/* seskey.c - make sesssion keys etc.
* Copyright (C) 1998, 1999, 2000, 2001, 2002, 2003, 2004,
* 2006 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., 51 Franklin Street, Fifth Floor, Boston, MA 02110-1301,
* USA.
*/
#include <config.h>
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <assert.h>
#include "util.h"
#include "cipher.h"
#include "mpi.h"
#include "main.h"
#include "i18n.h"
/****************
* Make a session key and put it into DEK
*/
void
make_session_key( DEK *dek )
{
CIPHER_HANDLE chd;
int i, rc;
dek->keylen = cipher_get_keylen( dek->algo ) / 8;
chd = cipher_open( dek->algo, CIPHER_MODE_AUTO_CFB, 1 );
randomize_buffer( dek->key, dek->keylen, 1 );
for(i=0; i < 16; i++ ) {
rc = cipher_setkey( chd, dek->key, dek->keylen );
if( !rc ) {
cipher_close( chd );
return;
}
log_info(_("weak key created - retrying\n") );
/* Renew the session key until we get a non-weak key. */
randomize_buffer( dek->key, dek->keylen, 1 );
}
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 = xmalloc_secure( nframe );
n = 0;
frame[n++] = 0;
frame[n++] = 2;
i = nframe - 6 - dek->keylen;
assert( i > 0 );
p = get_random_bits( i*8, 1, 1 );
/* 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 + 3; /* better get some more */
pp = get_random_bits( k*8, 1, 1);
for(j=0; j < i && k ;) {
if( !p[j] )
p[j] = pp[--k];
if (p[j])
j++;
}
xfree(pp);
}
memcpy( frame+n, p, i );
xfree(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 );
a = mpi_alloc_secure ( mpi_nlimb_hint_from_nbytes (nframe) );
mpi_set_buffer( a, frame, nframe, 0 );
xfree(frame);
return a;
}
static MPI
do_encode_md( MD_HANDLE 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 1 PAD(n bytes) 0 ASN(asnlen bytes) MD(len bytes)
*
* PAD consists of FF bytes.
*/
frame = md_is_secure(md)? xmalloc_secure( nframe ) : xmalloc( nframe );
n = 0;
frame[n++] = 0;
frame[n++] = 1; /* block type */
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, md_read(md, algo), len ); n += len;
assert( n == nframe );
a = (md_is_secure(md)
? mpi_alloc_secure ( mpi_nlimb_hint_from_nbytes (nframe) )
: mpi_alloc ( mpi_nlimb_hint_from_nbytes (nframe )));
mpi_set_buffer( a, frame, nframe, 0 );
xfree(frame);
/* Note that PGP before version 2.3 encoded the MD as:
*
* 0 1 MD(16 bytes) 0 PAD(n bytes) 1
*
* The MD is always 16 bytes here because it's always MD5. We do
* not support pre-v2.3 signatures, but I'm including this comment
* so the information is easily found in the future.
*/
return a;
}
/****************
* Encode a message digest into an MPI.
* If it's for a DSA signature, make sure that the hash is large
* enough to fill up q. If the hash is too big, take the leftmost
* bits.
*/
MPI
encode_md_value( PKT_public_key *pk, PKT_secret_key *sk,
MD_HANDLE md, int hash_algo )
{
MPI frame;
assert(hash_algo);
assert(pk || sk);
if((pk?pk->pubkey_algo:sk->pubkey_algo) == PUBKEY_ALGO_DSA)
{
/* It's a DSA signature, so find out the size of q. */
unsigned int qbytes=mpi_get_nbits(pk?pk->pkey[1]:sk->skey[1]);
/* Make sure it is a multiple of 8 bits. */
if(qbytes%8)
{
log_error(_("DSA requires the hash length to be a"
" multiple of 8 bits\n"));
return NULL;
}
/* Don't allow any q smaller than 160 bits. This might need a
revisit as the DSA2 design firms up, but for now, we don't
want someone to issue signatures from a key with a 16-bit q
or something like that, which would look correct but allow
trivial forgeries. Yes, I know this rules out using MD5 with
DSA. ;) */
if(qbytes<160)
{
log_error(_("DSA key %s uses an unsafe (%u bit) hash\n"),
pk?keystr_from_pk(pk):keystr_from_sk(sk),qbytes);
return NULL;
}
qbytes/=8;
/* Check if we're too short. Too long is safe as we'll
automatically left-truncate. */
if(md_digest_length(hash_algo) < qbytes)
{
log_error(_("DSA key %s requires a %u bit or larger hash\n"),
pk?keystr_from_pk(pk):keystr_from_sk(sk),qbytes*8);
return NULL;
}
frame = (md_is_secure(md)
? mpi_alloc_secure (mpi_nlimb_hint_from_nbytes (qbytes) )
: mpi_alloc ( mpi_nlimb_hint_from_nbytes (qbytes) ));
mpi_set_buffer( frame, md_read(md, hash_algo), qbytes, 0 );
}
else
{
const byte *asn;
size_t asnlen,mdlen;
asn = md_asn_oid( hash_algo, &asnlen, &mdlen );
frame = do_encode_md( md, hash_algo, mdlen,
mpi_get_nbits(pk?pk->pkey[0]:sk->skey[0]),
asn, asnlen );
}
return frame;
}