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gnupg/g10/seskey.c

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/* seskey.c - make sesssion keys etc.
* Copyright (C) 1998, 1999, 2000, 2001, 2003 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 "gpg.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 )
{
gcry_cipher_hd_t chd;
int i, rc;
dek->keylen = gcry_cipher_get_algo_keylen (dek->algo);
if (gcry_cipher_open (&chd, 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 (gpg_err_code (rc) != GPG_ERR_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)
*/
gcry_mpi_t
encode_session_key (DEK *dek, unsigned int nbits)
{
int nframe = (nbits+7) / 8;
byte *p;
byte *frame;
int i,n;
u16 csum;
gcry_mpi_t 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];
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 );
if (gcry_mpi_scan( &a, GCRYMPI_FMT_USG, frame, &nframe))
BUG();
xfree (frame);
return a;
}
static gcry_mpi_t
do_encode_md( gcry_md_hd_t md, int algo, size_t len, unsigned nbits,
const byte *asn, size_t asnlen, int v3compathack )
{
int nframe = (nbits+7) / 8;
byte *frame;
int i,n;
gcry_mpi_t 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)? xmalloc_secure (nframe): xmalloc (nframe);
n = 0;
frame[n++] = 0;
frame[n++] = v3compathack? algo : 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, gcry_md_read (md, algo), len ); n += len;
assert( n == nframe );
if (gcry_mpi_scan( &a, GCRYMPI_FMT_USG, frame, &nframe ))
BUG();
xfree (frame);
return a;
}
/****************
* Encode a message digest into an MPI.
* v3compathack is used to work around a bug in old GnuPG versions
* which did put the algo identifier inseatd of the block type 1 into
* the encoded value. Setting this flag forces the old behaviour.
*/
gcry_mpi_t
encode_md_value (int pubkey_algo, gcry_md_hd_t md, int hash_algo,
unsigned int nbits, int v3compathack )
{
int algo = hash_algo? hash_algo : gcry_md_get_algo (md);
gcry_mpi_t frame;
if (pubkey_algo == GCRY_PK_DSA)
{
size_t n = gcry_md_get_algo_dlen(hash_algo);
if (n != 20)
{
log_error (_("DSA requires the use of a 160 bit hash algorithm\n"));
return NULL;
}
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 = 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, v3compathack );
xfree (asn);
}
return frame;
}
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