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/* pubkey-enc.c - public key encoded packet handling
* Copyright (c) 1997 by Werner Koch (dd9jn)
*
* This file is part of G10.
*
* G10 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.
*
* G10 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 "util.h"
#include "memory.h"
#include "packet.h"
#include "mpi.h"
#include "keydb.h"
#include "cipher.h"
/****************
* Get the session key from a pubkey enc paket and return
* it in DEK, which should have been allocated in secure memory.
*/
int
get_session_key( PKT_pubkey_enc *k, DEK *dek )
{
int i, j, c, rc = 0;
RSA_secret_key *skey = m_alloc_secure( sizeof *skey );
MPI dek_frame = mpi_alloc_secure(40);
u16 csum, csum2;
if( k->pubkey_algo != PUBKEY_ALGO_RSA ) {
rc = G10ERR_PUBKEY_ALGO; /* unsupported algorithm */
goto leave;
}
/* get the secret key for the given public key
* and decode the rsa_integer
*/
if( (rc = get_seckey( skey, k->keyid )) )
goto leave;
if( DBG_CIPHER )
log_mpidump("Encr DEK frame:", k->d.rsa.rsa_integer );
rsa_secret( dek_frame, k->d.rsa.rsa_integer, skey );
/* Now get the DEK (data encryption key) from the dek_frame
*
* Old versions encode the DEK in in this format (msb is left):
*
* 0 1 DEK(16 bytes) CSUM(2 bytes) 0 RND(n bytes) 2
*
* Later versions encode the DEK like this:
*
* 0 2 RND(n bytes) 0 A DEK(k bytes) CSUM(2 bytes)
*
* RND are non-zero randow bytes.
* A is the cipher algorithm ( 1 for IDEA, 42 for blowfish )
* DEK is the encryption key (session key) with length k
* (16 for idea, 42 for blowfish)
* CSUM
*/
if( DBG_CIPHER )
log_mpidump("DEK frame:", dek_frame );
for(i=0; mpi_getbyte(dek_frame, i) != -1; i++ )
;
for(i--; i >= 0 && !(c=mpi_getbyte(dek_frame, i)); i--)
; /* Skip leading zeroes */
if( i < 16 )
{ rc = G10ERR_WRONG_SECKEY; goto leave; }
if( c == 1 && mpi_getbyte(dek_frame,0) == 2 ) {
log_error("old encoding of DEK is not supported\n");
rc = G10ERR_CIPHER_ALGO;
goto leave;
}
if( c != 2 ) /* somethink is wrong */
{ rc = G10ERR_WRONG_SECKEY; goto leave; }
/* look for the zeor byte */
for(i--; i > 4 ; i-- )
if( !mpi_getbyte(dek_frame,i) )
break;
if( i <= 4 ) /* zero byte not found */
{ rc = G10ERR_WRONG_SECKEY; goto leave; }
/* next byte indicates the used cipher */
switch( mpi_getbyte(dek_frame, --i ) ) {
case 1:
rc = G10ERR_NI_CIPHER;
goto leave;
case 42:
if( i != 22 ) /* length of blowfish is 20 (+2 bytes checksum) */
{ rc = G10ERR_WRONG_SECKEY; goto leave; }
dek->algo = CIPHER_ALGO_BLOWFISH;
break;
default:
rc = G10ERR_CIPHER_ALGO;
goto leave;
}
/* copy the key to DEK and compare the checksum */
csum = mpi_getbyte(dek_frame, 1) << 8;
csum |= mpi_getbyte(dek_frame, 0);
dek->keylen = i - 2;
for( i--, csum2=0, j=0; i > 1; i-- )
csum2 += dek->key[j++] = mpi_getbyte(dek_frame, i);
if( csum != csum2 ) {
rc = G10ERR_WRONG_SECKEY;
goto leave;
}
if( DBG_CIPHER )
log_hexdump("DEK is:", dek->key, dek->keylen );
leave:
mpi_free(dek_frame);
m_free(skey);
return rc;
}
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