUG();
    gcry_free(frame);

    return a;
}


static MPI
do_encode_md( GCRY_MD_HD md, int algo, size_t len, unsigned nbits,
				       const byte *asn, size_t asnlen )
{
    int nframe = (nbits+7) / 8;
    byte *frame;
    int i,n;
    MPI a;

    if( len + asnlen + 4  > nframe )
	log_bug("can't encode a %d bit MD into a %d bits frame\n",
		    (int)(len*8), (int)nbits);

    /* We encode the MD in this way:
     *
     *	   0  A PAD(n bytes)   0  ASN(asnlen bytes)  MD(len bytes)
     *
     * PAD consists of FF bytes.
     */
    frame = gcry_md_is_secure(md)? gcry_xmalloc_secure( nframe )
				 : gcry_xmalloc( nframe );
    n = 0;
    frame[n++] = 0;
    frame[n++] = algo;
    i = nframe - len - asnlen -3 ;
    assert( i > 1 );
    memset( frame+n, 0xff, i ); n += i;
    frame[n++] = 0;
    memcpy( frame+n, asn, asnlen ); n += asnlen;
    memcpy( frame+n, gcry_md_read(md, algo), len ); n += len;
    assert( n == nframe );
    if( gcry_mpi_scan( &a, GCRYMPI_FMT_USG, frame, &nframe ) )
	BUG();
    gcry_free(frame);
    return a;
}


MPI
encode_md_value( int pubkey_algo, GCRY_MD_HD md, int hash_algo, unsigned nbits )
{
    int algo = hash_algo? hash_algo : gcry_md_get_algo(md);
    MPI frame;

    if( pubkey_algo == GCRY_PK_DSA ) {
	size_t n = gcry_md_get_algo_dlen(hash_algo);
	if( gcry_mpi_scan( &frame, GCRYMPI_FMT_USG,
				   gcry_md_read(md, hash_algo), &n ) )
	    BUG();
    }
    else {
	byte *asn;
	size_t asnlen;

	if( gcry_md_algo_info( algo, GCRYCTL_GET_ASNOID, NULL, &asnlen ) )
	    log_fatal("can't get OID of algo %d: %s\n",
					    algo, gcry_strerror(-1));
	asn = gcry_xmalloc( asnlen );
	if( gcry_md_algo_info( algo, GCRYCTL_GET_ASNOID, asn, &asnlen ) )
	    BUG();
	frame = do_encode_md( md, algo, gcry_md_get_algo_dlen( algo ),
						     nbits, asn, asnlen );
	gcry_free( asn );
    }
    return frame;
}