3 * Handling of symmetric keysets
5 * (c) 2001 Straylight/Edgeware
8 /*----- Licensing notice --------------------------------------------------*
10 * This file is part of Trivial IP Encryption (TrIPE).
12 * TrIPE is free software; you can redistribute it and/or modify
13 * it under the terms of the GNU General Public License as published by
14 * the Free Software Foundation; either version 2 of the License, or
15 * (at your option) any later version.
17 * TrIPE is distributed in the hope that it will be useful,
18 * but WITHOUT ANY WARRANTY; without even the implied warranty of
19 * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
20 * GNU General Public License for more details.
22 * You should have received a copy of the GNU General Public License
23 * along with TrIPE; if not, write to the Free Software Foundation,
24 * Inc., 59 Temple Place - Suite 330, Boston, MA 02111-1307, USA.
27 /*----- Header files ------------------------------------------------------*/
31 /*----- Tunable parameters ------------------------------------------------*/
33 /* --- Note on size limits --- *
35 * For a 64-bit block cipher (e.g., Blowfish), the probability of a collision
36 * occurring after 32 MB is less than %$2^{-21}$%, and the probability of a
37 * collision occurring after 64 MB is less than %$2^{-19}$%. These could be
38 * adjusted dependent on the encryption scheme, but it's too much pain.
41 #define T_EXP MIN(60) /* Expiry time for a key */
42 #define T_REGEN MIN(45) /* Regeneration time for a key */
43 #define SZ_EXP MEG(64) /* Expiry data size for a key */
44 #define SZ_REGEN MEG(32) /* Data size threshold for regen */
46 /*----- Handy macros ------------------------------------------------------*/
48 #define KEYOK(ks, now) ((ks)->sz_exp > 0 && (ks)->t_exp > now)
50 #define SEQSZ 4 /* Size of sequence number packet */
52 /*----- Low-level packet encryption and decryption ------------------------*/
54 /* --- Encrypted data format --- *
56 * Let %$p_i$% be the %$i$%-th plaintext message, with type %$t$%. We first
59 * %$c_i = \mathcal{E}\textrm{-CBC}_{K_{\text{E}}}(p_i)$%
61 * as the CBC-ciphertext of %$p_i$%, and then
63 * %$\sigma_i = \mathcal{T}_{K_{\text{M}}}(t, i, c_i)$%
65 * as a MAC on the %%\emph{ciphertext}%%. The message sent is then the pair
66 * %$(\sigma_i, c_i)$%. This construction is provably secure in the NM-CCA
67 * sense (assuming that the cipher is IND-CPA, and the MAC is SUF-CMA)
68 * [Bellare and Namprempre].
70 * This also ensures that, assuming the key is good, we have a secure channel
71 * [Krawczyk]. Actually, [Krawczyk] shows that, if the cipher is either a
72 * simple stream cipher or a block cipher in CBC mode, we can use the MAC-
73 * then-encrypt scheme and still have a secure channel. However, I like the
74 * NM-CCA guarantee from [Bellare and Namprempre]. I'm less worried about
75 * the Horton Principle [Wagner and Schneier].
78 /* --- @doencrypt@ --- *
80 * Arguments: @keyset *ks@ = pointer to keyset to use
81 * @unsigned ty@ = type of message this is
82 * @buf *b@ = pointer to an input buffer
83 * @buf *bb@ = pointer to an output buffer
85 * Returns: Zero if OK; @KSERR_REGEN@ if it's time to generate new keys.
86 * Also returns zero if there was insufficient buffer space, but
87 * the buffer is broken in this case.
89 * Use: Encrypts a message with the given key. We assume that the
90 * keyset is OK to use.
93 static int doencrypt(keyset *ks, unsigned ty, buf *b, buf *bb)
96 gcipher *c = ks->cout;
97 const octet *p = BCUR(b);
99 octet *qmac, *qseq, *qiv, *qpk;
101 size_t ivsz = GC_CLASS(c)->blksz;
102 size_t tagsz = ks->tagsz;
107 /* --- Allocate the required buffer space --- */
109 if (buf_ensure(bb, tagsz + SEQSZ + ivsz + sz))
110 return (0); /* Caution! */
111 qmac = BCUR(bb); qseq = qmac + tagsz; qiv = qseq + SEQSZ; qpk = qiv + ivsz;
112 BSTEP(bb, tagsz + SEQSZ + ivsz + sz);
115 oseq = ks->oseq++; STORE32(qseq, oseq);
116 IF_TRACING(T_KEYSET, {
117 trace(T_KEYSET, "keyset: encrypting packet %lu using keyset %u",
118 (unsigned long)oseq, ks->seq);
119 trace_block(T_CRYPTO, "crypto: plaintext packet", p, sz);
122 /* --- Encrypt the packet --- */
125 rand_get(RAND_GLOBAL, qiv, ivsz);
127 IF_TRACING(T_KEYSET, {
128 trace_block(T_CRYPTO, "crypto: initialization vector", qiv, ivsz);
131 GC_ENCRYPT(c, p, qpk, sz);
132 IF_TRACING(T_KEYSET, {
133 trace_block(T_CRYPTO, "crypto: encrypted packet", qpk, sz);
136 /* --- Now compute the MAC --- */
139 h = GM_INIT(ks->mout);
140 GH_HASH(h, t, sizeof(t));
141 GH_HASH(h, qseq, SEQSZ + ivsz + sz);
142 memcpy(qmac, GH_DONE(h, 0), tagsz);
144 IF_TRACING(T_KEYSET, {
145 trace_block(T_CRYPTO, "crypto: computed MAC", qmac, tagsz);
149 /* --- Deduct the packet size from the key's data life --- */
156 if (osz >= SZ_REGEN && nsz < SZ_REGEN) {
157 T( trace(T_KEYSET, "keyset: keyset %u data regen limit exceeded -- "
158 "forcing exchange", ks->seq); )
165 /* --- @dodecrypt@ --- *
167 * Arguments: @keyset *ks@ = pointer to keyset to use
168 * @unsigned ty@ = expected type code
169 * @buf *b@ = pointer to an input buffer
170 * @buf *bb@ = pointer to an output buffer
171 * @uint32 *seq@ = where to store the sequence number
173 * Returns: Zero on success; @KSERR_DECRYPT@ on failure.
175 * Use: Attempts to decrypt a message with the given key. No other
176 * checking (e.g., sequence number checks) is performed. We
177 * assume that the keyset is OK to use, and that there is
178 * sufficient output buffer space reserved. If the decryption
179 * is successful, the buffer pointer is moved past the decrypted
180 * packet, and the packet's sequence number is stored in @*seq@.
183 static int dodecrypt(keyset *ks, unsigned ty, buf *b, buf *bb, uint32 *seq)
185 const octet *pmac, *piv, *pseq, *ppk;
186 size_t psz = BLEFT(b);
190 gcipher *c = ks->cin;
191 size_t ivsz = GC_CLASS(c)->blksz;
192 size_t tagsz = ks->tagsz;
197 /* --- Break up the packet into its components --- */
199 if (psz < ivsz + SEQSZ + tagsz) {
200 T( trace(T_KEYSET, "keyset: block too small for keyset %u", ks->seq); )
201 return (KSERR_DECRYPT);
203 sz = psz - ivsz - SEQSZ - tagsz;
204 pmac = BCUR(b); pseq = pmac + tagsz; piv = pseq + SEQSZ; ppk = piv + ivsz;
207 IF_TRACING(T_KEYSET, {
208 trace(T_KEYSET, "keyset: decrypting using keyset %u", ks->seq);
209 trace_block(T_CRYPTO, "crypto: ciphertext packet", ppk, sz);
212 /* --- Verify the MAC on the packet --- */
215 h = GM_INIT(ks->min);
216 GH_HASH(h, t, sizeof(t));
217 GH_HASH(h, pseq, SEQSZ + ivsz + sz);
219 eq = !memcmp(mac, pmac, tagsz);
220 IF_TRACING(T_KEYSET, {
221 trace_block(T_CRYPTO, "crypto: computed MAC", mac, tagsz);
225 IF_TRACING(T_KEYSET, {
226 trace(T_KEYSET, "keyset: incorrect MAC: decryption failed");
227 trace_block(T_CRYPTO, "crypto: expected MAC", pmac, tagsz);
229 return (KSERR_DECRYPT);
233 /* --- Decrypt the packet --- */
237 IF_TRACING(T_KEYSET, {
238 trace_block(T_CRYPTO, "crypto: initialization vector", piv, ivsz);
241 GC_DECRYPT(c, ppk, q, sz);
244 IF_TRACING(T_KEYSET, {
245 trace(T_KEYSET, "keyset: decrypted OK (sequence = %lu)",
246 (unsigned long)LOAD32(pseq));
247 trace_block(T_CRYPTO, "crypto: decrypted packet", q, sz);
253 /*----- Operations on a single keyset -------------------------------------*/
255 /* --- @ks_drop@ --- *
257 * Arguments: @keyset *ks@ = pointer to a keyset
261 * Use: Decrements a keyset's reference counter. If the counter hits
262 * zero, the keyset is freed.
265 void ks_drop(keyset *ks)
270 GC_DESTROY(ks->cout);
272 GM_DESTROY(ks->mout);
276 /* --- @ks_gen@ --- *
278 * Arguments: @const void *k@ = pointer to key material
279 * @size_t x, y, z@ = offsets into key material (see below)
280 * @peer *p@ = pointer to peer information
282 * Returns: A pointer to the new keyset.
284 * Use: Derives a new keyset from the given key material. The
285 * offsets @x@, @y@ and @z@ separate the key material into three
286 * parts. Between the @k@ and @k + x@ is `my' contribution to
287 * the key material; between @k + x@ and @k + y@ is `your'
288 * contribution; and between @k + y@ and @k + z@ is a shared
289 * value we made together. These are used to construct two
290 * pairs of symmetric keys. Each pair consists of an encryption
291 * key and a message authentication key. One pair is used for
292 * outgoing messages, the other for incoming messages.
294 * The new key is marked so that it won't be selected for output
295 * by @ksl_encrypt@. You can still encrypt data with it by
296 * calling @ks_encrypt@ directly.
299 keyset *ks_gen(const void *k, size_t x, size_t y, size_t z, peer *p)
303 keyset *ks = CREATE(keyset);
304 time_t now = time(0);
306 T( static unsigned seq = 0; )
308 T( trace(T_KEYSET, "keyset: adding new keyset %u", seq); )
310 /* --- Construct the various keys --- *
312 * This is done with macros, because it's quite tedious.
315 #define MINE GH_HASH(h, pp, x)
316 #define YOURS GH_HASH(h, pp + x, y - x)
317 #define OURS GH_HASH(h, pp + y, z - y)
319 #define HASH_in MINE; YOURS; OURS
320 #define HASH_out YOURS; MINE; OURS
321 #define INIT_c(k) GC_INIT(algs.c, (k), algs.cksz)
322 #define INIT_m(k) GM_KEY(algs.m, (k), algs.mksz)
323 #define STR_c "encryption"
324 #define STR_m "integrity"
325 #define STR_in "incoming"
326 #define STR_out "outgoing"
328 #define SETKEY(a, dir) do { \
329 h = GH_INIT(algs.h); \
330 HASH_STRING(h, "tripe-" STR_##a); \
332 hh = GH_DONE(h, 0); \
333 IF_TRACING(T_KEYSET, { \
334 trace_block(T_CRYPTO, "crypto: " STR_##dir " key " STR_##a, \
337 ks->a##dir = INIT_##a(hh); \
341 SETKEY(c, in); SETKEY(c, out);
342 SETKEY(m, in); SETKEY(m, out);
357 T( ks->seq = seq++; )
359 ks->t_exp = now + T_EXP;
362 seq_reset(&ks->iseq);
366 ks->tagsz = algs.tagsz;
370 /* --- @ks_tregen@ --- *
372 * Arguments: @keyset *ks@ = pointer to a keyset
374 * Returns: The time at which moves ought to be made to replace this key.
377 time_t ks_tregen(keyset *ks) { return (ks->t_exp - T_EXP + T_REGEN); }
379 /* --- @ks_activate@ --- *
381 * Arguments: @keyset *ks@ = pointer to a keyset
385 * Use: Activates a keyset, so that it can be used for encrypting
389 void ks_activate(keyset *ks)
391 if (ks->f & KSF_LISTEN) {
392 T( trace(T_KEYSET, "keyset: activating keyset %u", ks->seq); )
393 ks->f &= ~KSF_LISTEN;
397 /* --- @ks_encrypt@ --- *
399 * Arguments: @keyset *ks@ = pointer to a keyset
400 * @unsigned ty@ = message type
401 * @buf *b@ = pointer to input buffer
402 * @buf *bb@ = pointer to output buffer
404 * Returns: Zero if successful; @KSERR_REGEN@ if we should negotiate a
405 * new key; @KSERR_NOKEYS@ if the key is not usable. Also
406 * returns zero if there was insufficient buffer (but the output
407 * buffer is broken in this case).
409 * Use: Encrypts a block of data using the key. Note that the `key
410 * ought to be replaced' notification is only ever given once
411 * for each key. Also note that this call forces a keyset to be
412 * used even if it's marked as not for data output.
415 int ks_encrypt(keyset *ks, unsigned ty, buf *b, buf *bb)
417 time_t now = time(0);
419 if (!KEYOK(ks, now)) {
421 return (KSERR_NOKEYS);
423 return (doencrypt(ks, ty, b, bb));
426 /* --- @ks_decrypt@ --- *
428 * Arguments: @keyset *ks@ = pointer to a keyset
429 * @unsigned ty@ = expected type code
430 * @buf *b@ = pointer to an input buffer
431 * @buf *bb@ = pointer to an output buffer
433 * Returns: Zero on success; @KSERR_DECRYPT@ on failure. Also returns
434 * zero if there was insufficient buffer (but the output buffer
435 * is broken in this case).
437 * Use: Attempts to decrypt a message using a given key. Note that
438 * requesting decryption with a key directly won't clear a
439 * marking that it's not for encryption.
442 int ks_decrypt(keyset *ks, unsigned ty, buf *b, buf *bb)
444 time_t now = time(0);
447 if (!KEYOK(ks, now) ||
448 buf_ensure(bb, BLEN(b)) ||
449 dodecrypt(ks, ty, b, bb, &seq) ||
450 seq_check(&ks->iseq, seq, "SYMM"))
451 return (KSERR_DECRYPT);
455 /*----- Keyset list handling ----------------------------------------------*/
457 /* --- @ksl_free@ --- *
459 * Arguments: @keyset **ksroot@ = pointer to keyset list head
463 * Use: Frees (releases references to) all of the keys in a keyset.
466 void ksl_free(keyset **ksroot)
469 for (ks = *ksroot; ks; ks = ksn) {
476 /* --- @ksl_link@ --- *
478 * Arguments: @keyset **ksroot@ = pointer to keyset list head
479 * @keyset *ks@ = pointer to a keyset
483 * Use: Links a keyset into a list. A keyset can only be on one list
484 * at a time. Bad things happen otherwise.
487 void ksl_link(keyset **ksroot, keyset *ks)
489 assert(!(ks->f & KSF_LINK));
496 /* --- @ksl_prune@ --- *
498 * Arguments: @keyset **ksroot@ = pointer to keyset list head
502 * Use: Prunes the keyset list by removing keys which mustn't be used
506 void ksl_prune(keyset **ksroot)
508 time_t now = time(0);
511 keyset *ks = *ksroot;
513 if (ks->t_exp <= now) {
514 T( trace(T_KEYSET, "keyset: expiring keyset %u (time limit reached)",
517 } else if (ks->sz_exp == 0) {
518 T( trace(T_KEYSET, "keyset: expiring keyset %u (data limit reached)",
533 /* --- @ksl_encrypt@ --- *
535 * Arguments: @keyset **ksroot@ = pointer to keyset list head
536 * @unsigned ty@ = message type
537 * @buf *b@ = pointer to input buffer
538 * @buf *bb@ = pointer to output buffer
540 * Returns: Zero if successful; @KSERR_REGEN@ if it's time to negotiate a
541 * new key; @KSERR_NOKEYS@ if there are no suitable keys
542 * available. Also returns zero if there was insufficient
543 * buffer space (but the output buffer is broken in this case).
545 * Use: Encrypts a packet.
548 int ksl_encrypt(keyset **ksroot, unsigned ty, buf *b, buf *bb)
550 time_t now = time(0);
551 keyset *ks = *ksroot;
555 T( trace(T_KEYSET, "keyset: no suitable keysets found"); )
557 return (KSERR_NOKEYS);
559 if (KEYOK(ks, now) && !(ks->f & KSF_LISTEN))
564 return (doencrypt(ks, ty, b, bb));
567 /* --- @ksl_decrypt@ --- *
569 * Arguments: @keyset **ksroot@ = pointer to keyset list head
570 * @unsigned ty@ = expected type code
571 * @buf *b@ = pointer to input buffer
572 * @buf *bb@ = pointer to output buffer
574 * Returns: Zero on success; @KSERR_DECRYPT@ on failure. Also returns
575 * zero if there was insufficient buffer (but the output buffer
576 * is broken in this case).
578 * Use: Decrypts a packet.
581 int ksl_decrypt(keyset **ksroot, unsigned ty, buf *b, buf *bb)
583 time_t now = time(0);
587 if (buf_ensure(bb, BLEN(b)))
588 return (KSERR_DECRYPT);
590 for (ks = *ksroot; ks; ks = ks->next) {
593 if (!dodecrypt(ks, ty, b, bb, &seq)) {
594 if (ks->f & KSF_LISTEN) {
595 T( trace(T_KEYSET, "keyset: implicitly activating keyset %u",
597 ks->f &= ~KSF_LISTEN;
599 if (seq_check(&ks->iseq, seq, "SYMM"))
600 return (KSERR_DECRYPT);
605 T( trace(T_KEYSET, "keyset: no matching keys, or incorrect MAC"); )
606 return (KSERR_DECRYPT);
609 /*----- That's all, folks -------------------------------------------------*/