2 sptps.c -- Simple Peer-to-Peer Security
3 Copyright (C) 2011 Guus Sliepen <guus@tinc-vpn.org>,
5 This program is free software; you can redistribute it and/or modify
6 it under the terms of the GNU General Public License as published by
7 the Free Software Foundation; either version 2 of the License, or
8 (at your option) any later version.
10 This program is distributed in the hope that it will be useful,
11 but WITHOUT ANY WARRANTY; without even the implied warranty of
12 MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
13 GNU General Public License for more details.
15 You should have received a copy of the GNU General Public License along
16 with this program; if not, write to the Free Software Foundation, Inc.,
17 51 Franklin Street, Fifth Floor, Boston, MA 02110-1301 USA.
31 Nonce MUST be exchanged first (done)
32 Signatures MUST be done over both nonces, to guarantee the signature is fresh
33 Otherwise: if ECDHE key of one side is compromised, it can be reused!
35 Add explicit tag to beginning of structure to distinguish the client and server when signing. (done)
37 Sign all handshake messages up to ECDHE kex with long-term public keys. (done)
39 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?)
41 Explicit close message needs to be added.
43 Maybe do add some alert messages to give helpful error messages? Not more than TLS sends.
45 Use counter mode instead of OFB. (done)
47 Make sure ECC operations are fixed time (aka prevent side-channel attacks).
50 // Log an error message.
51 static bool error(sptps_t *s, int s_errno, const char *msg) {
52 fprintf(stderr, "SPTPS error: %s\n", msg);
57 // Send a record (datagram version, accepts all record types, handles encryption and authentication).
58 static bool send_record_priv_datagram(sptps_t *s, uint8_t type, const char *data, uint16_t len) {
59 char buffer[len + 23UL];
61 // Create header with sequence number, length and record type
62 uint32_t seqno = htonl(s->outseqno++);
63 uint16_t netlen = htons(len);
65 memcpy(buffer, &netlen, 2);
66 memcpy(buffer + 2, &seqno, 4);
69 // Add plaintext (TODO: avoid unnecessary copy)
70 memcpy(buffer + 7, data, len);
73 // If first handshake has finished, encrypt and HMAC
74 cipher_set_counter(&s->outcipher, &seqno, sizeof seqno);
75 if(!cipher_counter_xor(&s->outcipher, buffer + 6, len + 1UL, buffer + 6))
78 if(!digest_create(&s->outdigest, buffer, len + 7UL, buffer + 7UL + len))
81 return s->send_data(s->handle, buffer + 2, len + 21UL);
83 // Otherwise send as plaintext
84 return s->send_data(s->handle, buffer + 2, len + 5UL);
87 // Send a record (private version, accepts all record types, handles encryption and authentication).
88 static bool send_record_priv(sptps_t *s, uint8_t type, const char *data, uint16_t len) {
90 return send_record_priv_datagram(s, type, data, len);
92 char buffer[len + 23UL];
94 // Create header with sequence number, length and record type
95 uint32_t seqno = htonl(s->outseqno++);
96 uint16_t netlen = htons(len);
98 memcpy(buffer, &seqno, 4);
99 memcpy(buffer + 4, &netlen, 2);
102 // Add plaintext (TODO: avoid unnecessary copy)
103 memcpy(buffer + 7, data, len);
106 // If first handshake has finished, encrypt and HMAC
107 if(!cipher_counter_xor(&s->outcipher, buffer + 4, len + 3UL, buffer + 4))
110 if(!digest_create(&s->outdigest, buffer, len + 7UL, buffer + 7UL + len))
113 return s->send_data(s->handle, buffer + 4, len + 19UL);
115 // Otherwise send as plaintext
116 return s->send_data(s->handle, buffer + 4, len + 3UL);
120 // Send an application record.
121 bool sptps_send_record(sptps_t *s, uint8_t type, const char *data, uint16_t len) {
122 // Sanity checks: application cannot send data before handshake is finished,
123 // and only record types 0..127 are allowed.
125 return error(s, EINVAL, "Handshake phase not finished yet");
127 if(type >= SPTPS_HANDSHAKE)
128 return error(s, EINVAL, "Invalid application record type");
130 return send_record_priv(s, type, data, len);
133 // Send a Key EXchange record, containing a random nonce and an ECDHE public key.
134 static bool send_kex(sptps_t *s) {
135 size_t keylen = ECDH_SIZE;
137 // Make room for our KEX message, which we will keep around since send_sig() needs it.
140 s->mykex = realloc(s->mykex, 1 + 32 + keylen);
142 return error(s, errno, strerror(errno));
144 // Set version byte to zero.
145 s->mykex[0] = SPTPS_VERSION;
147 // Create a random nonce.
148 randomize(s->mykex + 1, 32);
150 // Create a new ECDH public key.
151 if(!ecdh_generate_public(&s->ecdh, s->mykex + 1 + 32))
154 return send_record_priv(s, SPTPS_HANDSHAKE, s->mykex, 1 + 32 + keylen);
157 // Send a SIGnature record, containing an ECDSA signature over both KEX records.
158 static bool send_sig(sptps_t *s) {
159 size_t keylen = ECDH_SIZE;
160 size_t siglen = ecdsa_size(&s->mykey);
162 // Concatenate both KEX messages, plus tag indicating if it is from the connection originator
163 char msg[(1 + 32 + keylen) * 2 + 1];
166 msg[0] = s->initiator;
167 memcpy(msg + 1, s->mykex, 1 + 32 + keylen);
168 memcpy(msg + 2 + 32 + keylen, s->hiskex, 1 + 32 + keylen);
171 if(!ecdsa_sign(&s->mykey, msg, sizeof msg, sig))
174 // Send the SIG exchange record.
175 return send_record_priv(s, SPTPS_HANDSHAKE, sig, sizeof sig);
178 // Generate key material from the shared secret created from the ECDHE key exchange.
179 static bool generate_key_material(sptps_t *s, const char *shared, size_t len) {
180 // Initialise cipher and digest structures if necessary
183 = cipher_open_by_name(&s->incipher, "aes-256-ecb")
184 && cipher_open_by_name(&s->outcipher, "aes-256-ecb")
185 && digest_open_by_name(&s->indigest, "sha256", 16)
186 && digest_open_by_name(&s->outdigest, "sha256", 16);
191 // Allocate memory for key material
192 size_t keylen = digest_keylength(&s->indigest) + digest_keylength(&s->outdigest) + cipher_keylength(&s->incipher) + cipher_keylength(&s->outcipher);
194 s->key = realloc(s->key, keylen);
196 return error(s, errno, strerror(errno));
198 // Create the HMAC seed, which is "key expansion" + session label + server nonce + client nonce
199 char seed[s->labellen + 64 + 13];
200 strcpy(seed, "key expansion");
202 memcpy(seed + 13, s->mykex + 1, 32);
203 memcpy(seed + 45, s->hiskex + 1, 32);
205 memcpy(seed + 13, s->hiskex + 1, 32);
206 memcpy(seed + 45, s->mykex + 1, 32);
208 memcpy(seed + 78, s->label, s->labellen);
210 // Use PRF to generate the key material
211 if(!prf(shared, len, seed, s->labellen + 64 + 13, s->key, keylen))
217 // Send an ACKnowledgement record.
218 static bool send_ack(sptps_t *s) {
219 return send_record_priv(s, SPTPS_HANDSHAKE, "", 0);
222 // Receive an ACKnowledgement record.
223 static bool receive_ack(sptps_t *s, const char *data, uint16_t len) {
225 return error(s, EIO, "Invalid ACK record length");
229 = cipher_set_counter_key(&s->incipher, s->key)
230 && digest_set_key(&s->indigest, s->key + cipher_keylength(&s->incipher), digest_keylength(&s->indigest));
235 = cipher_set_counter_key(&s->incipher, s->key + cipher_keylength(&s->outcipher) + digest_keylength(&s->outdigest))
236 && digest_set_key(&s->indigest, s->key + cipher_keylength(&s->outcipher) + digest_keylength(&s->outdigest) + cipher_keylength(&s->incipher), digest_keylength(&s->indigest));
248 // Receive a Key EXchange record, respond by sending a SIG record.
249 static bool receive_kex(sptps_t *s, const char *data, uint16_t len) {
250 // Verify length of the HELLO record
251 if(len != 1 + 32 + ECDH_SIZE)
252 return error(s, EIO, "Invalid KEX record length");
254 // Ignore version number for now.
256 // Make a copy of the KEX message, send_sig() and receive_sig() need it
259 s->hiskex = realloc(s->hiskex, len);
261 return error(s, errno, strerror(errno));
263 memcpy(s->hiskex, data, len);
268 // Receive a SIGnature record, verify it, if it passed, compute the shared secret and calculate the session keys.
269 static bool receive_sig(sptps_t *s, const char *data, uint16_t len) {
270 size_t keylen = ECDH_SIZE;
271 size_t siglen = ecdsa_size(&s->hiskey);
273 // Verify length of KEX record.
275 return error(s, EIO, "Invalid KEX record length");
277 // Concatenate both KEX messages, plus tag indicating if it is from the connection originator
278 char msg[(1 + 32 + keylen) * 2 + 1];
280 msg[0] = !s->initiator;
281 memcpy(msg + 1, s->hiskex, 1 + 32 + keylen);
282 memcpy(msg + 2 + 32 + keylen, s->mykex, 1 + 32 + keylen);
285 if(!ecdsa_verify(&s->hiskey, msg, sizeof msg, data))
288 // Compute shared secret.
289 char shared[ECDH_SHARED_SIZE];
290 if(!ecdh_compute_shared(&s->ecdh, s->hiskex + 1 + 32, shared))
293 // Generate key material from shared secret.
294 if(!generate_key_material(s, shared, sizeof shared))
303 // Send cipher change record
307 // TODO: only set new keys after ACK has been set/received
310 = cipher_set_counter_key(&s->outcipher, s->key + cipher_keylength(&s->incipher) + digest_keylength(&s->indigest))
311 && digest_set_key(&s->outdigest, s->key + cipher_keylength(&s->incipher) + digest_keylength(&s->indigest) + cipher_keylength(&s->outcipher), digest_keylength(&s->outdigest));
316 = cipher_set_counter_key(&s->outcipher, s->key)
317 && digest_set_key(&s->outdigest, s->key + cipher_keylength(&s->outcipher), digest_keylength(&s->outdigest));
327 // Force another Key EXchange (for testing purposes).
328 bool sptps_force_kex(sptps_t *s) {
329 if(!s->outstate || s->state != SPTPS_SECONDARY_KEX)
330 return error(s, EINVAL, "Cannot force KEX in current state");
332 s->state = SPTPS_KEX;
336 // Receive a handshake record.
337 static bool receive_handshake(sptps_t *s, const char *data, uint16_t len) {
338 // Only a few states to deal with handshaking.
339 fprintf(stderr, "Received handshake message, current state %d\n", s->state);
341 case SPTPS_SECONDARY_KEX:
342 // We receive a secondary KEX request, first respond by sending our own.
346 // We have sent our KEX request, we expect our peer to sent one as well.
347 if(!receive_kex(s, data, len))
349 s->state = SPTPS_SIG;
352 // If we already sent our secondary public ECDH key, we expect the peer to send his.
353 if(!receive_sig(s, data, len))
355 s->state = SPTPS_ACK;
358 // We expect a handshake message to indicate transition to the new keys.
359 if(!receive_ack(s, data, len))
361 s->receive_record(s->handle, SPTPS_HANDSHAKE, NULL, 0);
362 s->state = SPTPS_SECONDARY_KEX;
364 // TODO: split ACK into a VERify and ACK?
366 return error(s, EIO, "Invalid session state");
370 // Receive incoming data, datagram version.
371 static bool sptps_receive_data_datagram(sptps_t *s, const char *data, size_t len) {
372 if(len < (s->instate ? 21 : 5))
373 return error(s, EIO, "Received short packet");
376 memcpy(&seqno, data, 4);
377 seqno = ntohl(seqno);
380 if(seqno != s->inseqno) {
381 fprintf(stderr, "Received invalid packet seqno: %d != %d\n", seqno, s->inseqno);
382 return error(s, EIO, "Invalid packet seqno");
385 s->inseqno = seqno + 1;
387 uint8_t type = data[4];
389 if(type != SPTPS_HANDSHAKE)
390 return error(s, EIO, "Application record received before handshake finished");
392 return receive_handshake(s, data + 5, len - 5);
395 if(seqno < s->inseqno) {
396 fprintf(stderr, "Received late or replayed packet: %d < %d\n", seqno, s->inseqno);
400 if(seqno > s->inseqno)
401 fprintf(stderr, "Missed %d packets\n", seqno - s->inseqno);
403 s->inseqno = seqno + 1;
405 uint16_t netlen = htons(len - 21);
407 char buffer[len + 23];
409 memcpy(buffer, &netlen, 2);
410 memcpy(buffer + 2, data, len);
412 memcpy(&seqno, buffer + 2, 4);
414 // Check HMAC and decrypt.
415 if(!digest_verify(&s->indigest, buffer, len - 14, buffer + len - 14))
416 return error(s, EIO, "Invalid HMAC");
418 cipher_set_counter(&s->incipher, &seqno, sizeof seqno);
419 if(!cipher_counter_xor(&s->incipher, buffer + 6, len - 4, buffer + 6))
422 // Append a NULL byte for safety.
423 buffer[len - 14] = 0;
425 uint8_t type = buffer[6];
427 if(type < SPTPS_HANDSHAKE) {
429 return error(s, EIO, "Application record received before handshake finished");
430 if(!s->receive_record(s->handle, type, buffer + 7, len - 21))
433 return error(s, EIO, "Invalid record type");
438 // Receive incoming data. Check if it contains a complete record, if so, handle it.
439 bool sptps_receive_data(sptps_t *s, const char *data, size_t len) {
441 return sptps_receive_data_datagram(s, data, len);
444 // First read the 2 length bytes.
446 size_t toread = 6 - s->buflen;
450 memcpy(s->inbuf + s->buflen, data, toread);
456 // Exit early if we don't have the full length.
460 // Decrypt the length bytes
463 if(!cipher_counter_xor(&s->incipher, s->inbuf + 4, 2, &s->reclen))
466 memcpy(&s->reclen, s->inbuf + 4, 2);
469 s->reclen = ntohs(s->reclen);
471 // If we have the length bytes, ensure our buffer can hold the whole request.
472 s->inbuf = realloc(s->inbuf, s->reclen + 23UL);
474 return error(s, errno, strerror(errno));
476 // Add sequence number.
477 uint32_t seqno = htonl(s->inseqno++);
478 memcpy(s->inbuf, &seqno, 4);
480 // Exit early if we have no more data to process.
485 // Read up to the end of the record.
486 size_t toread = s->reclen + (s->instate ? 23UL : 7UL) - s->buflen;
490 memcpy(s->inbuf + s->buflen, data, toread);
495 // If we don't have a whole record, exit.
496 if(s->buflen < s->reclen + (s->instate ? 23UL : 7UL))
499 // Check HMAC and decrypt.
501 if(!digest_verify(&s->indigest, s->inbuf, s->reclen + 7UL, s->inbuf + s->reclen + 7UL))
502 return error(s, EIO, "Invalid HMAC");
504 if(!cipher_counter_xor(&s->incipher, s->inbuf + 6UL, s->reclen + 1UL, s->inbuf + 6UL))
508 // Append a NULL byte for safety.
509 s->inbuf[s->reclen + 7UL] = 0;
511 uint8_t type = s->inbuf[6];
513 if(type < SPTPS_HANDSHAKE) {
515 return error(s, EIO, "Application record received before handshake finished");
516 if(!s->receive_record(s->handle, type, s->inbuf + 7, s->reclen))
518 } else if(type == SPTPS_HANDSHAKE) {
519 if(!receive_handshake(s, s->inbuf + 7, s->reclen))
522 return error(s, EIO, "Invalid record type");
531 // Start a SPTPS session.
532 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) {
533 // Initialise struct sptps
534 memset(s, 0, sizeof *s);
537 s->initiator = initiator;
538 s->datagram = datagram;
542 s->label = malloc(labellen);
544 return error(s, errno, strerror(errno));
547 s->inbuf = malloc(7);
549 return error(s, errno, strerror(errno));
551 memset(s->inbuf, 0, 4);
554 memcpy(s->label, label, labellen);
555 s->labellen = labellen;
557 s->send_data = send_data;
558 s->receive_record = receive_record;
560 // Do first KEX immediately
561 s->state = SPTPS_KEX;
565 // Stop a SPTPS session.
566 bool sptps_stop(sptps_t *s) {
567 // Clean up any resources.