2 sptps.c -- Simple Peer-to-Peer Security
3 Copyright (C) 2011-2012 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(!ecdh_generate_public(&s->ecdh, 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
211 = cipher_open_by_name(&s->incipher, "aes-256-ecb")
212 && cipher_open_by_name(&s->outcipher, "aes-256-ecb")
213 && digest_open_by_name(&s->indigest, "sha256", 16)
214 && digest_open_by_name(&s->outdigest, "sha256", 16);
219 // Allocate memory for key material
220 size_t keylen = digest_keylength(&s->indigest) + digest_keylength(&s->outdigest) + cipher_keylength(&s->incipher) + cipher_keylength(&s->outcipher);
222 s->key = realloc(s->key, keylen);
224 return error(s, errno, strerror(errno));
226 // Create the HMAC seed, which is "key expansion" + session label + server nonce + client nonce
227 char seed[s->labellen + 64 + 13];
228 strcpy(seed, "key expansion");
230 memcpy(seed + 13, s->mykex + 1, 32);
231 memcpy(seed + 45, s->hiskex + 1, 32);
233 memcpy(seed + 13, s->hiskex + 1, 32);
234 memcpy(seed + 45, s->mykex + 1, 32);
236 memcpy(seed + 77, s->label, s->labellen);
238 // Use PRF to generate the key material
239 if(!prf(shared, len, seed, s->labellen + 64 + 13, s->key, keylen))
245 // Send an ACKnowledgement record.
246 static bool send_ack(sptps_t *s) {
247 return send_record_priv(s, SPTPS_HANDSHAKE, "", 0);
250 // Receive an ACKnowledgement record.
251 static bool receive_ack(sptps_t *s, const char *data, uint16_t len) {
253 return error(s, EIO, "Invalid ACK record length");
257 = cipher_set_counter_key(&s->incipher, s->key)
258 && digest_set_key(&s->indigest, s->key + cipher_keylength(&s->incipher), digest_keylength(&s->indigest));
263 = cipher_set_counter_key(&s->incipher, s->key + cipher_keylength(&s->outcipher) + digest_keylength(&s->outdigest))
264 && digest_set_key(&s->indigest, s->key + cipher_keylength(&s->outcipher) + digest_keylength(&s->outdigest) + cipher_keylength(&s->incipher), digest_keylength(&s->indigest));
276 // Receive a Key EXchange record, respond by sending a SIG record.
277 static bool receive_kex(sptps_t *s, const char *data, uint16_t len) {
278 // Verify length of the HELLO record
279 if(len != 1 + 32 + ECDH_SIZE)
280 return error(s, EIO, "Invalid KEX record length");
282 // Ignore version number for now.
284 // Make a copy of the KEX message, send_sig() and receive_sig() need it
287 s->hiskex = realloc(s->hiskex, len);
289 return error(s, errno, strerror(errno));
291 memcpy(s->hiskex, data, len);
296 // Receive a SIGnature record, verify it, if it passed, compute the shared secret and calculate the session keys.
297 static bool receive_sig(sptps_t *s, const char *data, uint16_t len) {
298 size_t keylen = ECDH_SIZE;
299 size_t siglen = ecdsa_size(&s->hiskey);
301 // Verify length of KEX record.
303 return error(s, EIO, "Invalid KEX record length");
305 // Concatenate both KEX messages, plus tag indicating if it is from the connection originator
306 char msg[(1 + 32 + keylen) * 2 + 1 + s->labellen];
308 msg[0] = !s->initiator;
309 memcpy(msg + 1, s->hiskex, 1 + 32 + keylen);
310 memcpy(msg + 1 + 33 + keylen, s->mykex, 1 + 32 + keylen);
311 memcpy(msg + 1 + 2 * (33 + keylen), s->label, s->labellen);
314 if(!ecdsa_verify(&s->hiskey, msg, sizeof msg, data))
317 // Compute shared secret.
318 char shared[ECDH_SHARED_SIZE];
319 if(!ecdh_compute_shared(&s->ecdh, s->hiskex + 1 + 32, shared))
322 // Generate key material from shared secret.
323 if(!generate_key_material(s, shared, sizeof shared))
332 // Send cipher change record
333 if(s->outstate && !send_ack(s))
336 // TODO: only set new keys after ACK has been set/received
339 = cipher_set_counter_key(&s->outcipher, s->key + cipher_keylength(&s->incipher) + digest_keylength(&s->indigest))
340 && digest_set_key(&s->outdigest, s->key + cipher_keylength(&s->incipher) + digest_keylength(&s->indigest) + cipher_keylength(&s->outcipher), digest_keylength(&s->outdigest));
345 = cipher_set_counter_key(&s->outcipher, s->key)
346 && digest_set_key(&s->outdigest, s->key + cipher_keylength(&s->outcipher), digest_keylength(&s->outdigest));
354 // Force another Key EXchange (for testing purposes).
355 bool sptps_force_kex(sptps_t *s) {
356 if(!s->outstate || s->state != SPTPS_SECONDARY_KEX)
357 return error(s, EINVAL, "Cannot force KEX in current state");
359 s->state = SPTPS_KEX;
363 // Receive a handshake record.
364 static bool receive_handshake(sptps_t *s, const char *data, uint16_t len) {
365 // Only a few states to deal with handshaking.
367 case SPTPS_SECONDARY_KEX:
368 // We receive a secondary KEX request, first respond by sending our own.
372 // We have sent our KEX request, we expect our peer to sent one as well.
373 if(!receive_kex(s, data, len))
375 s->state = SPTPS_SIG;
378 // If we already sent our secondary public ECDH key, we expect the peer to send his.
379 if(!receive_sig(s, data, len))
382 s->state = SPTPS_ACK;
385 if(!receive_ack(s, NULL, 0))
387 s->receive_record(s->handle, SPTPS_HANDSHAKE, NULL, 0);
388 s->state = SPTPS_SECONDARY_KEX;
393 // We expect a handshake message to indicate transition to the new keys.
394 if(!receive_ack(s, data, len))
396 s->receive_record(s->handle, SPTPS_HANDSHAKE, NULL, 0);
397 s->state = SPTPS_SECONDARY_KEX;
399 // TODO: split ACK into a VERify and ACK?
401 return error(s, EIO, "Invalid session state");
405 // Check datagram for valid HMAC
406 bool sptps_verify_datagram(sptps_t *s, const char *data, size_t len) {
407 if(!s->instate || len < 21)
410 char buffer[len + 23];
411 uint16_t netlen = htons(len - 21);
413 memcpy(buffer, &netlen, 2);
414 memcpy(buffer + 2, data, len);
416 return digest_verify(&s->indigest, buffer, len - 14, buffer + len - 14);
419 // Receive incoming data, datagram version.
420 static bool sptps_receive_data_datagram(sptps_t *s, const char *data, size_t len) {
421 if(len < (s->instate ? 21 : 5))
422 return error(s, EIO, "Received short packet");
425 memcpy(&seqno, data, 4);
426 seqno = ntohl(seqno);
429 if(seqno != s->inseqno)
430 return error(s, EIO, "Invalid packet seqno: %d != %d", seqno, s->inseqno);
432 s->inseqno = seqno + 1;
434 uint8_t type = data[4];
436 if(type != SPTPS_HANDSHAKE)
437 return error(s, EIO, "Application record received before handshake finished");
439 return receive_handshake(s, data + 5, len - 5);
443 uint16_t netlen = htons(len - 21);
445 char buffer[len + 23];
447 memcpy(buffer, &netlen, 2);
448 memcpy(buffer + 2, data, len);
450 memcpy(&seqno, buffer + 2, 4);
452 if(!digest_verify(&s->indigest, buffer, len - 14, buffer + len - 14))
453 return error(s, EIO, "Invalid HMAC");
455 // Replay protection using a sliding window of configurable size.
456 // s->inseqno is expected sequence number
457 // seqno is received sequence number
458 // s->late[] is a circular buffer, a 1 bit means a packet has not been received yet
459 // The circular buffer contains bits for sequence numbers from s->inseqno - s->replaywin * 8 to (but excluding) s->inseqno.
461 if(seqno != s->inseqno) {
462 if(seqno >= s->inseqno + s->replaywin * 8) {
463 // Prevent packets that jump far ahead of the queue from causing many others to be dropped.
464 if(s->farfuture++ < s->replaywin >> 2)
465 return error(s, EIO, "Packet is %d seqs in the future, dropped (%u)\n", seqno - s->inseqno, s->farfuture);
467 // Unless we have seen lots of them, in which case we consider the others lost.
468 warning(s, "Lost %d packets\n", seqno - s->inseqno);
469 memset(s->late, 0, s->replaywin);
470 } else if (seqno < s->inseqno) {
471 // If the sequence number is farther in the past than the bitmap goes, or if the packet was already received, drop it.
472 if((s->inseqno >= s->replaywin * 8 && seqno < s->inseqno - s->replaywin * 8) || !(s->late[(seqno / 8) % s->replaywin] & (1 << seqno % 8)))
473 return error(s, EIO, "Received late or replayed packet, seqno %d, last received %d\n", seqno, s->inseqno);
475 // We missed some packets. Mark them in the bitmap as being late.
476 for(int i = s->inseqno; i < seqno; i++)
477 s->late[(i / 8) % s->replaywin] |= 1 << i % 8;
481 // Mark the current packet as not being late.
482 s->late[(seqno / 8) % s->replaywin] &= ~(1 << seqno % 8);
486 if(seqno > s->inseqno)
487 s->inseqno = seqno + 1;
495 cipher_set_counter(&s->incipher, &seqno, sizeof seqno);
496 if(!cipher_counter_xor(&s->incipher, buffer + 6, len - 4, buffer + 6))
499 // Append a NULL byte for safety.
500 buffer[len - 14] = 0;
502 uint8_t type = buffer[6];
504 if(type < SPTPS_HANDSHAKE) {
506 return error(s, EIO, "Application record received before handshake finished");
507 if(!s->receive_record(s->handle, type, buffer + 7, len - 21))
509 } else if(type == SPTPS_HANDSHAKE) {
510 if(!receive_handshake(s, buffer + 7, len - 21))
513 return error(s, EIO, "Invalid record type");
519 // Receive incoming data. Check if it contains a complete record, if so, handle it.
520 bool sptps_receive_data(sptps_t *s, const char *data, size_t len) {
522 return sptps_receive_data_datagram(s, data, len);
525 // First read the 2 length bytes.
527 size_t toread = 6 - s->buflen;
531 memcpy(s->inbuf + s->buflen, data, toread);
537 // Exit early if we don't have the full length.
541 // Decrypt the length bytes
544 if(!cipher_counter_xor(&s->incipher, s->inbuf + 4, 2, &s->reclen))
547 memcpy(&s->reclen, s->inbuf + 4, 2);
550 s->reclen = ntohs(s->reclen);
552 // If we have the length bytes, ensure our buffer can hold the whole request.
553 s->inbuf = realloc(s->inbuf, s->reclen + 23UL);
555 return error(s, errno, strerror(errno));
557 // Add sequence number.
558 uint32_t seqno = htonl(s->inseqno++);
559 memcpy(s->inbuf, &seqno, 4);
561 // Exit early if we have no more data to process.
566 // Read up to the end of the record.
567 size_t toread = s->reclen + (s->instate ? 23UL : 7UL) - s->buflen;
571 memcpy(s->inbuf + s->buflen, data, toread);
576 // If we don't have a whole record, exit.
577 if(s->buflen < s->reclen + (s->instate ? 23UL : 7UL))
580 // Check HMAC and decrypt.
582 if(!digest_verify(&s->indigest, s->inbuf, s->reclen + 7UL, s->inbuf + s->reclen + 7UL))
583 return error(s, EIO, "Invalid HMAC");
585 if(!cipher_counter_xor(&s->incipher, s->inbuf + 6UL, s->reclen + 1UL, s->inbuf + 6UL))
589 // Append a NULL byte for safety.
590 s->inbuf[s->reclen + 7UL] = 0;
592 uint8_t type = s->inbuf[6];
594 if(type < SPTPS_HANDSHAKE) {
596 return error(s, EIO, "Application record received before handshake finished");
597 if(!s->receive_record(s->handle, type, s->inbuf + 7, s->reclen))
599 } else if(type == SPTPS_HANDSHAKE) {
600 if(!receive_handshake(s, s->inbuf + 7, s->reclen))
603 return error(s, EIO, "Invalid record type");
612 // Start a SPTPS session.
613 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) {
614 // Initialise struct sptps
615 memset(s, 0, sizeof *s);
618 s->initiator = initiator;
619 s->datagram = datagram;
622 s->replaywin = sptps_replaywin;
624 s->late = malloc(s->replaywin);
626 return error(s, errno, strerror(errno));
629 s->label = malloc(labellen);
631 return error(s, errno, strerror(errno));
634 s->inbuf = malloc(7);
636 return error(s, errno, strerror(errno));
638 memset(s->inbuf, 0, 4);
641 memcpy(s->label, label, labellen);
642 s->labellen = labellen;
644 s->send_data = send_data;
645 s->receive_record = receive_record;
647 // Do first KEX immediately
648 s->state = SPTPS_KEX;
652 // Stop a SPTPS session.
653 bool sptps_stop(sptps_t *s) {
654 // Clean up any resources.
655 cipher_close(&s->incipher);
656 cipher_close(&s->outcipher);
657 digest_close(&s->indigest);
658 digest_close(&s->outdigest);
666 memset(s, 0, sizeof *s);