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/* seskey.c - make session keys etc.
* Copyright (C) 1998, 1999, 2000, 2001, 2002, 2003, 2004,
* 2006, 2009, 2010 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 <https://www.gnu.org/licenses/>.
*/
#include <config.h>
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include "gpg.h"
#include "../common/util.h"
#include "options.h"
#include "main.h"
#include "../common/i18n.h"
/* Generate a new session key in *DEK that is appropriate for the
* algorithm DEK->ALGO (i.e., ensure that the key is not weak).
*
* This function overwrites DEK->KEYLEN, DEK->KEY. The rest of the
* fields are left as is. */
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 stored in DEK as an MPI in preparation to
* encrypt it with the public key algorithm OPENPGP_PK_ALGO with a key
* whose length (the size of the public key) is NBITS.
*
* On success, returns an MPI, which the caller must free using
* gcry_mpi_release(). */
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;
if (DBG_CRYPTO)
log_debug ("encode_session_key: encoding %d byte DEK", dek->keylen);
csum = 0;
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 granularity; 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));
/* alg+key+csum fit and the size is congruent to 8. */
log_assert (!(nframe%8) && nframe > 1 + dek->keylen + 2 );
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 of padded bytes. */
memset (frame+n, i, i); /* Use it as the value of each padded byte. */
log_assert (n+i == nframe);
if (DBG_CRYPTO)
log_debug ("encode_session_key: "
"[%d] %02x %02x %02x ... %02x %02x %02x\n",
(int) nframe, frame[0], frame[1], frame[2],
frame[nframe-3], frame[nframe-2], frame[nframe-1]);
return gcry_mpi_set_opaque (NULL, frame, 8*nframe);
}
/* 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 according to PKCS#1 v1.5 (see section
* 13.1.1 of RFC 4880):
*
* 0 2 RND(i bytes) 0 A DEK(k bytes) CSUM(2 bytes)
*
* (But how can we store the leading 0 - the external representation
* of MPIs doesn't allow leading zeroes =:-)
*
* RND are (at least 1) 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;
/* The number of random bytes are the number of otherwise unused
bytes. See diagram above. */
i = nframe - 6 - dek->keylen;
log_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;
log_assert (n == nframe);
return gcry_mpi_set_opaque (NULL, frame, 8*n);
}
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_error ("can't encode a %d bit MD into a %d bits frame, algo=%d\n",
(int)(len*8), (int)nbits, algo);
return NULL;
}
/* 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 ;
log_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;
log_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;
size_t mdlen;
log_assert (hash_algo);
log_assert (pk);
if (pk->pubkey_algo == PUBKEY_ALGO_EDDSA)
{
/* EdDSA signs data of arbitrary length. Thus no special
treatment is required. */
frame = gcry_mpi_set_opaque_copy (NULL, gcry_md_read (md, hash_algo),
8*gcry_md_get_algo_dlen (hash_algo));
}
else if (pk->pubkey_algo == PUBKEY_ALGO_DSA
|| pk->pubkey_algo == PUBKEY_ALGO_ECDSA)
{
/* It's a DSA signature, so find out the size of q. */
size_t qbits = gcry_mpi_get_nbits (pk->pkey[1]);
/* pkey[1] is Q for ECDSA, which is an uncompressed point,
i.e. 04 <x> <y> */
if (pk->pubkey_algo == PUBKEY_ALGO_ECDSA)
qbits = ecdsa_qbits_from_Q (qbits);
/* Make sure it is a multiple of 8 bits. */
if ((qbits%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 (qbits < 160)
{
log_error (_("%s key %s uses an unsafe (%zu bit) hash\n"),
openpgp_pk_algo_name (pk->pubkey_algo),
keystr_from_pk (pk), qbits);
return NULL;
}
/* ECDSA 521 is special has it is larger than the largest hash
we have (SHA-512). Thus we change the size for further
processing to 512. */
if (pk->pubkey_algo == PUBKEY_ALGO_ECDSA && qbits > 512)
qbits = 512;
/* Check if we're too short. Too long is safe as we'll
automatically left-truncate. */
mdlen = gcry_md_get_algo_dlen (hash_algo);
if (mdlen < qbits/8)
{
log_error (_("%s key %s requires a %zu bit or larger hash "
"(hash is %s)\n"),
openpgp_pk_algo_name (pk->pubkey_algo),
keystr_from_pk (pk), qbits,
gcry_md_algo_name (hash_algo));
return NULL;
}
/* Note that we do the truncation by passing QBITS/8 as length to
mpi_scan. */
if (gcry_mpi_scan (&frame, GCRYMPI_FMT_USG,
gcry_md_read (md, hash_algo), qbits/8, NULL))
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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