2 * Secure RTP with libgcrypt
3 * Copyright (C) 2007 RĂ©mi Denis-Courmont <rdenis # simphalempin , com>
5 * This library is free software; you can redistribute it and/or
6 * modify it under the terms of the GNU Lesser General Public
7 * License as published by the Free Software Foundation; either
8 * version 2.1 of the License, or (at your option) any later version.
10 * This library 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 GNU
13 * Lesser General Public License for more details.
15 * You should have received a copy of the GNU Lesser General Public
16 * License along with this library; if not, write to the Free Software
17 * Foundation, Inc., 51 Franklin St, Fifth Floor, Boston, MA 02110-1301 USA
36 #include <netinet/in.h>
41 * - ROC profil thingy (multicast really needs this)
44 * Requirements for conformance:
45 * - suites with NULL cipher
48 * Useless stuff (because nothing depends on it):
49 * - non-nul key derivation rate
53 typedef struct srtp_proto_t
55 gcry_cipher_hd_t cipher;
83 GCRY_THREAD_OPTION_PTHREAD_IMPL;
86 static bool libgcrypt_usable = false;
88 static void initonce_libgcrypt (void)
90 if ((gcry_check_version ("1.1.94") == NULL)
91 || gcry_control (GCRYCTL_DISABLE_SECMEM, 0)
92 || gcry_control (GCRYCTL_INITIALIZATION_FINISHED, 0)
94 || gcry_control (GCRYCTL_SET_THREAD_CBS, &gcry_threads_pthread)
99 libgcrypt_usable = true;
102 static int init_libgcrypt (void)
106 static pthread_mutex_t mutex = PTHREAD_MUTEX_INITIALIZER;
107 static pthread_once_t once = PTHREAD_ONCE_INIT;
109 pthread_mutex_lock (&mutex);
110 pthread_once (&once, initonce_libgcrypt);
111 retval = -libgcrypt_usable;
112 pthread_mutex_unlock (&mutex);
114 # warning FIXME: This is not thread-safe.
115 if (!libgcrypt_usable)
116 initonce_libgcrypt ();
117 retval = -libgcrypt_usable;
125 static void proto_destroy (srtp_proto_t *p)
127 gcry_md_close (p->mac);
128 gcry_cipher_close (p->cipher);
133 * Releases all resources associated with a Secure RTP session.
135 void srtp_destroy (srtp_session_t *s)
139 proto_destroy (&s->rtcp);
140 proto_destroy (&s->rtp);
145 static int proto_create (srtp_proto_t *p, int gcipher, int gmd)
147 if (gcry_cipher_open (&p->cipher, gcipher, GCRY_CIPHER_MODE_CTR, 0) == 0)
149 if (gcry_md_open (&p->mac, gmd, GCRY_MD_FLAG_HMAC) == 0)
151 gcry_cipher_close (p->cipher);
158 * Allocates a Secure RTP session.
160 * @param name cipher-suite name
161 * @param kdr key derivation rate
162 * @param winsize anti-replay windows size (between 64 and 32767 inclusive)
163 * 0 disable replay attack protection (OK for send only)
164 * @param flags OR'ed optional flags.
166 * @return NULL in case of error
169 srtp_create (const char *name, unsigned flags, unsigned kdr, uint16_t winsize)
171 assert (name != NULL);
174 return NULL; // FIXME: KDR not implemented yet
176 return NULL; // FIXME: replay protection not implemented yet
179 int cipher = GCRY_CIPHER_AES, md = GCRY_MD_SHA1;
181 if (strcmp (name, "AES_CM_128_HMAC_SHA1_80") == 0)
184 if (strcmp (name, "AES_CM_128_HMAC_SHA1_32") == 0)
187 // F8_128_HMAC_SHA1_80 is not implemented
190 if ((flags & ~SRTP_FLAGS_MASK) || (winsize > 32767) || init_libgcrypt ())
193 srtp_session_t *s = malloc (sizeof (*s));
197 memset (s, 0, sizeof (*s));
201 if (proto_create (&s->rtp, cipher, md) == 0)
203 if (proto_create (&s->rtcp, cipher, md) == 0)
205 proto_destroy (&s->rtp);
214 * AES-CM key derivation (saltlen = 14 bytes)
217 derive (gcry_cipher_hd_t prf, const void *salt,
218 const uint8_t *r, size_t rlen, uint8_t label,
219 void *out, size_t outlen)
223 memcpy (iv, salt, 14);
227 iv[13 - rlen] ^= label;
228 for (size_t i = 0; i < rlen; i++)
229 iv[sizeof (iv) - rlen + i] ^= r[i];
231 /* TODO: retry with CTR mode */
232 while (outlen >= sizeof (iv))
235 if (gcry_cipher_encrypt (prf, out, sizeof (iv), iv, sizeof (iv)))
237 outlen -= sizeof (iv);
238 out = ((uint8_t *)out) + sizeof (iv);
240 /* Increment IV in network byte order */
241 if (++iv[sizeof (iv) - 1] == 0)
242 ++iv[sizeof (iv) -2];
247 /* Truncated last AES output block */
248 if (gcry_cipher_encrypt (prf, iv, sizeof (iv), NULL, 0))
250 memcpy (out, iv, outlen);
258 proto_derive (srtp_proto_t *p, gcry_cipher_hd_t prf,
259 const void *salt, size_t saltlen,
260 const uint8_t *r, size_t rlen, bool rtcp)
265 uint8_t cipherkey[16];
266 uint8_t label = rtcp ? SRTCP_CRYPT : SRTP_CRYPT;
268 if (derive (prf, salt, r, rlen, label++, cipherkey, 16)
269 || gcry_cipher_setkey (p->cipher, cipherkey, 16)
270 || derive (prf, salt, r, rlen, label++, NULL, 0) /* FIXME HMAC */
271 || derive (prf, salt, r, rlen, label++, p->salt, 14))
279 * SRTP/SRTCP cipher/salt/MAC keys derivation.
282 srtp_derive (srtp_session_t *s, const void *key, size_t keylen,
283 const void *salt, size_t saltlen)
285 gcry_cipher_hd_t prf;
288 /* TODO: retry with CTR mode */
289 if (gcry_cipher_open (&prf, GCRY_CIPHER_AES, GCRY_CIPHER_MODE_ECB, 0)
290 || gcry_cipher_setkey (prf, key, keylen))
293 /* RTP key derivation */
296 uint64_t index = (((uint64_t)s->rtp_roc) << 16) | s->rtp_seq;
299 for (int i = sizeof (r) - 1; i >= 0; i--)
306 memset (r, 0, sizeof (r));
308 if (proto_derive (&s->rtp, prf, salt, saltlen, r, 6, false))
311 /* RTCP key derivation */
312 memcpy (r, &(uint32_t){ htonl (s->rtcp_index) }, 4);
313 if (proto_derive (&s->rtcp, prf, salt, saltlen, r, 4, true))
316 (void)gcry_cipher_close (prf);
323 * Sets (or resets) the master key and master salt for a SRTP session.
324 * This must be done at least once before using rtp_send(), rtp_recv(),
325 * rtcp_send() or rtcp_recv(). Also, rekeying is required every
326 * 2^48 RTP packets or 2^31 RTCP packets (whichever comes first),
327 * otherwise the protocol security might be broken.
329 * @return 0 on success, in case of error:
330 * EINVAL invalid or unsupported key/salt sizes combination
333 srtp_setkey (srtp_session_t *s, const void *key, size_t keylen,
334 const void *salt, size_t saltlen)
336 return srtp_derive (s, key, keylen, salt, saltlen) ? EINVAL : 0;
340 /** AES-CM encryption/decryption (ctr length = 16 bytes) */
342 encrypt (gcry_cipher_hd_t hd, uint32_t *ctr, uint8_t *data, size_t len)
344 const size_t ctrlen = 16;
345 while (len >= ctrlen)
347 if (gcry_cipher_setctr (hd, ctr, ctrlen)
348 || gcry_cipher_encrypt (hd, data, ctrlen, NULL, 0))
353 ctr[3] = htonl (ntohl (ctr[3]) + 1);
358 /* Truncated last block */
359 uint8_t dummy[ctrlen];
360 memcpy (dummy, data, len);
361 memset (dummy + len, 0, ctrlen - len);
363 if (gcry_cipher_setctr (hd, ctr, ctrlen)
364 || gcry_cipher_encrypt (hd, dummy, ctrlen, data, ctrlen))
366 memcpy (data, dummy, len);
373 /** AES-CM for RTP (salt = 14 bytes + 2 nul bytes) */
375 rtp_encrypt (gcry_cipher_hd_t hd, uint32_t ssrc, uint32_t roc, uint16_t seq,
376 const uint32_t *salt, uint8_t *data, size_t len)
378 /* Determines cryptographic counter (IV) */
380 counter[0] = salt[0];
381 counter[1] = salt[1] ^ ssrc;
382 counter[2] = salt[2] ^ htonl (roc);
383 counter[3] = salt[3] ^ htonl (seq << 16);
386 return encrypt (hd, counter, data, len);
391 * Encrypts/decrypts a RTP packet and updates SRTP context
392 * (CTR block cypher mode of operation has identical encryption and
393 * decryption function).
395 * @param buf RTP packet to be encrypted/digested
396 * @param len RTP packet length
398 * @return 0 on success, in case of error:
399 * EINVAL malformatted RTP packet
401 static int srtp_encrypt (srtp_session_t *s, uint8_t *buf, size_t len)
405 if ((len < 12) || ((buf[0] >> 6) != 2))
408 /* Computes encryption offset */
409 uint16_t offset = 12;
410 offset += (buf[0] & 0xf) * 4; // skips CSRC
420 memcpy (&extlen, buf + offset - 2, 2);
421 offset += htons (extlen);
427 /* Determines RTP 48-bits counter and SSRC */
429 memcpy (&ssrc, buf + 8, 4);
431 uint16_t seq = (buf[2] << 8) | buf[3];
432 if (((seq - s->rtp_seq) & 0xffff) < 32768)
434 if (seq < s->rtp_seq)
435 s->rtp_roc++; /* Sequence number wrap */
439 if (seq > s->rtp_seq)
445 if (s->flags & SRTP_UNENCRYPTED)
448 if (rtp_encrypt (s->rtp.cipher, ssrc, s->rtp_roc, seq, s->rtp.salt,
449 buf + offset, len - offset))
457 * Turns a RTP packet into a SRTP packet: encrypt it, then computes
458 * the authentication tag and appends it.
459 * Note that you can encrypt packet in disorder.
461 * @param buf RTP packet to be encrypted/digested
462 * @param lenp pointer to the RTP packet length on entry,
463 * set to the SRTP length on exit (undefined in case of error)
464 * @param bufsize size (bytes) of the packet buffer
466 * @return 0 on success, in case of error:
467 * EINVAL malformatted RTP packet
468 * ENOBUFS bufsize is too small
471 srtp_send (srtp_session_t *s, uint8_t *buf, size_t *lenp, size_t bufsize)
474 int val = srtp_encrypt (s, buf, len);
478 if (bufsize < (len + s->rtp.mac_len))
481 /* FIXME: HMAC and anti-replay */
487 * Turns a RTP packet into a SRTP packet: encrypt it, then computes
488 * the authentication tag and appends it.
489 * Note that you can encrypt packet in disorder.
491 * @param buf RTP packet to be decrypted/digested
492 * @param lenp pointer to the RTP packet length on entry,
493 * set to the SRTP length on exit (undefined in case of error)
495 * @return 0 on success, in case of error:
496 * EINVAL malformatted RTP packet
497 * EACCES authentication failed (spoofed packet or out-of-sync)
500 srtp_recv (srtp_session_t *s, uint8_t *buf, size_t *lenp)
503 int val = srtp_encrypt (s, buf, len);
507 /* FIXME: HMAC and anti-replay */