mirror of git://git.gnupg.org/gnupg.git
268 lines
7.2 KiB
C
268 lines
7.2 KiB
C
/* seskey.c - make sesssion keys etc.
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* Copyright (C) 1998, 1999, 2000, 2001, 2002, 2003, 2004, 2006,
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* 2007 Free Software Foundation, Inc.
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*
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* This file is part of GnuPG.
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*
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* GnuPG is free software; you can redistribute it and/or modify
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* it under the terms of the GNU General Public License as published by
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* the Free Software Foundation; either version 3 of the License, or
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* (at your option) any later version.
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*
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* GnuPG is distributed in the hope that it will be useful,
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* but WITHOUT ANY WARRANTY; without even the implied warranty of
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* MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
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* GNU General Public License for more details.
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*
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* You should have received a copy of the GNU General Public License
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* along with this program; if not, see <http://www.gnu.org/licenses/>.
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*/
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#include <config.h>
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#include <stdio.h>
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#include <stdlib.h>
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#include <string.h>
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#include <assert.h>
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#include "util.h"
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#include "cipher.h"
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#include "mpi.h"
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#include "main.h"
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#include "i18n.h"
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/****************
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* Make a session key and put it into DEK
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*/
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void
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make_session_key( DEK *dek )
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{
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CIPHER_HANDLE chd;
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int i, rc;
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dek->keylen = cipher_get_keylen( dek->algo ) / 8;
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chd = cipher_open( dek->algo, CIPHER_MODE_AUTO_CFB, 1 );
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randomize_buffer( dek->key, dek->keylen, 1 );
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for(i=0; i < 16; i++ ) {
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rc = cipher_setkey( chd, dek->key, dek->keylen );
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if( !rc ) {
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cipher_close( chd );
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return;
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}
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log_info(_("weak key created - retrying\n") );
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/* Renew the session key until we get a non-weak key. */
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randomize_buffer( dek->key, dek->keylen, 1 );
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}
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log_fatal(_(
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"cannot avoid weak key for symmetric cipher; tried %d times!\n"),
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i);
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}
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/****************
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* Encode the session key. NBITS is the number of bits which should be used
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* for packing the session key.
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* returns: A mpi with the session key (caller must free)
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*/
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MPI
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encode_session_key( DEK *dek, unsigned nbits )
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{
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int nframe = (nbits+7) / 8;
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byte *p;
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byte *frame;
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int i,n;
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u16 csum;
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MPI a;
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/* the current limitation is that we can only use a session key
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* whose length is a multiple of BITS_PER_MPI_LIMB
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* I think we can live with that.
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*/
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if( dek->keylen + 7 > nframe || !nframe )
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log_bug("can't encode a %d bit key in a %d bits frame\n",
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dek->keylen*8, nbits );
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/* We encode the session key in this way:
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*
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* 0 2 RND(n bytes) 0 A DEK(k bytes) CSUM(2 bytes)
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*
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* (But how can we store the leading 0 - the external representaion
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* of MPIs doesn't allow leading zeroes =:-)
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*
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* RND are non-zero random bytes.
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* A is the cipher algorithm
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* DEK is the encryption key (session key) length k depends on the
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* cipher algorithm (20 is used with blowfish160).
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* CSUM is the 16 bit checksum over the DEK
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*/
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csum = 0;
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for( p = dek->key, i=0; i < dek->keylen; i++ )
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csum += *p++;
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frame = xmalloc_secure( nframe );
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n = 0;
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frame[n++] = 0;
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frame[n++] = 2;
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i = nframe - 6 - dek->keylen;
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assert( i > 0 );
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p = get_random_bits( i*8, 1, 1 );
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/* replace zero bytes by new values */
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for(;;) {
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int j, k;
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byte *pp;
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/* count the zero bytes */
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for(j=k=0; j < i; j++ )
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if( !p[j] )
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k++;
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if( !k )
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break; /* okay: no zero bytes */
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k += k/128 + 3; /* better get some more */
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pp = get_random_bits( k*8, 1, 1);
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for(j=0; j < i && k ;) {
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if( !p[j] )
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p[j] = pp[--k];
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if (p[j])
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j++;
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}
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xfree(pp);
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}
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memcpy( frame+n, p, i );
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xfree(p);
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n += i;
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frame[n++] = 0;
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frame[n++] = dek->algo;
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memcpy( frame+n, dek->key, dek->keylen ); n += dek->keylen;
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frame[n++] = csum >>8;
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frame[n++] = csum;
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assert( n == nframe );
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a = mpi_alloc_secure ( mpi_nlimb_hint_from_nbytes (nframe) );
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mpi_set_buffer( a, frame, nframe, 0 );
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xfree(frame);
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return a;
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}
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MPI
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pkcs1_encode_md( MD_HANDLE md, int algo, size_t len, unsigned nbits,
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const byte *asn, size_t asnlen )
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{
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int nframe = (nbits+7) / 8;
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byte *frame;
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int i,n;
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MPI a;
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if( len + asnlen + 4 > nframe )
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log_bug("can't encode a %d bit MD into a %d bits frame\n",
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(int)(len*8), (int)nbits);
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/* We encode the MD in this way:
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*
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* 0 1 PAD(n bytes) 0 ASN(asnlen bytes) MD(len bytes)
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*
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* PAD consists of FF bytes.
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*/
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frame = md_is_secure(md)? xmalloc_secure( nframe ) : xmalloc( nframe );
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n = 0;
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frame[n++] = 0;
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frame[n++] = 1; /* block type */
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i = nframe - len - asnlen -3 ;
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assert( i > 1 );
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memset( frame+n, 0xff, i ); n += i;
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frame[n++] = 0;
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memcpy( frame+n, asn, asnlen ); n += asnlen;
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memcpy( frame+n, md_read(md, algo), len ); n += len;
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assert( n == nframe );
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a = (md_is_secure(md)
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? mpi_alloc_secure ( mpi_nlimb_hint_from_nbytes (nframe) )
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: mpi_alloc ( mpi_nlimb_hint_from_nbytes (nframe )));
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mpi_set_buffer( a, frame, nframe, 0 );
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xfree(frame);
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/* Note that PGP before version 2.3 encoded the MD as:
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*
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* 0 1 MD(16 bytes) 0 PAD(n bytes) 1
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*
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* The MD is always 16 bytes here because it's always MD5. We do
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* not support pre-v2.3 signatures, but I'm including this comment
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* so the information is easily found in the future.
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*/
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return a;
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}
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/****************
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* Encode a message digest into an MPI.
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* If it's for a DSA signature, make sure that the hash is large
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* enough to fill up q. If the hash is too big, take the leftmost
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* bits.
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*/
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MPI
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encode_md_value( PKT_public_key *pk, PKT_secret_key *sk,
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MD_HANDLE md, int hash_algo )
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{
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MPI frame;
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assert(hash_algo);
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assert(pk || sk);
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if((pk?pk->pubkey_algo:sk->pubkey_algo) == PUBKEY_ALGO_DSA)
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{
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/* It's a DSA signature, so find out the size of q. */
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unsigned int qbytes=mpi_get_nbits(pk?pk->pkey[1]:sk->skey[1]);
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/* Make sure it is a multiple of 8 bits. */
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if(qbytes%8)
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{
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log_error(_("DSA requires the hash length to be a"
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" multiple of 8 bits\n"));
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return NULL;
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}
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/* Don't allow any q smaller than 160 bits. This might need a
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revisit as the DSA2 design firms up, but for now, we don't
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want someone to issue signatures from a key with a 16-bit q
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or something like that, which would look correct but allow
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trivial forgeries. Yes, I know this rules out using MD5 with
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DSA. ;) */
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if(qbytes<160)
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{
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log_error(_("DSA key %s uses an unsafe (%u bit) hash\n"),
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pk?keystr_from_pk(pk):keystr_from_sk(sk),qbytes);
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return NULL;
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}
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qbytes/=8;
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/* Check if we're too short. Too long is safe as we'll
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automatically left-truncate. */
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if(md_digest_length(hash_algo) < qbytes)
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{
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log_error(_("DSA key %s requires a %u bit or larger hash\n"),
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pk?keystr_from_pk(pk):keystr_from_sk(sk),qbytes*8);
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return NULL;
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}
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frame = (md_is_secure(md)
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? mpi_alloc_secure (mpi_nlimb_hint_from_nbytes (qbytes) )
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: mpi_alloc ( mpi_nlimb_hint_from_nbytes (qbytes) ));
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mpi_set_buffer( frame, md_read(md, hash_algo), qbytes, 0 );
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}
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else
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{
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const byte *asn;
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size_t asnlen,mdlen;
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asn = md_asn_oid( hash_algo, &asnlen, &mdlen );
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frame = pkcs1_encode_md( md, hash_algo, mdlen,
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mpi_get_nbits(pk?pk->pkey[0]:sk->skey[0]),
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asn, asnlen );
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}
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return frame;
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}
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