g10/seskey.c

   1 /* seskey.c -  make sesssion keys etc.
   2  * Copyright (C) 1998, 1999, 2000, 2001, 2002, 2003, 2004,
   3  *               2006 Free Software Foundation, Inc.
   4  *
   5  * This file is part of GnuPG.
   6  *
   7  * GnuPG is free software; you can redistribute it and/or modify
   8  * it under the terms of the GNU General Public License as published by
   9  * the Free Software Foundation; either version 3 of the License, or
  10  * (at your option) any later version.
  11  *
  12  * GnuPG is distributed in the hope that it will be useful,
  13  * but WITHOUT ANY WARRANTY; without even the implied warranty of
  14  * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
  15  * GNU General Public License for more details.
  16  *
  17  * You should have received a copy of the GNU General Public License
  18  * along with this program; if not, see <http://www.gnu.org/licenses/>.
  19  */
  20
  21 #include <config.h>
  22 #include <stdio.h>
  23 #include <stdlib.h>
  24 #include <string.h>
  25 #include <assert.h>
  26
  27 #include "gpg.h"
  28 #include "util.h"
  29 #include "cipher.h"
  30 #include "main.h"
  31 #include "i18n.h"
  32
  33
  34 /****************
  35  * Make a session key and put it into DEK
  36  */
  37 void
  38 make_session_key( DEK *dek )
  39 {
  40     gcry_cipher_hd_t chd;
  41     int i, rc;
  42
  43     dek->keylen = gcry_cipher_get_algo_keylen (dek->algo);
  44
  45     if (gcry_cipher_open (&chd, dek->algo, GCRY_CIPHER_MODE_CFB,
  46                           (GCRY_CIPHER_SECURE
  47                            | (dek->algo >= 100 ?
  48                               0 : GCRY_CIPHER_ENABLE_SYNC))) )
  49       BUG();
  50     gcry_randomize (dek->key, dek->keylen, GCRY_STRONG_RANDOM );
  51     for (i=0; i < 16; i++ )
  52       {
  53         rc = gcry_cipher_setkey (chd, dek->key, dek->keylen);
  54         if (!rc)
  55           {
  56             gcry_cipher_close (chd);
  57             return;
  58           }
  59         if (gpg_err_code (rc) != GPG_ERR_WEAK_KEY)
  60           BUG();
  61         log_info(_("weak key created - retrying\n") );
  62         /* Renew the session key until we get a non-weak key. */
  63         gcry_randomize (dek->key, dek->keylen, GCRY_STRONG_RANDOM);
  64       }
  65     log_fatal (_("cannot avoid weak key for symmetric cipher; "
  66                  "tried %d times!\n"), i);
  67 }
  68
  69
  70 /****************
  71  * Encode the session key. NBITS is the number of bits which should be used
  72  * for packing the session key.
  73  * returns: A mpi with the session key (caller must free)
  74  */
  75 gcry_mpi_t
  76 encode_session_key (DEK *dek, unsigned int nbits)
  77 {
  78     size_t nframe = (nbits+7) / 8;
  79     byte *p;
  80     byte *frame;
  81     int i,n;
  82     u16 csum;
  83     gcry_mpi_t a;
  84
  85     /* The current limitation is that we can only use a session key
  86      * whose length is a multiple of BITS_PER_MPI_LIMB
  87      * I think we can live with that.
  88      */
  89     if( dek->keylen + 7 > nframe || !nframe )
  90         log_bug("can't encode a %d bit key in a %d bits frame\n",
  91                     dek->keylen*8, nbits );
  92
  93     /* We encode the session key in this way:
  94      *
  95      *     0  2  RND(n bytes)  0  A  DEK(k bytes)  CSUM(2 bytes)
  96      *
  97      * (But how can we store the leading 0 - the external representaion
  98      *  of MPIs doesn't allow leading zeroes =:-)
  99      *
 100      * RND are non-zero random bytes.
 101      * A   is the cipher algorithm
 102      * DEK is the encryption key (session key) length k depends on the
 103      *     cipher algorithm (20 is used with blowfish160).
 104      * CSUM is the 16 bit checksum over the DEK
 105      */
 106     csum = 0;
 107     for( p = dek->key, i=0; i < dek->keylen; i++ )
 108         csum += *p++;
 109
 110     frame = xmalloc_secure( nframe );
 111     n = 0;
 112     frame[n++] = 0;
 113     frame[n++] = 2;
 114     i = nframe - 6 - dek->keylen;
 115     assert( i > 0 );
 116     p = gcry_random_bytes_secure (i, GCRY_STRONG_RANDOM);
 117     /* Replace zero bytes by new values. */
 118     for(;;) {
 119         int j, k;
 120         byte *pp;
 121
 122         /* count the zero bytes */
 123         for(j=k=0; j < i; j++ )
 124             if( !p[j] )
 125                 k++;
 126         if( !k )
 127             break; /* okay: no zero bytes */
 128         k += k/128 + 3; /* better get some more */
 129         pp = gcry_random_bytes_secure (k, GCRY_STRONG_RANDOM);
 130         for(j=0; j < i && k ;) {
 131             if( !p[j] )
 132                 p[j] = pp[--k];
 133             if (p[j])
 134               j++;
 135         }
 136         xfree(pp);
 137     }
 138     memcpy( frame+n, p, i );
 139     xfree(p);
 140     n += i;
 141     frame[n++] = 0;
 142     frame[n++] = dek->algo;
 143     memcpy( frame+n, dek->key, dek->keylen ); n += dek->keylen;
 144     frame[n++] = csum >>8;
 145     frame[n++] = csum;
 146     assert( n == nframe );
 147     if (gcry_mpi_scan( &a, GCRYMPI_FMT_USG, frame, n, &nframe))
 148       BUG();
 149     xfree(frame);
 150     return a;
 151 }
 152
 153
 154 static gcry_mpi_t
 155 do_encode_md( gcry_md_hd_t md, int algo, size_t len, unsigned nbits,
 156               const byte *asn, size_t asnlen )
 157 {
 158     size_t nframe = (nbits+7) / 8;
 159     byte *frame;
 160     int i,n;
 161     gcry_mpi_t a;
 162
 163     if( len + asnlen + 4  > nframe )
 164         log_bug("can't encode a %d bit MD into a %d bits frame\n",
 165                     (int)(len*8), (int)nbits);
 166
 167     /* We encode the MD in this way:
 168      *
 169      *     0  1 PAD(n bytes)   0  ASN(asnlen bytes)  MD(len bytes)
 170      *
 171      * PAD consists of FF bytes.
 172      */
 173     frame = gcry_md_is_secure (md)? xmalloc_secure (nframe) : xmalloc (nframe);
 174     n = 0;
 175     frame[n++] = 0;
 176     frame[n++] = 1; /* block type */
 177     i = nframe - len - asnlen -3 ;
 178     assert( i > 1 );
 179     memset( frame+n, 0xff, i ); n += i;
 180     frame[n++] = 0;
 181     memcpy( frame+n, asn, asnlen ); n += asnlen;
 182     memcpy( frame+n, gcry_md_read (md, algo), len ); n += len;
 183     assert( n == nframe );
 184
 185     if (gcry_mpi_scan( &a, GCRYMPI_FMT_USG, frame, n, &nframe ))
 186         BUG();
 187     xfree(frame);
 188
 189     /* Note that PGP before version 2.3 encoded the MD as:
 190      *
 191      *   0   1   MD(16 bytes)   0   PAD(n bytes)   1
 192      *
 193      * The MD is always 16 bytes here because it's always MD5.  We do
 194      * not support pre-v2.3 signatures, but I'm including this comment
 195      * so the information is easily found in the future.
 196      */
 197
 198     return a;
 199 }
 200
 201
 202 /****************
 203  * Encode a message digest into an MPI.
 204  * If it's for a DSA signature, make sure that the hash is large
 205  * enough to fill up q.  If the hash is too big, take the leftmost
 206  * bits.
 207  */
 208 gcry_mpi_t
 209 encode_md_value (PKT_public_key *pk, PKT_secret_key *sk,
 210                  gcry_md_hd_t md, int hash_algo)
 211 {
 212   gcry_mpi_t frame;
 213
 214   assert(hash_algo);
 215   assert(pk || sk);
 216
 217   if((pk?pk->pubkey_algo:sk->pubkey_algo) == GCRY_PK_DSA)
 218     {
 219       /* It's a DSA signature, so find out the size of q. */
 220
 221       size_t qbytes = gcry_mpi_get_nbits (pk?pk->pkey[1]:sk->skey[1]);
 222
 223       /* Make sure it is a multiple of 8 bits. */
 224
 225       if(qbytes%8)
 226         {
 227           log_error(_("DSA requires the hash length to be a"
 228                       " multiple of 8 bits\n"));
 229           return NULL;
 230         }
 231
 232       /* Don't allow any q smaller than 160 bits.  This might need a
 233          revisit as the DSA2 design firms up, but for now, we don't
 234          want someone to issue signatures from a key with a 16-bit q
 235          or something like that, which would look correct but allow
 236          trivial forgeries.  Yes, I know this rules out using MD5 with
 237          DSA. ;) */
 238       if (qbytes < 160)
 239         {
 240           log_error (_("DSA key %s uses an unsafe (%u bit) hash\n"),
 241                      pk?keystr_from_pk(pk):keystr_from_sk(sk),
 242                      (unsigned int)qbytes);
 243           return NULL;
 244         }
 245
 246       qbytes/=8;
 247
 248       /* Check if we're too short.  Too long is safe as we'll
 249          automatically left-truncate. */
 250       if (gcry_md_get_algo_dlen (hash_algo) < qbytes)
 251         {
 252           log_error (_("DSA key %s requires a %u bit or larger hash\n"),
 253                      pk?keystr_from_pk(pk):keystr_from_sk(sk),
 254                      (unsigned int)(qbytes*8));
 255           return NULL;
 256         }
 257
 258       if (gcry_mpi_scan (&frame, GCRYMPI_FMT_USG,
 259                          gcry_md_read (md, hash_algo), qbytes, &qbytes))
 260         BUG();
 261     }
 262   else
 263     {
 264       gpg_error_t rc;
 265       byte *asn;
 266       size_t asnlen;
 267
 268       rc = gcry_md_test_algo (hash_algo);
 269       if (!rc)
 270         rc = gcry_md_algo_info (hash_algo, GCRYCTL_GET_ASNOID, NULL, &asnlen);
 271       if (rc)
 272         log_fatal ("can't get OID of algo %d: %s\n",
 273                    hash_algo, gpg_strerror (rc));
 274       asn = xmalloc (asnlen);
 275       if ( gcry_md_algo_info (hash_algo, GCRYCTL_GET_ASNOID, asn, &asnlen) )
 276         BUG();
 277       frame = do_encode_md (md, hash_algo, gcry_md_get_algo_dlen (hash_algo),
 278                             gcry_mpi_get_nbits (pk?pk->pkey[0]:sk->skey[0]),
 279                             asn, asnlen);
 280       xfree (asn);
 281     }
 282
 283   return frame;
 284 }