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 #define debug( ... ) (void)0
40 * - ROC profile thingy (multicast really needs this)
43 * Useless stuff (because nothing depends on it):
44 * - non-nul key derivation rate
48 typedef struct srtp_proto_t
50 gcry_cipher_hd_t cipher;
78 # include <winsock2.h>
80 # include <netinet/in.h>
82 GCRY_THREAD_OPTION_PTHREAD_IMPL;
85 static bool libgcrypt_usable = false;
87 static void initonce_libgcrypt (void)
89 if ((gcry_check_version ("1.1.94") == NULL)
90 || gcry_control (GCRYCTL_DISABLE_SECMEM, 0)
91 || gcry_control (GCRYCTL_INITIALIZATION_FINISHED, 0)
93 || gcry_control (GCRYCTL_SET_THREAD_CBS, &gcry_threads_pthread)
98 libgcrypt_usable = true;
101 static int init_libgcrypt (void)
105 static pthread_mutex_t mutex = PTHREAD_MUTEX_INITIALIZER;
106 static pthread_once_t once = PTHREAD_ONCE_INIT;
108 pthread_mutex_lock (&mutex);
109 pthread_once (&once, initonce_libgcrypt);
110 retval = -libgcrypt_usable;
111 pthread_mutex_unlock (&mutex);
113 # warning FIXME: This is not thread-safe.
114 if (!libgcrypt_usable)
115 initonce_libgcrypt ();
116 retval = -libgcrypt_usable;
124 static void proto_destroy (srtp_proto_t *p)
126 gcry_md_close (p->mac);
127 gcry_cipher_close (p->cipher);
132 * Releases all resources associated with a Secure RTP session.
134 void srtp_destroy (srtp_session_t *s)
138 proto_destroy (&s->rtcp);
139 proto_destroy (&s->rtp);
144 static int proto_create (srtp_proto_t *p, int gcipher, int gmd)
146 if (gcry_cipher_open (&p->cipher, gcipher, GCRY_CIPHER_MODE_CTR, 0) == 0)
148 if (gcry_md_open (&p->mac, gmd, GCRY_MD_FLAG_HMAC) == 0)
150 gcry_cipher_close (p->cipher);
157 * Allocates a Secure RTP one-way session.
158 * The same session cannot be used both ways because this would confuse
159 * internal cryptographic counters; it is however of course feasible to open
160 * multiple simultaneous sessions with the same master key.
162 * @param name cipher-suite name
163 * @param kdr key derivation rate
164 * @param winsize anti-replay windows size (between 64 and 32767 inclusive)
165 * 0 disable replay attack protection (OK for send only)
166 * @param flags OR'ed optional flags.
168 * @return NULL in case of error
171 srtp_create (const char *name, unsigned flags, unsigned kdr, uint16_t winsize)
173 assert (name != NULL);
176 return NULL; // FIXME: KDR not implemented yet
178 return NULL; // FIXME: replay protection not implemented yet
181 int cipher = GCRY_CIPHER_AES, md = GCRY_MD_SHA1;
183 if (strcmp (name, "AES_CM_128_HMAC_SHA1_80") == 0)
186 if (strcmp (name, "AES_CM_128_HMAC_SHA1_32") == 0)
189 // F8_128_HMAC_SHA1_80 is not implemented
192 if ((flags & ~SRTP_FLAGS_MASK) || (winsize > 32767) || init_libgcrypt ())
195 srtp_session_t *s = malloc (sizeof (*s));
199 memset (s, 0, sizeof (*s));
202 s->tag_len = tag_len;
204 if (proto_create (&s->rtp, cipher, md) == 0)
206 if (proto_create (&s->rtcp, cipher, md) == 0)
208 proto_destroy (&s->rtp);
217 * AES-CM key derivation (saltlen = 14 bytes)
220 derive (gcry_cipher_hd_t prf, const void *salt,
221 const uint8_t *r, size_t rlen, uint8_t label,
222 void *out, size_t outlen)
226 memcpy (iv, salt, 14);
230 iv[13 - rlen] ^= label;
231 for (size_t i = 0; i < rlen; i++)
232 iv[sizeof (iv) - rlen + i] ^= r[i];
234 /* TODO: retry with CTR mode */
235 while (outlen >= sizeof (iv))
238 if (gcry_cipher_encrypt (prf, out, sizeof (iv), iv, sizeof (iv)))
240 outlen -= sizeof (iv);
241 out = ((uint8_t *)out) + sizeof (iv);
243 /* Increment IV in network byte order */
244 if (++iv[sizeof (iv) - 1] == 0)
245 ++iv[sizeof (iv) -2];
250 /* Truncated last AES output block */
251 if (gcry_cipher_encrypt (prf, iv, sizeof (iv), NULL, 0))
253 memcpy (out, iv, outlen);
261 proto_derive (srtp_proto_t *p, gcry_cipher_hd_t prf,
262 const void *salt, size_t saltlen,
263 const uint8_t *r, size_t rlen, bool rtcp)
269 uint8_t label = rtcp ? SRTCP_CRYPT : SRTP_CRYPT;
271 if (derive (prf, salt, r, rlen, label++, keybuf, 16)
272 || gcry_cipher_setkey (p->cipher, keybuf, 16)
273 || derive (prf, salt, r, rlen, label++, keybuf, 20)
274 || gcry_md_setkey (p->mac, keybuf, 20)
275 || derive (prf, salt, r, rlen, label, p->salt, 14))
283 * SRTP/SRTCP cipher/salt/MAC keys derivation.
286 srtp_derive (srtp_session_t *s, const void *key, size_t keylen,
287 const void *salt, size_t saltlen)
289 gcry_cipher_hd_t prf;
292 /* TODO: retry with CTR mode */
293 if (gcry_cipher_open (&prf, GCRY_CIPHER_AES, GCRY_CIPHER_MODE_ECB, 0)
294 || gcry_cipher_setkey (prf, key, keylen))
297 /* RTP key derivation */
300 uint64_t index = (((uint64_t)s->rtp_roc) << 16) | s->rtp_seq;
303 for (int i = sizeof (r) - 1; i >= 0; i--)
310 memset (r, 0, sizeof (r));
312 if (proto_derive (&s->rtp, prf, salt, saltlen, r, 6, false))
315 /* RTCP key derivation */
316 memcpy (r, &(uint32_t){ htonl (s->rtcp_index) }, 4);
317 if (proto_derive (&s->rtcp, prf, salt, saltlen, r, 4, true))
320 (void)gcry_cipher_close (prf);
326 * Sets (or resets) the master key and master salt for a SRTP session.
327 * This must be done at least once before using rtp_send(), rtp_recv(),
328 * rtcp_send() or rtcp_recv(). Also, rekeying is required every
329 * 2^48 RTP packets or 2^31 RTCP packets (whichever comes first),
330 * otherwise the protocol security might be broken.
332 * @return 0 on success, in case of error:
333 * EINVAL invalid or unsupported key/salt sizes combination
336 srtp_setkey (srtp_session_t *s, const void *key, size_t keylen,
337 const void *salt, size_t saltlen)
339 return srtp_derive (s, key, keylen, salt, saltlen) ? EINVAL : 0;
343 /** AES-CM encryption/decryption (ctr length = 16 bytes) */
345 ctr_crypt (gcry_cipher_hd_t hd, uint32_t *ctr, uint8_t *data, size_t len)
347 const size_t ctrlen = 16;
348 while (len >= ctrlen)
350 if (gcry_cipher_setctr (hd, ctr, ctrlen)
351 || gcry_cipher_encrypt (hd, data, ctrlen, NULL, 0))
356 ctr[3] = htonl (ntohl (ctr[3]) + 1);
361 /* Truncated last block */
362 uint8_t dummy[ctrlen];
363 memcpy (dummy, data, len);
364 memset (dummy + len, 0, ctrlen - len);
366 if (gcry_cipher_setctr (hd, ctr, ctrlen)
367 || gcry_cipher_encrypt (hd, dummy, ctrlen, data, ctrlen))
369 memcpy (data, dummy, len);
376 /** AES-CM for RTP (salt = 14 bytes + 2 nul bytes) */
378 rtp_crypt (gcry_cipher_hd_t hd, uint32_t ssrc, uint32_t roc, uint16_t seq,
379 const uint32_t *salt, uint8_t *data, size_t len)
381 /* Determines cryptographic counter (IV) */
383 counter[0] = salt[0];
384 counter[1] = salt[1] ^ ssrc;
385 counter[2] = salt[2] ^ htonl (roc);
386 counter[3] = salt[3] ^ htonl (seq << 16);
389 return ctr_crypt (hd, counter, data, len);
393 /** Message Authentication and Integrity for RTP */
394 static const uint8_t *
395 rtp_digest (gcry_md_hd_t md, const void *data, size_t len, uint32_t roc)
398 gcry_md_write (md, data, len);
399 gcry_md_write (md, &(uint32_t){ htonl (roc) }, 4);
400 return gcry_md_read (md, 0);
405 * Encrypts/decrypts a RTP packet and updates SRTP context
406 * (CTR block cypher mode of operation has identical encryption and
407 * decryption function).
409 * @param buf RTP packet to be en-/decrypted
410 * @param len RTP packet length
412 * @return 0 on success, in case of error:
413 * EINVAL malformatted RTP packet
415 static int srtp_crypt (srtp_session_t *s, uint8_t *buf, size_t len)
419 if ((len < 12) || ((buf[0] >> 6) != 2))
422 /* Computes encryption offset */
423 uint16_t offset = 12;
424 offset += (buf[0] & 0xf) * 4; // skips CSRC
434 memcpy (&extlen, buf + offset - 2, 2);
435 offset += htons (extlen); // skips RTP extension header
441 /* Determines RTP 48-bits counter and SSRC */
443 memcpy (&ssrc, buf + 8, 4);
445 uint16_t seq = (buf[2] << 8) | buf[3];
446 if (((seq - s->rtp_seq) & 0xffff) < 32768)
448 if (seq < s->rtp_seq)
449 s->rtp_roc++; /* Sequence number wrap */
453 if (seq > s->rtp_seq)
459 if (s->flags & SRTP_UNENCRYPTED)
462 if (rtp_crypt (s->rtp.cipher, ssrc, s->rtp_roc, seq, s->rtp.salt,
463 buf + offset, len - offset))
471 * Turns a RTP packet into a SRTP packet: encrypt it, then computes
472 * the authentication tag and appends it.
473 * Note that you can encrypt packet in disorder.
475 * @param buf RTP packet to be encrypted/digested
476 * @param lenp pointer to the RTP packet length on entry,
477 * set to the SRTP length on exit (undefined in case of error)
478 * @param bufsize size (bytes) of the packet buffer
480 * @return 0 on success, in case of error:
481 * EINVAL malformatted RTP packet or internal error
482 * ENOSPC bufsize is too small (to add authentication tag)
485 srtp_send (srtp_session_t *s, uint8_t *buf, size_t *lenp, size_t bufsize)
488 int val = srtp_crypt (s, buf, len);
492 if (s->flags & SRTP_UNAUTHENTICATED)
495 if (bufsize < (len + s->tag_len))
498 const uint8_t *tag = rtp_digest (s->rtp.mac, buf, len, s->rtp_roc);
499 memcpy (buf + len, tag, s->tag_len);
500 *lenp = len + s->tag_len;
507 * Turns a SRTP packet into a RTP packet: authenticates the packet,
510 * @param buf RTP packet to be digested/decrypted
511 * @param lenp pointer to the SRTP packet length on entry,
512 * set to the RTP length on exit (undefined in case of error)
514 * @return 0 on success, in case of error:
515 * EINVAL malformatted SRTP packet
516 * EACCES authentication failed (spoofed packet or out-of-sync)
519 srtp_recv (srtp_session_t *s, uint8_t *buf, size_t *lenp)
522 /* FIXME: anti-replay */
524 if (!(s->flags & SRTP_UNAUTHENTICATED))
526 if (len < s->tag_len)
530 const uint8_t *tag = rtp_digest (s->rtp.mac, buf, len, s->rtp_roc);
531 if (memcmp (buf + len, tag, s->tag_len))
537 return srtp_crypt (s, buf, len);
541 /** AES-CM for RTCP (salt = 14 bytes + 2 nul bytes) */
543 rtcp_crypt (gcry_cipher_hd_t hd, uint32_t ssrc, uint32_t index,
544 const uint32_t *salt, uint8_t *data, size_t len)
546 return rtp_crypt (hd, ssrc, index >> 16, index & 0xffff, salt, data, len);
550 /** Message Authentication and Integrity for RTCP */
551 static const uint8_t *
552 rtcp_digest (gcry_md_hd_t md, const void *data, size_t len)
555 gcry_md_write (md, data, len);
556 return gcry_md_read (md, 0);
561 * Encrypts/decrypts a RTCP packet and updates SRTCP context
562 * (CTR block cypher mode of operation has identical encryption and
563 * decryption function).
565 * @param buf RTCP packet to be en-/decrypted
566 * @param len RTCP packet length
568 * @return 0 on success, in case of error:
569 * EINVAL malformatted RTCP packet
571 static int srtcp_crypt (srtp_session_t *s, uint8_t *buf, size_t len)
575 /* 8-bytes unencrypted header, and 4-bytes unencrypted footer */
576 if ((len < 12) || ((buf[0] >> 6) != 2))
579 uint32_t index = s->rtcp_index++;
580 if (index == 0x7fffffff)
581 s->rtcp_index = 0; /* 31-bit wrap */
583 if (s->flags & SRTCP_UNENCRYPTED)
587 memcpy (&ssrc, buf + 4, 4);
589 if (rtcp_crypt (s->rtcp.cipher, ssrc, index, s->rtp.salt,
597 * Turns a RTCP packet into a SRTCP packet: encrypt it, then computes
598 * the authentication tag and appends it.
600 * @param buf RTCP packet to be encrypted/digested
601 * @param lenp pointer to the RTCP packet length on entry,
602 * set to the SRTCP length on exit (undefined in case of error)
603 * @param bufsize size (bytes) of the packet buffer
605 * @return 0 on success, in case of error:
606 * EINVAL malformatted RTCP packet or internal error
607 * ENOSPC bufsize is too small (to add index and authentication tag)
610 srtcp_send (srtp_session_t *s, uint8_t *buf, size_t *lenp, size_t bufsize)
613 if (bufsize < (len + 4 + s->tag_len))
616 uint32_t index = s->rtcp_index;
617 if ((s->flags & SRTCP_UNENCRYPTED) == 0)
618 index |= 0x80000000; /* Set Encrypted bit */
619 memcpy (buf + len, &(uint32_t){ htonl (index) }, 4);
621 int val = srtcp_crypt (s, buf, len);
625 len += 4; /* Digest SRTCP index too */
627 const uint8_t *tag = rtcp_digest (s->rtp.mac, buf, len);
628 memcpy (buf + len, tag, s->tag_len);
629 *lenp = len + s->tag_len;
635 * Turns a SRTCP packet into a RTCP packet: authenticates the packet,
638 * @param buf RTCP packet to be digested/decrypted
639 * @param lenp pointer to the SRTCP packet length on entry,
640 * set to the RTCP length on exit (undefined in case of error)
642 * @return 0 on success, in case of error:
643 * EINVAL malformatted SRTCP packet
644 * EACCES authentication failed (spoofed packet or out-of-sync)
647 srtcp_recv (srtp_session_t *s, uint8_t *buf, size_t *lenp)
650 /* FIXME: anti-replay ?? */
652 if (len < (4u + s->tag_len))
656 const uint8_t *tag = rtcp_digest (s->rtp.mac, buf, len);
657 if (memcmp (buf + len, tag, s->tag_len))
660 len -= 4; /* Remove SRTCP index befor decryption */
663 return srtp_crypt (s, buf, len);