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)
42 * Useless stuff (because nothing depends on it):
43 * - non-nul key derivation rate
47 typedef struct srtp_proto_t
49 gcry_cipher_hd_t cipher;
80 # include <winsock2.h>
82 # include <netinet/in.h>
84 GCRY_THREAD_OPTION_PTHREAD_IMPL;
87 static bool libgcrypt_usable = false;
89 static void initonce_libgcrypt (void)
91 if ((gcry_check_version ("1.1.94") == NULL)
92 || gcry_control (GCRYCTL_DISABLE_SECMEM, 0)
93 || gcry_control (GCRYCTL_INITIALIZATION_FINISHED, 0)
95 || gcry_control (GCRYCTL_SET_THREAD_CBS, &gcry_threads_pthread)
100 libgcrypt_usable = true;
103 static int init_libgcrypt (void)
107 static pthread_mutex_t mutex = PTHREAD_MUTEX_INITIALIZER;
108 static pthread_once_t once = PTHREAD_ONCE_INIT;
110 pthread_mutex_lock (&mutex);
111 pthread_once (&once, initonce_libgcrypt);
112 retval = -libgcrypt_usable;
113 pthread_mutex_unlock (&mutex);
115 # warning FIXME: This is not thread-safe.
116 if (!libgcrypt_usable)
117 initonce_libgcrypt ();
118 retval = -libgcrypt_usable;
126 static void proto_destroy (srtp_proto_t *p)
128 gcry_md_close (p->mac);
129 gcry_cipher_close (p->cipher);
134 * Releases all resources associated with a Secure RTP session.
136 void srtp_destroy (srtp_session_t *s)
140 proto_destroy (&s->rtcp);
141 proto_destroy (&s->rtp);
146 static int proto_create (srtp_proto_t *p, int gcipher, int gmd)
148 if (gcry_cipher_open (&p->cipher, gcipher, GCRY_CIPHER_MODE_CTR, 0) == 0)
150 if (gcry_md_open (&p->mac, gmd, GCRY_MD_FLAG_HMAC) == 0)
152 gcry_cipher_close (p->cipher);
159 * Allocates a Secure RTP one-way session.
160 * The same session cannot be used both ways because this would confuse
161 * internal cryptographic counters; it is however of course feasible to open
162 * multiple simultaneous sessions with the same master key.
164 * @param encr encryption algorithm number
165 * @param auth authentication algortihm number
166 * @param tag_len authentication tag byte length (NOT including RCC)
167 * @param flags OR'ed optional flags.
169 * @return NULL in case of error
172 srtp_create (int encr, int auth, unsigned tag_len, int prf, unsigned flags)
174 if ((flags & ~SRTP_FLAGS_MASK) || init_libgcrypt ())
181 cipher = GCRY_CIPHER_NONE;
184 case SRTP_ENCR_AES_CM:
185 cipher = GCRY_CIPHER_AES;
198 case SRTP_AUTH_HMAC_SHA1:
206 if (tag_len > gcry_md_get_algo_dlen (auth))
209 if (prf != SRTP_PRF_AES_CM)
212 srtp_session_t *s = malloc (sizeof (*s));
216 memset (s, 0, sizeof (*s));
218 s->tag_len = tag_len;
220 if (proto_create (&s->rtp, cipher, md) == 0)
222 if (proto_create (&s->rtcp, cipher, md) == 0)
224 proto_destroy (&s->rtp);
233 * AES-CM key derivation (saltlen = 14 bytes)
236 derive (gcry_cipher_hd_t prf, const void *salt,
237 const uint8_t *r, size_t rlen, uint8_t label,
238 void *out, size_t outlen)
242 memcpy (iv, salt, 14);
246 iv[13 - rlen] ^= label;
247 for (size_t i = 0; i < rlen; i++)
248 iv[sizeof (iv) - rlen + i] ^= r[i];
250 /* TODO: retry with CTR mode */
251 while (outlen >= sizeof (iv))
254 if (gcry_cipher_encrypt (prf, out, sizeof (iv), iv, sizeof (iv)))
256 outlen -= sizeof (iv);
257 out = ((uint8_t *)out) + sizeof (iv);
259 /* Increment IV in network byte order */
260 if (++iv[sizeof (iv) - 1] == 0)
261 ++iv[sizeof (iv) -2];
266 /* Truncated last AES output block */
267 if (gcry_cipher_encrypt (prf, iv, sizeof (iv), NULL, 0))
269 memcpy (out, iv, outlen);
277 proto_derive (srtp_proto_t *p, gcry_cipher_hd_t prf,
278 const void *salt, size_t saltlen,
279 const uint8_t *r, size_t rlen, bool rtcp)
285 uint8_t label = rtcp ? SRTCP_CRYPT : SRTP_CRYPT;
287 if (derive (prf, salt, r, rlen, label++, keybuf, 16)
288 || gcry_cipher_setkey (p->cipher, keybuf, 16)
289 || derive (prf, salt, r, rlen, label++, keybuf, 20)
290 || gcry_md_setkey (p->mac, keybuf, 20)
291 || derive (prf, salt, r, rlen, label, p->salt, 14))
299 * SRTP/SRTCP cipher/salt/MAC keys derivation.
302 srtp_derive (srtp_session_t *s, const void *key, size_t keylen,
303 const void *salt, size_t saltlen)
305 gcry_cipher_hd_t prf;
308 /* TODO: retry with CTR mode */
309 if (gcry_cipher_open (&prf, GCRY_CIPHER_AES, GCRY_CIPHER_MODE_ECB, 0)
310 || gcry_cipher_setkey (prf, key, keylen))
313 /* RTP key derivation */
316 uint64_t index = (((uint64_t)s->rtp_roc) << 16) | s->rtp_seq;
319 for (int i = sizeof (r) - 1; i >= 0; i--)
326 memset (r, 0, sizeof (r));
328 if (proto_derive (&s->rtp, prf, salt, saltlen, r, 6, false))
331 /* RTCP key derivation */
332 memcpy (r, &(uint32_t){ htonl (s->rtcp_index) }, 4);
333 if (proto_derive (&s->rtcp, prf, salt, saltlen, r, 4, true))
336 (void)gcry_cipher_close (prf);
342 * Sets (or resets) the master key and master salt for a SRTP session.
343 * This must be done at least once before using rtp_send(), rtp_recv(),
344 * rtcp_send() or rtcp_recv(). Also, rekeying is required every
345 * 2^48 RTP packets or 2^31 RTCP packets (whichever comes first),
346 * otherwise the protocol security might be broken.
348 * @return 0 on success, in case of error:
349 * EINVAL invalid or unsupported key/salt sizes combination
352 srtp_setkey (srtp_session_t *s, const void *key, size_t keylen,
353 const void *salt, size_t saltlen)
355 return srtp_derive (s, key, keylen, salt, saltlen) ? EINVAL : 0;
360 * Sets Roll-over-Counter Carry (RCC) rate for the SRTP session. If not
361 * specified (through this function), the default rate of ONE is assumed
362 * (i.e. every RTP packets will carry the RoC). RCC rate is ignored if none
363 * of the RCC mode has been selected.
365 * The RCC mode is selected through one of these flags for srtp_create():
366 * SRTP_RCC_MODE1: integrity protection only for RoC carrying packets
367 * SRTP_RCC_MODE2: integrity protection for all packets
368 * SRTP_RCC_MODE3: no integrity protection
370 * RCC mode 3 is insecure. Compared to plain RTP, it provides confidentiality
371 * (through encryption) but is much more prone to DoS. It can only be used if
372 * anti-spoofing protection is provided by lower network layers (e.g. IPsec,
373 * or trusted routers and proper source address filtering).
375 * If RCC rate is 1, RCC mode 1 and 2 are functionally identical.
377 * @param rate RoC Carry rate (MUST NOT be zero)
379 void srtp_setrcc_rate (srtp_session_t *s, uint16_t rate)
386 /** AES-CM encryption/decryption (ctr length = 16 bytes) */
388 ctr_crypt (gcry_cipher_hd_t hd, uint32_t *ctr, uint8_t *data, size_t len)
390 const size_t ctrlen = 16;
391 while (len >= ctrlen)
393 if (gcry_cipher_setctr (hd, ctr, ctrlen)
394 || gcry_cipher_encrypt (hd, data, ctrlen, NULL, 0))
399 ctr[3] = htonl (ntohl (ctr[3]) + 1);
404 /* Truncated last block */
405 uint8_t dummy[ctrlen];
406 memcpy (dummy, data, len);
407 memset (dummy + len, 0, ctrlen - len);
409 if (gcry_cipher_setctr (hd, ctr, ctrlen)
410 || gcry_cipher_encrypt (hd, dummy, ctrlen, data, ctrlen))
412 memcpy (data, dummy, len);
419 /** AES-CM for RTP (salt = 14 bytes + 2 nul bytes) */
421 rtp_crypt (gcry_cipher_hd_t hd, uint32_t ssrc, uint32_t roc, uint16_t seq,
422 const uint32_t *salt, uint8_t *data, size_t len)
424 /* Determines cryptographic counter (IV) */
426 counter[0] = salt[0];
427 counter[1] = salt[1] ^ ssrc;
428 counter[2] = salt[2] ^ htonl (roc);
429 counter[3] = salt[3] ^ htonl (seq << 16);
432 return ctr_crypt (hd, counter, data, len);
436 /** Determines SRTP Roll-Over-Counter (in host-byte order) */
438 srtp_compute_roc (const srtp_session_t *s, uint16_t seq)
440 uint32_t roc = s->rtp_roc;
442 if (((seq - s->rtp_seq) & 0xffff) < 0x8000)
444 /* Sequence is ahead, good */
445 if (seq < s->rtp_seq)
446 roc++; /* Sequence number wrap */
450 /* Sequence is late, bad */
451 if (seq > s->rtp_seq)
452 roc--; /* Wrap back */
458 /** Returns RTP sequence (in host-byte order) */
459 static inline uint16_t rtp_seq (const uint8_t *buf)
461 return (buf[2] << 8) | buf[3];
465 /** Message Authentication and Integrity for RTP */
466 static const uint8_t *
467 rtp_digest (srtp_session_t *s, const uint8_t *data, size_t len)
469 const gcry_md_hd_t md = s->rtp.mac;
470 uint32_t roc = htonl (srtp_compute_roc (s, rtp_seq (data)));
473 gcry_md_write (md, data, len);
474 gcry_md_write (md, &roc, 4);
475 return gcry_md_read (md, 0);
480 * Encrypts/decrypts a RTP packet and updates SRTP context
481 * (CTR block cypher mode of operation has identical encryption and
482 * decryption function).
484 * @param buf RTP packet to be en-/decrypted
485 * @param len RTP packet length
487 * @return 0 on success, in case of error:
488 * EINVAL malformatted RTP packet
489 * EACCES replayed packet or out-of-window or sync lost
491 static int srtp_crypt (srtp_session_t *s, uint8_t *buf, size_t len)
495 if ((len < 12) || ((buf[0] >> 6) != 2))
498 /* Computes encryption offset */
499 uint16_t offset = 12;
500 offset += (buf[0] & 0xf) * 4; // skips CSRC
510 memcpy (&extlen, buf + offset - 2, 2);
511 offset += htons (extlen); // skips RTP extension header
517 /* Determines RTP 48-bits counter and SSRC */
518 uint16_t seq = rtp_seq (buf);
519 uint32_t roc = srtp_compute_roc (s, seq), ssrc;
520 memcpy (&ssrc, buf + 8, 4);
522 /* Updates ROC and sequence (it's safe now) */
523 int16_t diff = seq - s->rtp_seq;
526 /* Sequence in the future, good */
527 s->rtp.window = s->rtp.window << diff;
529 s->rtp_seq = seq, s->rtp_roc = roc;
533 /* Sequence in the past/present, bad */
535 if ((diff >= 64) || ((s->rtp.window >> diff) & 1))
536 return EACCES; /* Replay attack */
537 s->rtp.window |= 1 << diff;
540 /* Encrypt/Decrypt */
541 if (s->flags & SRTP_UNENCRYPTED)
544 if (rtp_crypt (s->rtp.cipher, ssrc, roc, seq, s->rtp.salt,
545 buf + offset, len - offset))
553 * Turns a RTP packet into a SRTP packet: encrypt it, then computes
554 * the authentication tag and appends it.
555 * Note that you can encrypt packet in disorder.
557 * @param buf RTP packet to be encrypted/digested
558 * @param lenp pointer to the RTP packet length on entry,
559 * set to the SRTP length on exit (undefined in case of error)
560 * @param bufsize size (bytes) of the packet buffer
562 * @return 0 on success, in case of error:
563 * EINVAL malformatted RTP packet or internal error
564 * ENOSPC bufsize is too small (to add authentication tag)
565 * EACCES packet would trigger a replay error on receiver
568 srtp_send (srtp_session_t *s, uint8_t *buf, size_t *lenp, size_t bufsize)
571 int val = srtp_crypt (s, buf, len);
575 if (!(s->flags & SRTP_UNAUTHENTICATED))
577 if (bufsize < (len + s->tag_len))
580 const uint8_t *tag = rtp_digest (s, buf, len);
581 memcpy (buf + len, tag, s->tag_len);
582 *lenp = len + s->tag_len;
590 * Turns a SRTP packet into a RTP packet: authenticates the packet,
593 * @param buf RTP packet to be digested/decrypted
594 * @param lenp pointer to the SRTP packet length on entry,
595 * set to the RTP length on exit (undefined in case of error)
597 * @return 0 on success, in case of error:
598 * EINVAL malformatted SRTP packet
599 * EACCES authentication failed (spoofed packet or out-of-sync)
602 srtp_recv (srtp_session_t *s, uint8_t *buf, size_t *lenp)
606 if (!(s->flags & SRTP_UNAUTHENTICATED))
608 if (len < (12u + s->tag_len))
612 const uint8_t *tag = rtp_digest (s, buf, len);
613 if (memcmp (buf + len, tag, s->tag_len))
619 return srtp_crypt (s, buf, len);
623 /** AES-CM for RTCP (salt = 14 bytes + 2 nul bytes) */
625 rtcp_crypt (gcry_cipher_hd_t hd, uint32_t ssrc, uint32_t index,
626 const uint32_t *salt, uint8_t *data, size_t len)
628 return rtp_crypt (hd, ssrc, index >> 16, index & 0xffff, salt, data, len);
632 /** Message Authentication and Integrity for RTCP */
633 static const uint8_t *
634 rtcp_digest (gcry_md_hd_t md, const void *data, size_t len)
637 gcry_md_write (md, data, len);
638 return gcry_md_read (md, 0);
643 * Encrypts/decrypts a RTCP packet and updates SRTCP context
644 * (CTR block cypher mode of operation has identical encryption and
645 * decryption function).
647 * @param buf RTCP packet to be en-/decrypted
648 * @param len RTCP packet length
650 * @return 0 on success, in case of error:
651 * EINVAL malformatted RTCP packet
653 static int srtcp_crypt (srtp_session_t *s, uint8_t *buf, size_t len)
657 /* 8-bytes unencrypted header, and 4-bytes unencrypted footer */
658 if ((len < 12) || ((buf[0] >> 6) != 2))
662 memcpy (&index, buf + len, 4);
663 index = ntohl (index);
664 if (((index >> 31) != 0) != ((s->flags & SRTCP_UNENCRYPTED) == 0))
665 return EINVAL; // E-bit mismatch
667 index &= ~(1 << 31); // clear E-bit for counter
669 /* Updates SRTCP index (safe here) */
670 int32_t diff = index - s->rtcp_index;
673 /* Packet in the future, good */
674 s->rtcp.window = s->rtcp.window << diff;
676 s->rtcp_index = index;
680 /* Packet in the past/present, bad */
682 if ((diff >= 64) || ((s->rtcp.window >> diff) & 1))
683 return EACCES; // replay attack!
684 s->rtp.window |= 1 << diff;
688 if (s->flags & SRTCP_UNENCRYPTED)
692 memcpy (&ssrc, buf + 4, 4);
694 if (rtcp_crypt (s->rtcp.cipher, ssrc, index, s->rtp.salt,
702 * Turns a RTCP packet into a SRTCP packet: encrypt it, then computes
703 * the authentication tag and appends it.
705 * @param buf RTCP packet to be encrypted/digested
706 * @param lenp pointer to the RTCP packet length on entry,
707 * set to the SRTCP length on exit (undefined in case of error)
708 * @param bufsize size (bytes) of the packet buffer
710 * @return 0 on success, in case of error:
711 * EINVAL malformatted RTCP packet or internal error
712 * ENOSPC bufsize is too small (to add index and authentication tag)
715 srtcp_send (srtp_session_t *s, uint8_t *buf, size_t *lenp, size_t bufsize)
718 if (bufsize < (len + 4 + s->tag_len))
721 uint32_t index = ++s->rtcp_index;
723 s->rtcp_index = index = 0; /* 31-bit wrap */
725 if ((s->flags & SRTCP_UNENCRYPTED) == 0)
726 index |= 0x80000000; /* Set Encrypted bit */
727 memcpy (buf + len, &(uint32_t){ htonl (index) }, 4);
729 int val = srtcp_crypt (s, buf, len);
733 len += 4; /* Digests SRTCP index too */
735 const uint8_t *tag = rtcp_digest (s->rtp.mac, buf, len);
736 memcpy (buf + len, tag, s->tag_len);
737 *lenp = len + s->tag_len;
743 * Turns a SRTCP packet into a RTCP packet: authenticates the packet,
746 * @param buf RTCP packet to be digested/decrypted
747 * @param lenp pointer to the SRTCP packet length on entry,
748 * set to the RTCP length on exit (undefined in case of error)
750 * @return 0 on success, in case of error:
751 * EINVAL malformatted SRTCP packet
752 * EACCES authentication failed (spoofed packet or out-of-sync)
755 srtcp_recv (srtp_session_t *s, uint8_t *buf, size_t *lenp)
759 if (len < (4u + s->tag_len))
763 const uint8_t *tag = rtcp_digest (s->rtp.mac, buf, len);
764 if (memcmp (buf + len, tag, s->tag_len))
767 len -= 4; /* Remove SRTCP index before decryption */
769 return srtp_crypt (s, buf, len);