mirror of
git://git.gnupg.org/gnupg.git
synced 2024-11-04 20:38:50 +01:00
272 lines
7.4 KiB
C
272 lines
7.4 KiB
C
/* 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( (nframe+BYTES_PER_MPI_LIMB-1) / BYTES_PER_MPI_LIMB );
|
|
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( (nframe+BYTES_PER_MPI_LIMB-1) / BYTES_PER_MPI_LIMB )
|
|
: mpi_alloc( (nframe+BYTES_PER_MPI_LIMB-1) / BYTES_PER_MPI_LIMB );
|
|
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((qbytes+BYTES_PER_MPI_LIMB-1)
|
|
/ BYTES_PER_MPI_LIMB )
|
|
: mpi_alloc((qbytes+BYTES_PER_MPI_LIMB-1) / BYTES_PER_MPI_LIMB );
|
|
|
|
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;
|
|
}
|