2 sptps.c -- Simple Peer-to-Peer Security
3 Copyright (C) 2011-2013 Guus Sliepen <guus@tinc-vpn.org>,
4 2010 Brandon L. Black <blblack@gmail.com>
6 This program is free software; you can redistribute it and/or modify
7 it under the terms of the GNU General Public License as published by
8 the Free Software Foundation; either version 2 of the License, or
9 (at your option) any later version.
11 This program is distributed in the hope that it will be useful,
12 but WITHOUT ANY WARRANTY; without even the implied warranty of
13 MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
14 GNU General Public License for more details.
16 You should have received a copy of the GNU General Public License along
17 with this program; if not, write to the Free Software Foundation, Inc.,
18 51 Franklin Street, Fifth Floor, Boston, MA 02110-1301 USA.
32 unsigned int sptps_replaywin = 16;
35 Nonce MUST be exchanged first (done)
36 Signatures MUST be done over both nonces, to guarantee the signature is fresh
37 Otherwise: if ECDHE key of one side is compromised, it can be reused!
39 Add explicit tag to beginning of structure to distinguish the client and server when signing. (done)
41 Sign all handshake messages up to ECDHE kex with long-term public keys. (done)
43 HMACed KEX finished message to prevent downgrade attacks and prove you have the right key material (done by virtue of ECDSA over the whole ECDHE exchange?)
45 Explicit close message needs to be added.
47 Maybe do add some alert messages to give helpful error messages? Not more than TLS sends.
49 Use counter mode instead of OFB. (done)
51 Make sure ECC operations are fixed time (aka prevent side-channel attacks).
54 void sptps_log_quiet(sptps_t *s, int s_errno, const char *format, va_list ap) {
57 void sptps_log_stderr(sptps_t *s, int s_errno, const char *format, va_list ap) {
58 vfprintf(stderr, format, ap);
62 void (*sptps_log)(sptps_t *s, int s_errno, const char *format, va_list ap) = sptps_log_stderr;
64 // Log an error message.
65 static bool error(sptps_t *s, int s_errno, const char *format, ...) {
69 sptps_log(s, s_errno, format, ap);
77 static void warning(sptps_t *s, const char *format, ...) {
80 sptps_log(s, 0, format, ap);
84 // Send a record (datagram version, accepts all record types, handles encryption and authentication).
85 static bool send_record_priv_datagram(sptps_t *s, uint8_t type, const char *data, uint16_t len) {
86 char buffer[len + 23UL];
88 // Create header with sequence number, length and record type
89 uint32_t seqno = htonl(s->outseqno++);
90 uint16_t netlen = htons(len);
92 memcpy(buffer, &netlen, 2);
93 memcpy(buffer + 2, &seqno, 4);
96 // Add plaintext (TODO: avoid unnecessary copy)
97 memcpy(buffer + 7, data, len);
100 // If first handshake has finished, encrypt and HMAC
101 cipher_set_counter(s->outcipher, &seqno, sizeof seqno);
102 if(!cipher_counter_xor(s->outcipher, buffer + 6, len + 1UL, buffer + 6))
105 if(!digest_create(s->outdigest, buffer, len + 7UL, buffer + 7UL + len))
108 return s->send_data(s->handle, type, buffer + 2, len + 21UL);
110 // Otherwise send as plaintext
111 return s->send_data(s->handle, type, buffer + 2, len + 5UL);
114 // Send a record (private version, accepts all record types, handles encryption and authentication).
115 static bool send_record_priv(sptps_t *s, uint8_t type, const char *data, uint16_t len) {
117 return send_record_priv_datagram(s, type, data, len);
119 char buffer[len + 23UL];
121 // Create header with sequence number, length and record type
122 uint32_t seqno = htonl(s->outseqno++);
123 uint16_t netlen = htons(len);
125 memcpy(buffer, &seqno, 4);
126 memcpy(buffer + 4, &netlen, 2);
129 // Add plaintext (TODO: avoid unnecessary copy)
130 memcpy(buffer + 7, data, len);
133 // If first handshake has finished, encrypt and HMAC
134 if(!cipher_counter_xor(s->outcipher, buffer + 4, len + 3UL, buffer + 4))
137 if(!digest_create(s->outdigest, buffer, len + 7UL, buffer + 7UL + len))
140 return s->send_data(s->handle, type, buffer + 4, len + 19UL);
142 // Otherwise send as plaintext
143 return s->send_data(s->handle, type, buffer + 4, len + 3UL);
147 // Send an application record.
148 bool sptps_send_record(sptps_t *s, uint8_t type, const char *data, uint16_t len) {
149 // Sanity checks: application cannot send data before handshake is finished,
150 // and only record types 0..127 are allowed.
152 return error(s, EINVAL, "Handshake phase not finished yet");
154 if(type >= SPTPS_HANDSHAKE)
155 return error(s, EINVAL, "Invalid application record type");
157 return send_record_priv(s, type, data, len);
160 // Send a Key EXchange record, containing a random nonce and an ECDHE public key.
161 static bool send_kex(sptps_t *s) {
162 size_t keylen = ECDH_SIZE;
164 // Make room for our KEX message, which we will keep around since send_sig() needs it.
167 s->mykex = realloc(s->mykex, 1 + 32 + keylen);
169 return error(s, errno, strerror(errno));
171 // Set version byte to zero.
172 s->mykex[0] = SPTPS_VERSION;
174 // Create a random nonce.
175 randomize(s->mykex + 1, 32);
177 // Create a new ECDH public key.
178 if(!(s->ecdh = ecdh_generate_public(s->mykex + 1 + 32)))
181 return send_record_priv(s, SPTPS_HANDSHAKE, s->mykex, 1 + 32 + keylen);
184 // Send a SIGnature record, containing an ECDSA signature over both KEX records.
185 static bool send_sig(sptps_t *s) {
186 size_t keylen = ECDH_SIZE;
187 size_t siglen = ecdsa_size(s->mykey);
189 // Concatenate both KEX messages, plus tag indicating if it is from the connection originator, plus label
190 char msg[(1 + 32 + keylen) * 2 + 1 + s->labellen];
193 msg[0] = s->initiator;
194 memcpy(msg + 1, s->mykex, 1 + 32 + keylen);
195 memcpy(msg + 1 + 33 + keylen, s->hiskex, 1 + 32 + keylen);
196 memcpy(msg + 1 + 2 * (33 + keylen), s->label, s->labellen);
199 if(!ecdsa_sign(s->mykey, msg, sizeof msg, sig))
202 // Send the SIG exchange record.
203 return send_record_priv(s, SPTPS_HANDSHAKE, sig, sizeof sig);
206 // Generate key material from the shared secret created from the ECDHE key exchange.
207 static bool generate_key_material(sptps_t *s, const char *shared, size_t len) {
208 // Initialise cipher and digest structures if necessary
210 s->incipher = cipher_open_by_name("aes-256-ecb");
211 s->outcipher = cipher_open_by_name("aes-256-ecb");
212 s->indigest = digest_open_by_name("sha256", 16);
213 s->outdigest = digest_open_by_name("sha256", 16);
214 if(!s->incipher || !s->outcipher || !s->indigest || !s->outdigest)
218 // Allocate memory for key material
219 size_t keylen = digest_keylength(s->indigest) + digest_keylength(s->outdigest) + cipher_keylength(s->incipher) + cipher_keylength(s->outcipher);
221 s->key = realloc(s->key, keylen);
223 return error(s, errno, strerror(errno));
225 // Create the HMAC seed, which is "key expansion" + session label + server nonce + client nonce
226 char seed[s->labellen + 64 + 13];
227 strcpy(seed, "key expansion");
229 memcpy(seed + 13, s->mykex + 1, 32);
230 memcpy(seed + 45, s->hiskex + 1, 32);
232 memcpy(seed + 13, s->hiskex + 1, 32);
233 memcpy(seed + 45, s->mykex + 1, 32);
235 memcpy(seed + 77, s->label, s->labellen);
237 // Use PRF to generate the key material
238 if(!prf(shared, len, seed, s->labellen + 64 + 13, s->key, keylen))
244 // Send an ACKnowledgement record.
245 static bool send_ack(sptps_t *s) {
246 return send_record_priv(s, SPTPS_HANDSHAKE, "", 0);
249 // Receive an ACKnowledgement record.
250 static bool receive_ack(sptps_t *s, const char *data, uint16_t len) {
252 return error(s, EIO, "Invalid ACK record length");
256 = cipher_set_counter_key(s->incipher, s->key)
257 && digest_set_key(s->indigest, s->key + cipher_keylength(s->incipher), digest_keylength(s->indigest));
262 = cipher_set_counter_key(s->incipher, s->key + cipher_keylength(s->outcipher) + digest_keylength(s->outdigest))
263 && digest_set_key(s->indigest, s->key + cipher_keylength(s->outcipher) + digest_keylength(s->outdigest) + cipher_keylength(s->incipher), digest_keylength(s->indigest));
275 // Receive a Key EXchange record, respond by sending a SIG record.
276 static bool receive_kex(sptps_t *s, const char *data, uint16_t len) {
277 // Verify length of the HELLO record
278 if(len != 1 + 32 + ECDH_SIZE)
279 return error(s, EIO, "Invalid KEX record length");
281 // Ignore version number for now.
283 // Make a copy of the KEX message, send_sig() and receive_sig() need it
286 s->hiskex = realloc(s->hiskex, len);
288 return error(s, errno, strerror(errno));
290 memcpy(s->hiskex, data, len);
295 // Receive a SIGnature record, verify it, if it passed, compute the shared secret and calculate the session keys.
296 static bool receive_sig(sptps_t *s, const char *data, uint16_t len) {
297 size_t keylen = ECDH_SIZE;
298 size_t siglen = ecdsa_size(s->hiskey);
300 // Verify length of KEX record.
302 return error(s, EIO, "Invalid KEX record length");
304 // Concatenate both KEX messages, plus tag indicating if it is from the connection originator
305 char msg[(1 + 32 + keylen) * 2 + 1 + s->labellen];
307 msg[0] = !s->initiator;
308 memcpy(msg + 1, s->hiskex, 1 + 32 + keylen);
309 memcpy(msg + 1 + 33 + keylen, s->mykex, 1 + 32 + keylen);
310 memcpy(msg + 1 + 2 * (33 + keylen), s->label, s->labellen);
313 if(!ecdsa_verify(s->hiskey, msg, sizeof msg, data))
316 // Compute shared secret.
317 char shared[ECDH_SHARED_SIZE];
318 if(!ecdh_compute_shared(s->ecdh, s->hiskex + 1 + 32, shared))
321 // Generate key material from shared secret.
322 if(!generate_key_material(s, shared, sizeof shared))
331 // Send cipher change record
332 if(s->outstate && !send_ack(s))
335 // TODO: only set new keys after ACK has been set/received
338 = cipher_set_counter_key(s->outcipher, s->key + cipher_keylength(s->incipher) + digest_keylength(s->indigest))
339 && digest_set_key(s->outdigest, s->key + cipher_keylength(s->incipher) + digest_keylength(s->indigest) + cipher_keylength(s->outcipher), digest_keylength(s->outdigest));
344 = cipher_set_counter_key(s->outcipher, s->key)
345 && digest_set_key(s->outdigest, s->key + cipher_keylength(s->outcipher), digest_keylength(s->outdigest));
353 // Force another Key EXchange (for testing purposes).
354 bool sptps_force_kex(sptps_t *s) {
355 if(!s->outstate || s->state != SPTPS_SECONDARY_KEX)
356 return error(s, EINVAL, "Cannot force KEX in current state");
358 s->state = SPTPS_KEX;
362 // Receive a handshake record.
363 static bool receive_handshake(sptps_t *s, const char *data, uint16_t len) {
364 // Only a few states to deal with handshaking.
366 case SPTPS_SECONDARY_KEX:
367 // We receive a secondary KEX request, first respond by sending our own.
371 // We have sent our KEX request, we expect our peer to sent one as well.
372 if(!receive_kex(s, data, len))
374 s->state = SPTPS_SIG;
377 // If we already sent our secondary public ECDH key, we expect the peer to send his.
378 if(!receive_sig(s, data, len))
381 s->state = SPTPS_ACK;
384 if(!receive_ack(s, NULL, 0))
386 s->receive_record(s->handle, SPTPS_HANDSHAKE, NULL, 0);
387 s->state = SPTPS_SECONDARY_KEX;
392 // We expect a handshake message to indicate transition to the new keys.
393 if(!receive_ack(s, data, len))
395 s->receive_record(s->handle, SPTPS_HANDSHAKE, NULL, 0);
396 s->state = SPTPS_SECONDARY_KEX;
398 // TODO: split ACK into a VERify and ACK?
400 return error(s, EIO, "Invalid session state");
404 // Check datagram for valid HMAC
405 bool sptps_verify_datagram(sptps_t *s, const char *data, size_t len) {
406 if(!s->instate || len < 21)
409 char buffer[len + 23];
410 uint16_t netlen = htons(len - 21);
412 memcpy(buffer, &netlen, 2);
413 memcpy(buffer + 2, data, len);
415 return digest_verify(s->indigest, buffer, len - 14, buffer + len - 14);
418 // Receive incoming data, datagram version.
419 static bool sptps_receive_data_datagram(sptps_t *s, const char *data, size_t len) {
420 if(len < (s->instate ? 21 : 5))
421 return error(s, EIO, "Received short packet");
424 memcpy(&seqno, data, 4);
425 seqno = ntohl(seqno);
428 if(seqno != s->inseqno)
429 return error(s, EIO, "Invalid packet seqno: %d != %d", seqno, s->inseqno);
431 s->inseqno = seqno + 1;
433 uint8_t type = data[4];
435 if(type != SPTPS_HANDSHAKE)
436 return error(s, EIO, "Application record received before handshake finished");
438 return receive_handshake(s, data + 5, len - 5);
442 uint16_t netlen = htons(len - 21);
444 char buffer[len + 23];
446 memcpy(buffer, &netlen, 2);
447 memcpy(buffer + 2, data, len);
449 if(!digest_verify(s->indigest, buffer, len - 14, buffer + len - 14))
450 return error(s, EIO, "Invalid HMAC");
452 // Replay protection using a sliding window of configurable size.
453 // s->inseqno is expected sequence number
454 // seqno is received sequence number
455 // s->late[] is a circular buffer, a 1 bit means a packet has not been received yet
456 // The circular buffer contains bits for sequence numbers from s->inseqno - s->replaywin * 8 to (but excluding) s->inseqno.
458 if(seqno != s->inseqno) {
459 if(seqno >= s->inseqno + s->replaywin * 8) {
460 // Prevent packets that jump far ahead of the queue from causing many others to be dropped.
461 if(s->farfuture++ < s->replaywin >> 2)
462 return error(s, EIO, "Packet is %d seqs in the future, dropped (%u)\n", seqno - s->inseqno, s->farfuture);
464 // Unless we have seen lots of them, in which case we consider the others lost.
465 warning(s, "Lost %d packets\n", seqno - s->inseqno);
466 memset(s->late, 0, s->replaywin);
467 } else if (seqno < s->inseqno) {
468 // If the sequence number is farther in the past than the bitmap goes, or if the packet was already received, drop it.
469 if((s->inseqno >= s->replaywin * 8 && seqno < s->inseqno - s->replaywin * 8) || !(s->late[(seqno / 8) % s->replaywin] & (1 << seqno % 8)))
470 return error(s, EIO, "Received late or replayed packet, seqno %d, last received %d\n", seqno, s->inseqno);
472 // We missed some packets. Mark them in the bitmap as being late.
473 for(int i = s->inseqno; i < seqno; i++)
474 s->late[(i / 8) % s->replaywin] |= 1 << i % 8;
478 // Mark the current packet as not being late.
479 s->late[(seqno / 8) % s->replaywin] &= ~(1 << seqno % 8);
483 if(seqno > s->inseqno)
484 s->inseqno = seqno + 1;
492 memcpy(&seqno, buffer + 2, 4);
493 cipher_set_counter(s->incipher, &seqno, sizeof seqno);
494 if(!cipher_counter_xor(s->incipher, buffer + 6, len - 4, buffer + 6))
497 // Append a NULL byte for safety.
498 buffer[len - 14] = 0;
500 uint8_t type = buffer[6];
502 if(type < SPTPS_HANDSHAKE) {
504 return error(s, EIO, "Application record received before handshake finished");
505 if(!s->receive_record(s->handle, type, buffer + 7, len - 21))
507 } else if(type == SPTPS_HANDSHAKE) {
508 if(!receive_handshake(s, buffer + 7, len - 21))
511 return error(s, EIO, "Invalid record type");
517 // Receive incoming data. Check if it contains a complete record, if so, handle it.
518 bool sptps_receive_data(sptps_t *s, const char *data, size_t len) {
520 return sptps_receive_data_datagram(s, data, len);
523 // First read the 2 length bytes.
525 size_t toread = 6 - s->buflen;
529 memcpy(s->inbuf + s->buflen, data, toread);
535 // Exit early if we don't have the full length.
539 // Decrypt the length bytes
542 if(!cipher_counter_xor(s->incipher, s->inbuf + 4, 2, &s->reclen))
545 memcpy(&s->reclen, s->inbuf + 4, 2);
548 s->reclen = ntohs(s->reclen);
550 // If we have the length bytes, ensure our buffer can hold the whole request.
551 s->inbuf = realloc(s->inbuf, s->reclen + 23UL);
553 return error(s, errno, strerror(errno));
555 // Add sequence number.
556 uint32_t seqno = htonl(s->inseqno++);
557 memcpy(s->inbuf, &seqno, 4);
559 // Exit early if we have no more data to process.
564 // Read up to the end of the record.
565 size_t toread = s->reclen + (s->instate ? 23UL : 7UL) - s->buflen;
569 memcpy(s->inbuf + s->buflen, data, toread);
574 // If we don't have a whole record, exit.
575 if(s->buflen < s->reclen + (s->instate ? 23UL : 7UL))
578 // Check HMAC and decrypt.
580 if(!digest_verify(s->indigest, s->inbuf, s->reclen + 7UL, s->inbuf + s->reclen + 7UL))
581 return error(s, EIO, "Invalid HMAC");
583 if(!cipher_counter_xor(s->incipher, s->inbuf + 6UL, s->reclen + 1UL, s->inbuf + 6UL))
587 // Append a NULL byte for safety.
588 s->inbuf[s->reclen + 7UL] = 0;
590 uint8_t type = s->inbuf[6];
592 if(type < SPTPS_HANDSHAKE) {
594 return error(s, EIO, "Application record received before handshake finished");
595 if(!s->receive_record(s->handle, type, s->inbuf + 7, s->reclen))
597 } else if(type == SPTPS_HANDSHAKE) {
598 if(!receive_handshake(s, s->inbuf + 7, s->reclen))
601 return error(s, EIO, "Invalid record type");
610 // Start a SPTPS session.
611 bool sptps_start(sptps_t *s, void *handle, bool initiator, bool datagram, ecdsa_t *mykey, ecdsa_t *hiskey, const char *label, size_t labellen, send_data_t send_data, receive_record_t receive_record) {
612 // Initialise struct sptps
613 memset(s, 0, sizeof *s);
616 s->initiator = initiator;
617 s->datagram = datagram;
620 s->replaywin = sptps_replaywin;
622 s->late = malloc(s->replaywin);
624 return error(s, errno, strerror(errno));
627 s->label = malloc(labellen);
629 return error(s, errno, strerror(errno));
632 s->inbuf = malloc(7);
634 return error(s, errno, strerror(errno));
636 memset(s->inbuf, 0, 4);
639 memcpy(s->label, label, labellen);
640 s->labellen = labellen;
642 s->send_data = send_data;
643 s->receive_record = receive_record;
645 // Do first KEX immediately
646 s->state = SPTPS_KEX;
650 // Stop a SPTPS session.
651 bool sptps_stop(sptps_t *s) {
652 // Clean up any resources.
653 cipher_close(s->incipher);
654 cipher_close(s->outcipher);
655 digest_close(s->indigest);
656 digest_close(s->outdigest);
664 memset(s, 0, sizeof *s);