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/* seskey.c - make sesssion keys etc.
* Copyright (C) 1998, 1999, 2000, 2001, 2002, 2003, 2004,
* 2006, 2009 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 3 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, see <http://www.gnu.org/licenses/>.
*/
#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 "options.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 = openpgp_cipher_get_algo_keylen (dek->algo);
if (openpgp_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 (int openpgp_pk_algo, DEK *dek, unsigned int nbits)
{
size_t nframe = (nbits+7) / 8;
byte *p;
byte *frame;
int i,n;
u16 csum = 0;
gcry_mpi_t a;
if( DBG_CIPHER )
log_debug("encode_session_key: encoding %d byte DEK", dek->keylen);
for( p = dek->key, i=0; i < dek->keylen; i++ )
csum += *p++;
/* Shortcut for ECDH. It's padding is minimal to simply make the output be a multiple of 8 bytes. */
if( openpgp_pk_algo == PUBKEY_ALGO_ECDH ) {
/* pad to 8 byte granulatiry; the padding byte is the number of padded bytes.
* A DEK(k bytes) CSUM(2 bytes) 0x 0x 0x 0x ... 0x
* +---- x times ---+
*/
nframe = ( 1 + dek->keylen + 2 /* the value so far is always odd */ + 7 ) & (~7);
assert( !(nframe%8) && nframe > 1 + dek->keylen + 2 ); /* alg+key+csum fit and the size is congruent to 8 */
frame = xmalloc_secure( nframe );
n = 0;
frame[n++] = dek->algo;
memcpy( frame+n, dek->key, dek->keylen ); n += dek->keylen;
frame[n++] = csum >>8;
frame[n++] = csum;
i = nframe - n; /* number padded bytes */
memset( frame+n, i, i );/* use it as the value of each padded byte */
assert( n+i == nframe );
if( DBG_CIPHER )
log_debug("encode_session_key: [%d] %02x %02x %02x ... %02x %02x %02x", nframe, frame[0],frame[1],frame[2], frame[nframe-3],frame[nframe-2],frame[nframe-1]);
if (gcry_mpi_scan( &a, GCRYMPI_FMT_USG, frame, nframe, &nframe))
BUG();
xfree(frame);
return 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
*/
frame = 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 + 3; /* better get some more */
pp = gcry_random_bytes_secure (k, GCRY_STRONG_RANDOM);
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 );
if (gcry_mpi_scan( &a, GCRYMPI_FMT_USG, frame, n, &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 )
{
size_t 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, algo=%d\n",
(int)(len*8), (int)nbits, algo);
/* 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 = gcry_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, gcry_md_read (md, algo), len ); n += len;
assert( n == nframe );
if (gcry_mpi_scan( &a, GCRYMPI_FMT_USG, frame, n, &nframe ))
BUG();
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.
*/
gcry_mpi_t
encode_md_value (PKT_public_key *pk, gcry_md_hd_t md, int hash_algo)
{
gcry_mpi_t frame;
int gcry_pkalgo;
assert (hash_algo);
assert (pk);
gcry_pkalgo = map_pk_openpgp_to_gcry( pk->pubkey_algo );
if (gcry_pkalgo == GCRY_PK_DSA || gcry_pkalgo == GCRY_PK_ECDSA )
{
/* It's a DSA signature, so find out the size of q. */
size_t qbytes = gcry_mpi_get_nbits (pk->pkey[1]);
/* pkey[1] is Q for ECDSA, which is an uncompressed point, i.e. 04 <x> <y> */
if( gcry_pkalgo==GCRY_PK_ECDSA )
qbytes = ecdsa_qbits_from_Q( qbytes );
/* 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 (_("%s key %s uses an unsafe (%zu bit) hash\n"),
gcry_pk_algo_name( gcry_pkalgo ),
keystr_from_pk (pk), qbytes);
return NULL;
}
qbytes /= 8;
/* Check if we're too short. Too long is safe as we'll
automatically left-truncate. */
/* This checks would require the use of SHA512 with ECDSA 512. I think this is overkill to fail in this case.
* Therefore, relax the check, but only for ECDSA keys. We may need to adjust it later for general case.
* ( Note that the check will never pass for ECDSA 521 anyway as the only hash that intended to match it is SHA 512, but 512 < 521 ).
*/
//if (gcry_md_get_algo_dlen (hash_algo) < qbytes )
if (gcry_md_get_algo_dlen (hash_algo) < ((gcry_pkalgo==GCRY_PK_ECDSA && qbytes>(521)/8) ? 512/8 : qbytes) )
{
log_error (_("%s key %s requires a %zu bit or larger hash, used hash-algo=%d\n"),
gcry_pk_algo_name( gcry_pkalgo ),
keystr_from_pk(pk), qbytes*8, hash_algo);
return NULL;
}
if (gcry_mpi_scan (&frame, GCRYMPI_FMT_USG,
gcry_md_read (md, hash_algo), qbytes, &qbytes))
BUG();
}
else
{
gpg_error_t rc;
byte *asn;
size_t asnlen;
rc = gcry_md_algo_info (hash_algo, GCRYCTL_GET_ASNOID, NULL, &asnlen);
if (rc)
log_fatal ("can't get OID of digest algorithm %d: %s\n",
hash_algo, gpg_strerror (rc));
asn = xtrymalloc (asnlen);
if (!asn)
return NULL;
if ( gcry_md_algo_info (hash_algo, GCRYCTL_GET_ASNOID, asn, &asnlen) )
BUG();
frame = do_encode_md (md, hash_algo, gcry_md_get_algo_dlen (hash_algo),
gcry_mpi_get_nbits (pk->pkey[0]), asn, asnlen);
xfree (asn);
}
return frame;
}

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