3 * $Id: keyset.c,v 1.5 2001/06/19 22:07:43 mdw Exp $
5 * Handling of symmetric keysets
7 * (c) 2001 Straylight/Edgeware
10 /*----- Licensing notice --------------------------------------------------*
12 * This file is part of Trivial IP Encryption (TrIPE).
14 * TrIPE is free software; you can redistribute it and/or modify
15 * it under the terms of the GNU General Public License as published by
16 * the Free Software Foundation; either version 2 of the License, or
17 * (at your option) any later version.
19 * TrIPE is distributed in the hope that it will be useful,
20 * but WITHOUT ANY WARRANTY; without even the implied warranty of
21 * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
22 * GNU General Public License for more details.
24 * You should have received a copy of the GNU General Public License
25 * along with TrIPE; if not, write to the Free Software Foundation,
26 * Inc., 59 Temple Place - Suite 330, Boston, MA 02111-1307, USA.
29 /*----- Revision history --------------------------------------------------*
32 * Revision 1.5 2001/06/19 22:07:43 mdw
33 * Change the encrypted packet format to be non-malleable.
35 * Revision 1.4 2001/06/16 14:06:40 mdw
36 * Quantify collision probabilities for the stated data volume bounds.
38 * Revision 1.3 2001/02/16 21:39:55 mdw
39 * Major overhaul. Separate functions for manipulating keysets from
40 * functions for manipulating keyset lists. Introduce a concept of
41 * listening-only keys.
43 * Revision 1.2 2001/02/05 19:53:23 mdw
44 * Add sequence number protection.
46 * Revision 1.1 2001/02/03 20:26:37 mdw
51 /*----- Header files ------------------------------------------------------*/
55 /*----- Tunable parameters ------------------------------------------------*/
57 /* --- Note on size limits --- *
59 * For a 64-bit block cipher (e.g., Blowfish), the probability of a collision
60 * occurring after 32 MB is less than %$2^{-21}$%, and the probability of a
61 * collision occurring after 64 MB is less than %$2^{-19}$%.
64 #define T_EXP MIN(60) /* Expiry time for a key */
65 #define T_REGEN MIN(45) /* Regeneration time for a key */
66 #define SZ_EXP MEG(64) /* Expiry data size for a key */
67 #define SZ_REGEN MEG(32) /* Data size threshold for regen */
69 /*----- Handy macros ------------------------------------------------------*/
71 #define KEYOK(ks, now) ((ks)->sz_exp > 0 && (ks)->t_exp > now)
73 /*----- Low-level packet encryption and decryption ------------------------*/
75 /* --- Encrypted data format --- *
77 * Let %$p_i$% be the %$i$%-th plaintext message. We first compute
79 * %$c_i = \mathcal{E}\textrm{-CBC}_{K_{\text{E}}}(p_i)$%
81 * as the CBC-ciphertext of %$p_i$%, and then
83 * %$\sigma_i = \mathcal{T}_{K_{\text{M}}}(i, c_i)$%
85 * as a MAC on the %%\emph{ciphertext}%%. The message sent is then the pair
86 * %$(\sigma_i, c_i)$%. This construction is provably secure in the NM-CCA
87 * sense (assuming that the cipher is IND-CPA, and the MAC is SUF-CMA)
88 * [Bellare and Namprempre].
90 * This also ensures that, assuming the key is good, we have a secure channel
91 * [Krawczyk]. Actually, [Krawczyk] shows that, if the cipher is either a
92 * simple stream cipher or a block cipher in CBC mode, we can use the MAC-
93 * then-encrypt scheme and still have a secure channel. However, I like the
94 * NM-CCA guarantee from [Bellare and Namprempre]. I'm less worried about
95 * the Horton Principle [Wagner and Schneier].
98 /* --- @doencrypt@ --- *
100 * Arguments: @keyset *ks@ = pointer to keyset to use
101 * @buf *b@ = pointer to an input buffer
102 * @buf *bb@ = pointer to an output buffer
104 * Returns: Zero if OK, nonzero if a new key is required.
106 * Use: Encrypts a message with the given key. We assume that the
107 * keyset is OK to use.
110 static int doencrypt(keyset *ks, buf *b, buf *bb)
114 const octet *p = BCUR(b);
115 size_t sz = BLEFT(b);
116 octet *qmac, *qseq, *qiv, *qpk;
121 /* --- Allocate the required buffer space --- */
124 if (buf_ensure(bb, MACSZ + SEQSZ + IVSZ + sz))
125 return (0); /* Caution! */
126 qmac = BCUR(bb); qseq = qmac + MACSZ; qiv = qseq + SEQSZ; qpk = qiv + IVSZ;
127 BSTEP(bb, MACSZ + SEQSZ + IVSZ + sz);
129 /* --- Encrypt the packet --- */
131 oseq = ks->oseq++; STORE32(qseq, oseq);
132 rand_get(RAND_GLOBAL, qiv, IVSZ);
133 c->ops->setiv(c, qiv);
134 c->ops->encrypt(c, p, qpk, sz);
135 IF_TRACING(T_KEYSET, {
136 trace(T_KEYSET, "keyset: encrypting packet %lu using keyset %u",
137 (unsigned long)oseq, ks->seq);
138 trace_block(T_CRYPTO, "crypto: encrypted packet", qpk, sz);
141 /* --- Now compute the MAC --- */
143 h = ks->mout->ops->init(ks->mout);
144 h->ops->hash(h, qseq, SEQSZ + IVSZ + sz);
145 memcpy(qmac, h->ops->done(h, 0), MACSZ);
147 IF_TRACING(T_KEYSET, {
148 trace_block(T_CRYPTO, "crypto: computed MAC", qmac, MACSZ);
151 /* --- Deduct the packet size from the key's data life --- */
158 if (osz >= SZ_REGEN && nsz < SZ_REGEN) {
159 T( trace(T_KEYSET, "keyset: keyset %u data regen limit exceeded -- "
160 "forcing exchange", ks->seq); )
167 /* --- @dodecrypt@ --- *
169 * Arguments: @keyset *ks@ = pointer to keyset to use
170 * @buf *b@ = pointer to an input buffer
171 * @buf *bb@ = pointer to an output buffer
172 * @uint32 *seq@ = where to store the sequence number
174 * Returns: Zero if OK, nonzero if it failed.
176 * Use: Attempts to decrypt a message with the given key. No other
177 * checking (e.g., sequence number checks) is performed. We
178 * assume that the keyset is OK to use, and that there is
179 * sufficient output buffer space reserved. If the decryption
180 * is successful, the buffer pointer is moved past the decrypted
181 * packet, and the packet's sequence number is stored in @*seq@.
184 static int dodecrypt(keyset *ks, buf *b, buf *bb, uint32 *seq)
186 const octet *pmac, *piv, *pseq, *ppk;
187 size_t psz = BLEFT(b);
191 gcipher *c = ks->cin;
192 size_t ivsz = c->ops->c->blksz;
196 /* --- Break up the packet into its components --- */
198 if (psz < ivsz + 4) {
199 T( trace(T_KEYSET, "keyset: block too small for keyset %u", ks->seq); )
202 sz = psz - IVSZ - SEQSZ - MACSZ;
203 pmac = BCUR(b); pseq = pmac + MACSZ; piv = pseq + SEQSZ; ppk = piv + IVSZ;
205 /* --- Verify the MAC on the packet --- */
207 h = ks->min->ops->init(ks->min);
208 h->ops->hash(h, pseq, SEQSZ + IVSZ + sz);
209 mac = h->ops->done(h, 0);
210 eq = !memcmp(mac, pmac, MACSZ);
211 IF_TRACING(T_KEYSET, {
212 trace(T_KEYSET, "keyset: decrypting using keyset %u", ks->seq);
213 trace_block(T_CRYPTO, "crypto: computed MAC", mac, MACSZ);
217 IF_TRACING(T_KEYSET, {
218 trace(T_KEYSET, "keyset: decryption failed");
219 trace_block(T_CRYPTO, "crypto: expected MAC", pmac, MACSZ);
224 /* --- Decrypt the packet --- */
226 c->ops->setiv(c, piv);
227 c->ops->decrypt(c, ppk, q, sz);
230 IF_TRACING(T_KEYSET, {
231 trace(T_KEYSET, "keyset: decrypted OK (sequence = %lu)",
232 (unsigned long)LOAD32(pseq));
233 trace_block(T_CRYPTO, "crypto: decrypted packet", q, sz);
239 /* --- @dosequence@ --- *
241 * Arguments: @keyset *ks@ = pointer to a keyset
242 * @uint32 seq@ = a sequence number from a packet
244 * Returns: Zero if the sequence number is OK, nonzero if it's not.
246 * Use: Checks a sequence number. The data in the keyset which keeps
247 * track of valid sequence numbers is updated if the sequence
248 * number given is good. It's assumed that the sequence number
249 * has already been checked for authenticity.
252 static int dosequence(keyset *ks, uint32 seq)
257 if (seq < ks->iseq) {
258 a_warn("received packet has old sequence number (possible replay)");
261 if (seq >= ks->iseq + KS_SEQWINSZ) {
262 n = seq - (ks->iseq + KS_SEQWINSZ - 1);
269 seqbit = 1 << (seq - ks->iseq);
270 if (ks->iwin & seqbit) {
271 a_warn("received packet repeats old sequence number");
278 /*----- Operations on a single keyset -------------------------------------*/
280 /* --- @ks_drop@ --- *
282 * Arguments: @keyset *ks@ = pointer to a keyset
286 * Use: Decrements a keyset's reference counter. If the counter hits
287 * zero, the keyset is freed.
290 void ks_drop(keyset *ks)
294 ks->cin->ops->destroy(ks->cin);
295 ks->cout->ops->destroy(ks->cout);
296 ks->min->ops->destroy(ks->min);
297 ks->mout->ops->destroy(ks->mout);
301 /* --- @ks_gen@ --- *
303 * Arguments: @const void *k@ = pointer to key material
304 * @size_t x, y, z@ = offsets into key material (see below)
306 * Returns: A pointer to the new keyset.
308 * Use: Derives a new keyset from the given key material. The
309 * offsets @x@, @y@ and @z@ separate the key material into three
310 * parts. Between the @k@ and @k + x@ is `my' contribution to
311 * the key material; between @k + x@ and @k + y@ is `your'
312 * contribution; and between @k + y@ and @k + z@ is a shared
313 * value we made together. These are used to construct two
314 * pairs of symmetric keys. Each pair consists of an encryption
315 * key and a message authentication key. One pair is used for
316 * outgoing messages, the other for incoming messages.
318 * The new key is marked so that it won't be selected for output
319 * by @ksl_encrypt@. You can still encrypt data with it by
320 * calling @ks_encrypt@ directly.
323 keyset *ks_gen(const void *k, size_t x, size_t y, size_t z)
327 keyset *ks = CREATE(keyset);
328 time_t now = time(0);
330 T( static unsigned seq = 0; )
332 T( trace(T_KEYSET, "keyset: adding new keyset %u", seq); )
334 /* --- Construct the various keys --- *
336 * This is done with macros, because it's quite tedious.
339 #define MINE HASH(&h, p, x)
340 #define YOURS HASH(&h, p + x, y - x)
341 #define OURS HASH(&h, p + y, z - y)
343 #define IN MINE; YOURS; OURS
344 #define OUT YOURS; MINE; OURS
345 #define STR_IN "incoming"
346 #define STR_OUT "outgoing"
348 #define GETHASH(str, dir) do { \
350 HASH_STRING(&h, "tripe-" str); \
352 HASH_DONE(&h, buf); \
353 IF_TRACING(T_KEYSET, { \
354 trace_block(T_CRYPTO, "crypto: " STR_##dir " key " str, \
359 GETHASH("encryption", IN); ks->cin = CIPHER->init(buf, sizeof(buf));
360 GETHASH("integrity", IN); ks->min = MAC->key(buf, sizeof(buf));
361 GETHASH("encryption", OUT); ks->cout = CIPHER->init(buf, sizeof(buf));
362 GETHASH("integrity", OUT); ks->mout = MAC->key(buf, sizeof(buf));
373 T( ks->seq = seq++; )
374 ks->t_exp = now + T_EXP;
376 ks->oseq = ks->iseq = 0;
384 /* --- @ks_tregen@ --- *
386 * Arguments: @keyset *ks@ = pointer to a keyset
388 * Returns: The time at which moves ought to be made to replace this key.
391 time_t ks_tregen(keyset *ks) { return (ks->t_exp - T_EXP + T_REGEN); }
393 /* --- @ks_activate@ --- *
395 * Arguments: @keyset *ks@ = pointer to a keyset
399 * Use: Activates a keyset, so that it can be used for encrypting
403 void ks_activate(keyset *ks)
405 if (ks->f & KSF_LISTEN) {
406 T( trace(T_KEYSET, "keyset: activating keyset %u", ks->seq); )
407 ks->f &= ~KSF_LISTEN;
411 /* --- @ks_encrypt@ --- *
413 * Arguments: @keyset *ks@ = pointer to a keyset
414 * @buf *b@ = pointer to input buffer
415 * @buf *bb@ = pointer to output buffer
417 * Returns: Zero if OK, nonzero if the key needs replacing. If the
418 * encryption failed, the output buffer is broken and zero is
421 * Use: Encrypts a block of data using the key. Note that the `key
422 * ought to be replaced' notification is only ever given once
423 * for each key. Also note that this call forces a keyset to be
424 * used even if it's marked as not for data output.
427 int ks_encrypt(keyset *ks, buf *b, buf *bb)
429 time_t now = time(0);
431 if (!KEYOK(ks, now)) {
435 return (doencrypt(ks, b, bb));
438 /* --- @ks_decrypt@ --- *
440 * Arguments: @keyset *ks@ = pointer to a keyset
441 * @buf *b@ = pointer to an input buffer
442 * @buf *bb@ = pointer to an output buffer
444 * Returns: Zero on success, or nonzero if there was some problem.
446 * Use: Attempts to decrypt a message using a given key. Note that
447 * requesting decryption with a key directly won't clear a
448 * marking that it's not for encryption.
451 int ks_decrypt(keyset *ks, buf *b, buf *bb)
453 time_t now = time(0);
456 if (!KEYOK(ks, now) ||
457 buf_ensure(bb, BLEN(b)) ||
458 dodecrypt(ks, b, bb, &seq) ||
464 /*----- Keyset list handling ----------------------------------------------*/
466 /* --- @ksl_free@ --- *
468 * Arguments: @keyset **ksroot@ = pointer to keyset list head
472 * Use: Frees (releases references to) all of the keys in a keyset.
475 void ksl_free(keyset **ksroot)
478 for (ks = *ksroot; ks; ks = ksn) {
485 /* --- @ksl_link@ --- *
487 * Arguments: @keyset **ksroot@ = pointer to keyset list head
488 * @keyset *ks@ = pointer to a keyset
492 * Use: Links a keyset into a list. A keyset can only be on one list
493 * at a time. Bad things happen otherwise.
496 void ksl_link(keyset **ksroot, keyset *ks)
498 assert(!(ks->f & KSF_LINK));
505 /* --- @ksl_prune@ --- *
507 * Arguments: @keyset **ksroot@ = pointer to keyset list head
511 * Use: Prunes the keyset list by removing keys which mustn't be used
515 void ksl_prune(keyset **ksroot)
517 time_t now = time(0);
520 keyset *ks = *ksroot;
522 if (ks->t_exp <= now) {
523 T( trace(T_KEYSET, "keyset: expiring keyset %u (time limit reached)",
526 } else if (ks->sz_exp == 0) {
527 T( trace(T_KEYSET, "keyset: expiring keyset %u (data limit reached)",
542 /* --- @ksl_encrypt@ --- *
544 * Arguments: @keyset **ksroot@ = pointer to keyset list head
545 * @buf *b@ = pointer to input buffer
546 * @buf *bb@ = pointer to output buffer
548 * Returns: Nonzero if a new key is needed.
550 * Use: Encrypts a packet.
553 int ksl_encrypt(keyset **ksroot, buf *b, buf *bb)
555 time_t now = time(0);
556 keyset *ks = *ksroot;
560 T( trace(T_KEYSET, "keyset: no suitable keysets found"); )
564 if (KEYOK(ks, now) && !(ks->f & KSF_LISTEN))
569 return (doencrypt(ks, b, bb));
572 /* --- @ksl_decrypt@ --- *
574 * Arguments: @keyset **ksroot@ = pointer to keyset list head
575 * @buf *b@ = pointer to input buffer
576 * @buf *bb@ = pointer to output buffer
578 * Returns: Nonzero if the packet couldn't be decrypted.
580 * Use: Decrypts a packet.
583 int ksl_decrypt(keyset **ksroot, buf *b, buf *bb)
585 time_t now = time(0);
589 if (buf_ensure(bb, BLEN(b)))
592 for (ks = *ksroot; ks; ks = ks->next) {
595 if (!dodecrypt(ks, b, bb, &seq)) {
596 if (ks->f & KSF_LISTEN) {
597 T( trace(T_KEYSET, "keyset: implicitly activating keyset %u",
599 ks->f &= ~KSF_LISTEN;
601 return (dosequence(ks, seq));
604 T( trace(T_KEYSET, "keyset: no matching keys"); )
608 /*----- That's all, folks -------------------------------------------------*/