Line data Source code
1 : /* seskey.c - make sesssion keys etc.
2 : * Copyright (C) 1998, 1999, 2000, 2001, 2002, 2003, 2004,
3 : * 2006, 2009, 2010 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 "options.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
38 236 : make_session_key( DEK *dek )
39 : {
40 : gcry_cipher_hd_t chd;
41 : int i, rc;
42 :
43 236 : dek->keylen = openpgp_cipher_get_algo_keylen (dek->algo);
44 :
45 236 : if (openpgp_cipher_open (&chd, dek->algo, GCRY_CIPHER_MODE_CFB,
46 : (GCRY_CIPHER_SECURE
47 : | (dek->algo >= 100 ?
48 : 0 : GCRY_CIPHER_ENABLE_SYNC))) )
49 0 : BUG();
50 236 : gcry_randomize (dek->key, dek->keylen, GCRY_STRONG_RANDOM );
51 236 : for (i=0; i < 16; i++ )
52 : {
53 236 : rc = gcry_cipher_setkey (chd, dek->key, dek->keylen);
54 236 : if (!rc)
55 : {
56 236 : gcry_cipher_close (chd);
57 236 : return;
58 : }
59 0 : if (gpg_err_code (rc) != GPG_ERR_WEAK_KEY)
60 0 : BUG();
61 0 : log_info(_("weak key created - retrying\n") );
62 : /* Renew the session key until we get a non-weak key. */
63 0 : gcry_randomize (dek->key, dek->keylen, GCRY_STRONG_RANDOM);
64 : }
65 0 : 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
76 236 : encode_session_key (int openpgp_pk_algo, DEK *dek, unsigned int nbits)
77 : {
78 236 : 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 :
85 236 : if (DBG_CRYPTO)
86 0 : log_debug ("encode_session_key: encoding %d byte DEK", dek->keylen);
87 :
88 236 : csum = 0;
89 6076 : for (p = dek->key, i=0; i < dek->keylen; i++)
90 5840 : csum += *p++;
91 :
92 : /* Shortcut for ECDH. It's padding is minimal to simply make the
93 : output be a multiple of 8 bytes. */
94 236 : if (openpgp_pk_algo == PUBKEY_ALGO_ECDH)
95 : {
96 : /* Pad to 8 byte granulatiry; the padding byte is the number of
97 : * padded bytes.
98 : *
99 : * A DEK(k bytes) CSUM(2 bytes) 0x 0x 0x 0x ... 0x
100 : * +---- x times ---+
101 : */
102 42 : nframe = (( 1 + dek->keylen + 2 /* The value so far is always odd. */
103 21 : + 7 ) & (~7));
104 :
105 : /* alg+key+csum fit and the size is congruent to 8. */
106 21 : assert (!(nframe%8) && nframe > 1 + dek->keylen + 2 );
107 :
108 21 : frame = xmalloc_secure (nframe);
109 21 : n = 0;
110 21 : frame[n++] = dek->algo;
111 21 : memcpy (frame+n, dek->key, dek->keylen);
112 21 : n += dek->keylen;
113 21 : frame[n++] = csum >> 8;
114 21 : frame[n++] = csum;
115 21 : i = nframe - n; /* Number of padded bytes. */
116 21 : memset (frame+n, i, i); /* Use it as the value of each padded byte. */
117 21 : assert (n+i == nframe);
118 :
119 21 : if (DBG_CRYPTO)
120 0 : log_debug ("encode_session_key: "
121 : "[%d] %02x %02x %02x ... %02x %02x %02x\n",
122 0 : (int) nframe, frame[0], frame[1], frame[2],
123 0 : frame[nframe-3], frame[nframe-2], frame[nframe-1]);
124 :
125 21 : if (gcry_mpi_scan (&a, GCRYMPI_FMT_USG, frame, nframe, &nframe))
126 0 : BUG();
127 21 : xfree(frame);
128 21 : return a;
129 : }
130 :
131 : /* The current limitation is that we can only use a session key
132 : * whose length is a multiple of BITS_PER_MPI_LIMB
133 : * I think we can live with that.
134 : */
135 215 : if (dek->keylen + 7 > nframe || !nframe)
136 0 : log_bug ("can't encode a %d bit key in a %d bits frame\n",
137 0 : dek->keylen*8, nbits );
138 :
139 : /* We encode the session key in this way:
140 : *
141 : * 0 2 RND(n bytes) 0 A DEK(k bytes) CSUM(2 bytes)
142 : *
143 : * (But how can we store the leading 0 - the external representaion
144 : * of MPIs doesn't allow leading zeroes =:-)
145 : *
146 : * RND are non-zero random bytes.
147 : * A is the cipher algorithm
148 : * DEK is the encryption key (session key) length k depends on the
149 : * cipher algorithm (20 is used with blowfish160).
150 : * CSUM is the 16 bit checksum over the DEK
151 : */
152 :
153 215 : frame = xmalloc_secure( nframe );
154 215 : n = 0;
155 215 : frame[n++] = 0;
156 215 : frame[n++] = 2;
157 215 : i = nframe - 6 - dek->keylen;
158 215 : assert( i > 0 );
159 215 : p = gcry_random_bytes_secure (i, GCRY_STRONG_RANDOM);
160 : /* Replace zero bytes by new values. */
161 : for (;;)
162 : {
163 : int j, k;
164 : byte *pp;
165 :
166 : /* Count the zero bytes. */
167 22352 : for (j=k=0; j < i; j++ )
168 22088 : if (!p[j])
169 60 : k++;
170 264 : if (!k)
171 215 : break; /* Okay: no zero bytes. */
172 49 : k += k/128 + 3; /* Better get some more. */
173 49 : pp = gcry_random_bytes_secure (k, GCRY_STRONG_RANDOM);
174 4100 : for (j=0; j < i && k ;)
175 : {
176 4002 : if (!p[j])
177 60 : p[j] = pp[--k];
178 4002 : if (p[j])
179 4002 : j++;
180 : }
181 49 : xfree (pp);
182 49 : }
183 215 : memcpy (frame+n, p, i);
184 215 : xfree (p);
185 215 : n += i;
186 215 : frame[n++] = 0;
187 215 : frame[n++] = dek->algo;
188 215 : memcpy (frame+n, dek->key, dek->keylen );
189 215 : n += dek->keylen;
190 215 : frame[n++] = csum >>8;
191 215 : frame[n++] = csum;
192 215 : assert (n == nframe);
193 215 : if (gcry_mpi_scan( &a, GCRYMPI_FMT_USG, frame, n, &nframe))
194 0 : BUG();
195 215 : xfree (frame);
196 215 : return a;
197 : }
198 :
199 :
200 : static gcry_mpi_t
201 36 : do_encode_md( gcry_md_hd_t md, int algo, size_t len, unsigned nbits,
202 : const byte *asn, size_t asnlen )
203 : {
204 36 : size_t nframe = (nbits+7) / 8;
205 : byte *frame;
206 : int i,n;
207 : gcry_mpi_t a;
208 :
209 36 : if( len + asnlen + 4 > nframe )
210 0 : log_bug ("can't encode a %d bit MD into a %d bits frame, algo=%d\n",
211 : (int)(len*8), (int)nbits, algo);
212 :
213 : /* We encode the MD in this way:
214 : *
215 : * 0 1 PAD(n bytes) 0 ASN(asnlen bytes) MD(len bytes)
216 : *
217 : * PAD consists of FF bytes.
218 : */
219 36 : frame = gcry_md_is_secure (md)? xmalloc_secure (nframe) : xmalloc (nframe);
220 36 : n = 0;
221 36 : frame[n++] = 0;
222 36 : frame[n++] = 1; /* block type */
223 36 : i = nframe - len - asnlen -3 ;
224 36 : assert( i > 1 );
225 36 : memset( frame+n, 0xff, i ); n += i;
226 36 : frame[n++] = 0;
227 36 : memcpy( frame+n, asn, asnlen ); n += asnlen;
228 36 : memcpy( frame+n, gcry_md_read (md, algo), len ); n += len;
229 36 : assert( n == nframe );
230 :
231 36 : if (gcry_mpi_scan( &a, GCRYMPI_FMT_USG, frame, n, &nframe ))
232 0 : BUG();
233 36 : xfree(frame);
234 :
235 : /* Note that PGP before version 2.3 encoded the MD as:
236 : *
237 : * 0 1 MD(16 bytes) 0 PAD(n bytes) 1
238 : *
239 : * The MD is always 16 bytes here because it's always MD5. We do
240 : * not support pre-v2.3 signatures, but I'm including this comment
241 : * so the information is easily found in the future.
242 : */
243 :
244 36 : return a;
245 : }
246 :
247 :
248 : /****************
249 : * Encode a message digest into an MPI.
250 : * If it's for a DSA signature, make sure that the hash is large
251 : * enough to fill up q. If the hash is too big, take the leftmost
252 : * bits.
253 : */
254 : gcry_mpi_t
255 367 : encode_md_value (PKT_public_key *pk, gcry_md_hd_t md, int hash_algo)
256 : {
257 : gcry_mpi_t frame;
258 : size_t mdlen;
259 :
260 367 : assert (hash_algo);
261 367 : assert (pk);
262 :
263 367 : if (pk->pubkey_algo == PUBKEY_ALGO_EDDSA)
264 : {
265 : /* EdDSA signs data of arbitrary length. Thus no special
266 : treatment is required. */
267 0 : frame = gcry_mpi_set_opaque_copy (NULL, gcry_md_read (md, hash_algo),
268 0 : 8*gcry_md_get_algo_dlen (hash_algo));
269 : }
270 367 : else if (pk->pubkey_algo == PUBKEY_ALGO_DSA
271 73 : || pk->pubkey_algo == PUBKEY_ALGO_ECDSA)
272 331 : {
273 : /* It's a DSA signature, so find out the size of q. */
274 :
275 331 : size_t qbits = gcry_mpi_get_nbits (pk->pkey[1]);
276 :
277 : /* pkey[1] is Q for ECDSA, which is an uncompressed point,
278 : i.e. 04 <x> <y> */
279 331 : if (pk->pubkey_algo == PUBKEY_ALGO_ECDSA)
280 37 : qbits = ecdsa_qbits_from_Q (qbits);
281 :
282 : /* Make sure it is a multiple of 8 bits. */
283 331 : if ((qbits%8))
284 : {
285 0 : log_error(_("DSA requires the hash length to be a"
286 : " multiple of 8 bits\n"));
287 0 : return NULL;
288 : }
289 :
290 : /* Don't allow any q smaller than 160 bits. This might need a
291 : revisit as the DSA2 design firms up, but for now, we don't
292 : want someone to issue signatures from a key with a 16-bit q
293 : or something like that, which would look correct but allow
294 : trivial forgeries. Yes, I know this rules out using MD5 with
295 : DSA. ;) */
296 331 : if (qbits < 160)
297 : {
298 0 : log_error (_("%s key %s uses an unsafe (%zu bit) hash\n"),
299 0 : openpgp_pk_algo_name (pk->pubkey_algo),
300 : keystr_from_pk (pk), qbits);
301 0 : return NULL;
302 : }
303 :
304 :
305 : /* ECDSA 521 is special has it is larger than the largest hash
306 : we have (SHA-512). Thus we chnage the size for further
307 : processing to 512. */
308 331 : if (pk->pubkey_algo == PUBKEY_ALGO_ECDSA && qbits > 512)
309 11 : qbits = 512;
310 :
311 : /* Check if we're too short. Too long is safe as we'll
312 : automatically left-truncate. */
313 331 : mdlen = gcry_md_get_algo_dlen (hash_algo);
314 331 : if (mdlen < qbits/8)
315 : {
316 0 : log_error (_("%s key %s requires a %zu bit or larger hash "
317 : "(hash is %s)\n"),
318 0 : openpgp_pk_algo_name (pk->pubkey_algo),
319 : keystr_from_pk (pk), qbits,
320 : gcry_md_algo_name (hash_algo));
321 0 : return NULL;
322 : }
323 :
324 : /* Note that we do the truncation by passing QBITS/8 as length to
325 : mpi_scan. */
326 662 : if (gcry_mpi_scan (&frame, GCRYMPI_FMT_USG,
327 331 : gcry_md_read (md, hash_algo), qbits/8, NULL))
328 0 : BUG();
329 : }
330 : else
331 : {
332 : gpg_error_t rc;
333 : byte *asn;
334 : size_t asnlen;
335 :
336 36 : rc = gcry_md_algo_info (hash_algo, GCRYCTL_GET_ASNOID, NULL, &asnlen);
337 36 : if (rc)
338 0 : log_fatal ("can't get OID of digest algorithm %d: %s\n",
339 : hash_algo, gpg_strerror (rc));
340 36 : asn = xtrymalloc (asnlen);
341 36 : if (!asn)
342 0 : return NULL;
343 36 : if ( gcry_md_algo_info (hash_algo, GCRYCTL_GET_ASNOID, asn, &asnlen) )
344 0 : BUG();
345 36 : frame = do_encode_md (md, hash_algo, gcry_md_get_algo_dlen (hash_algo),
346 : gcry_mpi_get_nbits (pk->pkey[0]), asn, asnlen);
347 36 : xfree (asn);
348 : }
349 :
350 367 : return frame;
351 : }
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