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
21 * Useless stuff (because nothing depends on it):
22 * - non-nul key derivation rate
43 # include <winsock2.h>
45 # include <netinet/in.h>
47 GCRY_THREAD_OPTION_PTHREAD_IMPL;
50 #define debug( ... ) (void)0
52 typedef struct srtp_proto_t
54 gcry_cipher_hd_t cipher;
84 static inline unsigned rcc_mode (const srtp_session_t *s)
86 return (s->flags >> 4) & 3;
89 static bool libgcrypt_usable = false;
91 static void initonce_libgcrypt (void)
94 gcry_control (GCRYCTL_SET_THREAD_CBS, &gcry_threads_pthread);
97 if ((gcry_check_version ("1.1.94") == NULL)
98 || gcry_control (GCRYCTL_DISABLE_SECMEM, 0)
99 || gcry_control (GCRYCTL_INITIALIZATION_FINISHED, 0))
102 libgcrypt_usable = true;
105 static int init_libgcrypt (void)
109 static pthread_mutex_t mutex = PTHREAD_MUTEX_INITIALIZER;
110 static pthread_once_t once = PTHREAD_ONCE_INIT;
112 pthread_mutex_lock (&mutex);
113 pthread_once (&once, initonce_libgcrypt);
115 # warning FIXME: This is not thread-safe.
116 if (!libgcrypt_usable)
117 initonce_libgcrypt ();
120 retval = libgcrypt_usable ? 0 : -1;
123 pthread_mutex_unlock (&mutex);
131 static void proto_destroy (srtp_proto_t *p)
133 gcry_md_close (p->mac);
134 gcry_cipher_close (p->cipher);
139 * Releases all resources associated with a Secure RTP session.
141 void srtp_destroy (srtp_session_t *s)
145 proto_destroy (&s->rtcp);
146 proto_destroy (&s->rtp);
151 static int proto_create (srtp_proto_t *p, int gcipher, int gmd)
153 if (gcry_cipher_open (&p->cipher, gcipher, GCRY_CIPHER_MODE_CTR, 0) == 0)
155 if (gcry_md_open (&p->mac, gmd, GCRY_MD_FLAG_HMAC) == 0)
157 gcry_cipher_close (p->cipher);
164 * Allocates a Secure RTP one-way session.
165 * The same session cannot be used both ways because this would confuse
166 * internal cryptographic counters; it is however of course feasible to open
167 * multiple simultaneous sessions with the same master key.
169 * @param encr encryption algorithm number
170 * @param auth authentication algortihm number
171 * @param tag_len authentication tag byte length (NOT including RCC)
172 * @param flags OR'ed optional flags.
174 * @return NULL in case of error
177 srtp_create (int encr, int auth, unsigned tag_len, int prf, unsigned flags)
179 if ((flags & ~SRTP_FLAGS_MASK) || init_libgcrypt ())
186 cipher = GCRY_CIPHER_NONE;
189 case SRTP_ENCR_AES_CM:
190 cipher = GCRY_CIPHER_AES;
203 case SRTP_AUTH_HMAC_SHA1:
211 if (tag_len > gcry_md_get_algo_dlen (auth))
214 if (prf != SRTP_PRF_AES_CM)
217 srtp_session_t *s = malloc (sizeof (*s));
221 memset (s, 0, sizeof (*s));
223 s->tag_len = tag_len;
224 s->rtp_rcc = 1; /* Default RCC rate */
231 if (proto_create (&s->rtp, cipher, md) == 0)
233 if (proto_create (&s->rtcp, cipher, md) == 0)
235 proto_destroy (&s->rtp);
245 * Counter Mode encryption/decryption (ctr length = 16 bytes)
246 * with non-padded (truncated) text
249 ctr_crypt (gcry_cipher_hd_t hd, const void *ctr, uint8_t *data, size_t len)
251 const size_t ctrlen = 16;
252 div_t d = div (len, ctrlen);
254 if (gcry_cipher_setctr (hd, ctr, ctrlen)
255 || gcry_cipher_encrypt (hd, data, d.quot * ctrlen, NULL, 0))
260 /* Truncated last block */
261 uint8_t dummy[ctrlen];
262 data += d.quot * ctrlen;
263 memcpy (dummy, data, d.rem);
264 memset (dummy + d.rem, 0, ctrlen - d.rem);
266 if (gcry_cipher_encrypt (hd, dummy, ctrlen, data, ctrlen))
268 memcpy (data, dummy, d.rem);
276 * AES-CM key derivation (saltlen = 14 bytes)
279 derive (gcry_cipher_hd_t prf, const void *salt,
280 const uint8_t *r, size_t rlen, uint8_t label,
281 void *out, size_t outlen)
285 memcpy (iv, salt, 14);
289 iv[13 - rlen] ^= label;
290 for (size_t i = 0; i < rlen; i++)
291 iv[sizeof (iv) - rlen + i] ^= r[i];
293 memset (out, 0, outlen);
294 return ctr_crypt (prf, iv, out, outlen);
299 proto_derive (srtp_proto_t *p, gcry_cipher_hd_t prf,
300 const void *salt, size_t saltlen,
301 const uint8_t *r, size_t rlen, bool rtcp)
307 uint8_t label = rtcp ? SRTCP_CRYPT : SRTP_CRYPT;
309 if (derive (prf, salt, r, rlen, label++, keybuf, 16)
310 || gcry_cipher_setkey (p->cipher, keybuf, 16)
311 || derive (prf, salt, r, rlen, label++, keybuf, 20)
312 || gcry_md_setkey (p->mac, keybuf, 20)
313 || derive (prf, salt, r, rlen, label, p->salt, 14))
321 * SRTP/SRTCP cipher/salt/MAC keys derivation.
324 srtp_derive (srtp_session_t *s, const void *key, size_t keylen,
325 const void *salt, size_t saltlen)
327 gcry_cipher_hd_t prf;
330 if (gcry_cipher_open (&prf, GCRY_CIPHER_AES, GCRY_CIPHER_MODE_CTR, 0)
331 || gcry_cipher_setkey (prf, key, keylen))
334 /* RTP key derivation */
337 uint64_t index = (((uint64_t)s->rtp_roc) << 16) | s->rtp_seq;
340 for (int i = sizeof (r) - 1; i >= 0; i--)
347 memset (r, 0, sizeof (r));
349 if (proto_derive (&s->rtp, prf, salt, saltlen, r, 6, false))
352 /* RTCP key derivation */
353 memcpy (r, &(uint32_t){ htonl (s->rtcp_index) }, 4);
354 if (proto_derive (&s->rtcp, prf, salt, saltlen, r, 4, true))
357 (void)gcry_cipher_close (prf);
363 * Sets (or resets) the master key and master salt for a SRTP session.
364 * This must be done at least once before using rtp_send(), rtp_recv(),
365 * rtcp_send() or rtcp_recv(). Also, rekeying is required every
366 * 2^48 RTP packets or 2^31 RTCP packets (whichever comes first),
367 * otherwise the protocol security might be broken.
369 * @return 0 on success, in case of error:
370 * EINVAL invalid or unsupported key/salt sizes combination
373 srtp_setkey (srtp_session_t *s, const void *key, size_t keylen,
374 const void *salt, size_t saltlen)
376 return srtp_derive (s, key, keylen, salt, saltlen) ? EINVAL : 0;
381 * Sets Roll-over-Counter Carry (RCC) rate for the SRTP session. If not
382 * specified (through this function), the default rate of ONE is assumed
383 * (i.e. every RTP packets will carry the RoC). RCC rate is ignored if none
384 * of the RCC mode has been selected.
386 * The RCC mode is selected through one of these flags for srtp_create():
387 * SRTP_RCC_MODE1: integrity protection only for RoC carrying packets
388 * SRTP_RCC_MODE2: integrity protection for all packets
389 * SRTP_RCC_MODE3: no integrity protection
391 * RCC mode 3 is insecure. Compared to plain RTP, it provides confidentiality
392 * (through encryption) but is much more prone to DoS. It can only be used if
393 * anti-spoofing protection is provided by lower network layers (e.g. IPsec,
394 * or trusted routers and proper source address filtering).
396 * If RCC rate is 1, RCC mode 1 and 2 are functionally identical.
398 * @param rate RoC Carry rate (MUST NOT be zero)
400 void srtp_setrcc_rate (srtp_session_t *s, uint16_t rate)
407 /** AES-CM for RTP (salt = 14 bytes + 2 nul bytes) */
409 rtp_crypt (gcry_cipher_hd_t hd, uint32_t ssrc, uint32_t roc, uint16_t seq,
410 const uint32_t *salt, uint8_t *data, size_t len)
412 /* Determines cryptographic counter (IV) */
414 counter[0] = salt[0];
415 counter[1] = salt[1] ^ ssrc;
416 counter[2] = salt[2] ^ htonl (roc);
417 counter[3] = salt[3] ^ htonl (seq << 16);
420 return ctr_crypt (hd, counter, data, len);
424 /** Determines SRTP Roll-Over-Counter (in host-byte order) */
426 srtp_compute_roc (const srtp_session_t *s, uint16_t seq)
428 uint32_t roc = s->rtp_roc;
430 if (((seq - s->rtp_seq) & 0xffff) < 0x8000)
432 /* Sequence is ahead, good */
433 if (seq < s->rtp_seq)
434 roc++; /* Sequence number wrap */
438 /* Sequence is late, bad */
439 if (seq > s->rtp_seq)
440 roc--; /* Wrap back */
446 /** Returns RTP sequence (in host-byte order) */
447 static inline uint16_t rtp_seq (const uint8_t *buf)
449 return (buf[2] << 8) | buf[3];
453 /** Message Authentication and Integrity for RTP */
454 static const uint8_t *
455 rtp_digest (srtp_session_t *s, const uint8_t *data, size_t len,
458 const gcry_md_hd_t md = s->rtp.mac;
461 gcry_md_write (md, data, len);
462 gcry_md_write (md, &(uint32_t){ htonl (roc) }, 4);
463 return gcry_md_read (md, 0);
468 * Encrypts/decrypts a RTP packet and updates SRTP context
469 * (CTR block cypher mode of operation has identical encryption and
470 * decryption function).
472 * @param buf RTP packet to be en-/decrypted
473 * @param len RTP packet length
475 * @return 0 on success, in case of error:
476 * EINVAL malformatted RTP packet
477 * EACCES replayed packet or out-of-window or sync lost
479 static int srtp_crypt (srtp_session_t *s, uint8_t *buf, size_t len)
483 if ((len < 12) || ((buf[0] >> 6) != 2))
486 /* Computes encryption offset */
487 uint16_t offset = 12;
488 offset += (buf[0] & 0xf) * 4; // skips CSRC
498 memcpy (&extlen, buf + offset - 2, 2);
499 offset += htons (extlen); // skips RTP extension header
505 /* Determines RTP 48-bits counter and SSRC */
506 uint16_t seq = rtp_seq (buf);
507 uint32_t roc = srtp_compute_roc (s, seq), ssrc;
508 memcpy (&ssrc, buf + 8, 4);
510 /* Updates ROC and sequence (it's safe now) */
511 int16_t diff = seq - s->rtp_seq;
514 /* Sequence in the future, good */
515 s->rtp.window = s->rtp.window << diff;
517 s->rtp_seq = seq, s->rtp_roc = roc;
521 /* Sequence in the past/present, bad */
523 if ((diff >= 64) || ((s->rtp.window >> diff) & 1))
524 return EACCES; /* Replay attack */
525 s->rtp.window |= 1 << diff;
528 /* Encrypt/Decrypt */
529 if (s->flags & SRTP_UNENCRYPTED)
532 if (rtp_crypt (s->rtp.cipher, ssrc, roc, seq, s->rtp.salt,
533 buf + offset, len - offset))
541 * Turns a RTP packet into a SRTP packet: encrypt it, then computes
542 * the authentication tag and appends it.
543 * Note that you can encrypt packet in disorder.
545 * @param buf RTP packet to be encrypted/digested
546 * @param lenp pointer to the RTP packet length on entry,
547 * set to the SRTP length on exit (undefined on non-ENOSPC error)
548 * @param bufsize size (bytes) of the packet buffer
550 * @return 0 on success, in case of error:
551 * EINVAL malformatted RTP packet or internal error
552 * ENOSPC bufsize is too small to add authentication tag
553 * (<lenp> will hold the required byte size)
554 * EACCES packet would trigger a replay error on receiver
557 srtp_send (srtp_session_t *s, uint8_t *buf, size_t *lenp, size_t bufsize)
560 int val = srtp_crypt (s, buf, len);
564 if (!(s->flags & SRTP_UNAUTHENTICATED))
566 size_t tag_len = s->tag_len;
567 *lenp = len + tag_len;
568 if (bufsize < (len + tag_len))
571 uint32_t roc = srtp_compute_roc (s, rtp_seq (buf));
572 const uint8_t *tag = rtp_digest (s, buf, len, roc);
576 if ((rtp_seq (buf) % s->rtp_rcc) == 0)
578 memcpy (buf + len, &(uint32_t){ htonl (s->rtp_roc) }, 4);
580 if (rcc_mode (s) == 3)
587 if (rcc_mode (s) & 1)
591 memcpy (buf + len, tag, tag_len);
599 * Turns a SRTP packet into a RTP packet: authenticates the packet,
602 * @param buf RTP packet to be digested/decrypted
603 * @param lenp pointer to the SRTP packet length on entry,
604 * set to the RTP length on exit (undefined in case of error)
606 * @return 0 on success, in case of error:
607 * EINVAL malformatted SRTP packet
608 * EACCES authentication failed (spoofed packet or out-of-sync)
611 srtp_recv (srtp_session_t *s, uint8_t *buf, size_t *lenp)
617 if (!(s->flags & SRTP_UNAUTHENTICATED))
619 size_t tag_len = s->tag_len, roc_len = 0;
622 if ((rtp_seq (buf) % s->rtp_rcc) == 0)
625 if (rcc_mode (s) == 3)
632 if (rcc_mode (s) & 1)
633 tag_len = 0; // RCC mode 1 or 3: no auth
637 if (len < (12u + roc_len + tag_len))
639 len -= roc_len + tag_len;
641 uint32_t roc = srtp_compute_roc (s, rtp_seq (buf)), rcc;
644 assert (roc_len == 4);
645 memcpy (&rcc, buf + len, 4);
651 const uint8_t *tag = rtp_digest (s, buf, len, rcc);
652 if (memcmp (buf + len + roc_len, tag, s->tag_len))
657 /* Authenticated packet carried a Roll-Over-Counter */
658 s->rtp_roc += rcc - roc;
659 assert (srtp_compute_roc (s, rtp_seq (buf)) == rcc);
664 return srtp_crypt (s, buf, len);
668 /** AES-CM for RTCP (salt = 14 bytes + 2 nul bytes) */
670 rtcp_crypt (gcry_cipher_hd_t hd, uint32_t ssrc, uint32_t index,
671 const uint32_t *salt, uint8_t *data, size_t len)
673 return rtp_crypt (hd, ssrc, index >> 16, index & 0xffff, salt, data, len);
677 /** Message Authentication and Integrity for RTCP */
678 static const uint8_t *
679 rtcp_digest (gcry_md_hd_t md, const void *data, size_t len)
682 gcry_md_write (md, data, len);
683 return gcry_md_read (md, 0);
688 * Encrypts/decrypts a RTCP packet and updates SRTCP context
689 * (CTR block cypher mode of operation has identical encryption and
690 * decryption function).
692 * @param buf RTCP packet to be en-/decrypted
693 * @param len RTCP packet length
695 * @return 0 on success, in case of error:
696 * EINVAL malformatted RTCP packet
698 static int srtcp_crypt (srtp_session_t *s, uint8_t *buf, size_t len)
702 /* 8-bytes unencrypted header, and 4-bytes unencrypted footer */
703 if ((len < 12) || ((buf[0] >> 6) != 2))
707 memcpy (&index, buf + len, 4);
708 index = ntohl (index);
709 if (((index >> 31) != 0) != ((s->flags & SRTCP_UNENCRYPTED) == 0))
710 return EINVAL; // E-bit mismatch
712 index &= ~(1 << 31); // clear E-bit for counter
714 /* Updates SRTCP index (safe here) */
715 int32_t diff = index - s->rtcp_index;
718 /* Packet in the future, good */
719 s->rtcp.window = s->rtcp.window << diff;
721 s->rtcp_index = index;
725 /* Packet in the past/present, bad */
727 if ((diff >= 64) || ((s->rtcp.window >> diff) & 1))
728 return EACCES; // replay attack!
729 s->rtp.window |= 1 << diff;
733 if (s->flags & SRTCP_UNENCRYPTED)
737 memcpy (&ssrc, buf + 4, 4);
739 if (rtcp_crypt (s->rtcp.cipher, ssrc, index, s->rtp.salt,
747 * Turns a RTCP packet into a SRTCP packet: encrypt it, then computes
748 * the authentication tag and appends it.
750 * @param buf RTCP packet to be encrypted/digested
751 * @param lenp pointer to the RTCP packet length on entry,
752 * set to the SRTCP length on exit (undefined in case of error)
753 * @param bufsize size (bytes) of the packet buffer
755 * @return 0 on success, in case of error:
756 * EINVAL malformatted RTCP packet or internal error
757 * ENOSPC bufsize is too small (to add index and authentication tag)
760 srtcp_send (srtp_session_t *s, uint8_t *buf, size_t *lenp, size_t bufsize)
763 if (bufsize < (len + 4 + s->tag_len))
766 uint32_t index = ++s->rtcp_index;
768 s->rtcp_index = index = 0; /* 31-bit wrap */
770 if ((s->flags & SRTCP_UNENCRYPTED) == 0)
771 index |= 0x80000000; /* Set Encrypted bit */
772 memcpy (buf + len, &(uint32_t){ htonl (index) }, 4);
774 int val = srtcp_crypt (s, buf, len);
778 len += 4; /* Digests SRTCP index too */
780 const uint8_t *tag = rtcp_digest (s->rtp.mac, buf, len);
781 memcpy (buf + len, tag, s->tag_len);
782 *lenp = len + s->tag_len;
788 * Turns a SRTCP packet into a RTCP packet: authenticates the packet,
791 * @param buf RTCP packet to be digested/decrypted
792 * @param lenp pointer to the SRTCP packet length on entry,
793 * set to the RTCP length on exit (undefined in case of error)
795 * @return 0 on success, in case of error:
796 * EINVAL malformatted SRTCP packet
797 * EACCES authentication failed (spoofed packet or out-of-sync)
800 srtcp_recv (srtp_session_t *s, uint8_t *buf, size_t *lenp)
804 if (len < (4u + s->tag_len))
808 const uint8_t *tag = rtcp_digest (s->rtp.mac, buf, len);
809 if (memcmp (buf + len, tag, s->tag_len))
812 len -= 4; /* Remove SRTCP index before decryption */
814 return srtp_crypt (s, buf, len);