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)
93 if ((gcry_check_version ("1.1.94") == NULL)
94 || gcry_control (GCRYCTL_DISABLE_SECMEM, 0)
95 || gcry_control (GCRYCTL_INITIALIZATION_FINISHED, 0)
97 || gcry_control (GCRYCTL_SET_THREAD_CBS, &gcry_threads_pthread)
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);
114 retval = -libgcrypt_usable;
115 pthread_mutex_unlock (&mutex);
117 # warning FIXME: This is not thread-safe.
118 if (!libgcrypt_usable)
119 initonce_libgcrypt ();
120 retval = -libgcrypt_usable;
128 static void proto_destroy (srtp_proto_t *p)
130 gcry_md_close (p->mac);
131 gcry_cipher_close (p->cipher);
136 * Releases all resources associated with a Secure RTP session.
138 void srtp_destroy (srtp_session_t *s)
142 proto_destroy (&s->rtcp);
143 proto_destroy (&s->rtp);
148 static int proto_create (srtp_proto_t *p, int gcipher, int gmd)
150 if (gcry_cipher_open (&p->cipher, gcipher, GCRY_CIPHER_MODE_CTR, 0) == 0)
152 if (gcry_md_open (&p->mac, gmd, GCRY_MD_FLAG_HMAC) == 0)
154 gcry_cipher_close (p->cipher);
161 * Allocates a Secure RTP one-way session.
162 * The same session cannot be used both ways because this would confuse
163 * internal cryptographic counters; it is however of course feasible to open
164 * multiple simultaneous sessions with the same master key.
166 * @param encr encryption algorithm number
167 * @param auth authentication algortihm number
168 * @param tag_len authentication tag byte length (NOT including RCC)
169 * @param flags OR'ed optional flags.
171 * @return NULL in case of error
174 srtp_create (int encr, int auth, unsigned tag_len, int prf, unsigned flags)
176 if ((flags & ~SRTP_FLAGS_MASK) || init_libgcrypt ())
183 cipher = GCRY_CIPHER_NONE;
186 case SRTP_ENCR_AES_CM:
187 cipher = GCRY_CIPHER_AES;
200 case SRTP_AUTH_HMAC_SHA1:
208 if (tag_len > gcry_md_get_algo_dlen (auth))
211 if (prf != SRTP_PRF_AES_CM)
214 srtp_session_t *s = malloc (sizeof (*s));
218 memset (s, 0, sizeof (*s));
220 s->tag_len = tag_len;
221 s->rtp_rcc = 1; /* Default RCC rate */
228 if (proto_create (&s->rtp, cipher, md) == 0)
230 if (proto_create (&s->rtcp, cipher, md) == 0)
232 proto_destroy (&s->rtp);
242 * Counter Mode encryption/decryption (ctr length = 16 bytes)
243 * with non-padded (truncated) text
246 ctr_crypt (gcry_cipher_hd_t hd, const void *ctr, uint8_t *data, size_t len)
248 const size_t ctrlen = 16;
249 div_t d = div (len, ctrlen);
251 if (gcry_cipher_setctr (hd, ctr, ctrlen)
252 || gcry_cipher_encrypt (hd, data, d.quot * ctrlen, NULL, 0))
257 /* Truncated last block */
258 uint8_t dummy[ctrlen];
259 data += d.quot * ctrlen;
260 memcpy (dummy, data, d.rem);
261 memset (dummy + d.rem, 0, ctrlen - d.rem);
263 if (gcry_cipher_encrypt (hd, dummy, ctrlen, data, ctrlen))
265 memcpy (data, dummy, d.rem);
273 * AES-CM key derivation (saltlen = 14 bytes)
276 derive (gcry_cipher_hd_t prf, const void *salt,
277 const uint8_t *r, size_t rlen, uint8_t label,
278 void *out, size_t outlen)
282 memcpy (iv, salt, 14);
286 iv[13 - rlen] ^= label;
287 for (size_t i = 0; i < rlen; i++)
288 iv[sizeof (iv) - rlen + i] ^= r[i];
290 memset (out, 0, outlen);
291 return ctr_crypt (prf, iv, out, outlen);
296 proto_derive (srtp_proto_t *p, gcry_cipher_hd_t prf,
297 const void *salt, size_t saltlen,
298 const uint8_t *r, size_t rlen, bool rtcp)
304 uint8_t label = rtcp ? SRTCP_CRYPT : SRTP_CRYPT;
306 if (derive (prf, salt, r, rlen, label++, keybuf, 16)
307 || gcry_cipher_setkey (p->cipher, keybuf, 16)
308 || derive (prf, salt, r, rlen, label++, keybuf, 20)
309 || gcry_md_setkey (p->mac, keybuf, 20)
310 || derive (prf, salt, r, rlen, label, p->salt, 14))
318 * SRTP/SRTCP cipher/salt/MAC keys derivation.
321 srtp_derive (srtp_session_t *s, const void *key, size_t keylen,
322 const void *salt, size_t saltlen)
324 gcry_cipher_hd_t prf;
327 if (gcry_cipher_open (&prf, GCRY_CIPHER_AES, GCRY_CIPHER_MODE_CTR, 0)
328 || gcry_cipher_setkey (prf, key, keylen))
331 /* RTP key derivation */
334 uint64_t index = (((uint64_t)s->rtp_roc) << 16) | s->rtp_seq;
337 for (int i = sizeof (r) - 1; i >= 0; i--)
344 memset (r, 0, sizeof (r));
346 if (proto_derive (&s->rtp, prf, salt, saltlen, r, 6, false))
349 /* RTCP key derivation */
350 memcpy (r, &(uint32_t){ htonl (s->rtcp_index) }, 4);
351 if (proto_derive (&s->rtcp, prf, salt, saltlen, r, 4, true))
354 (void)gcry_cipher_close (prf);
360 * Sets (or resets) the master key and master salt for a SRTP session.
361 * This must be done at least once before using rtp_send(), rtp_recv(),
362 * rtcp_send() or rtcp_recv(). Also, rekeying is required every
363 * 2^48 RTP packets or 2^31 RTCP packets (whichever comes first),
364 * otherwise the protocol security might be broken.
366 * @return 0 on success, in case of error:
367 * EINVAL invalid or unsupported key/salt sizes combination
370 srtp_setkey (srtp_session_t *s, const void *key, size_t keylen,
371 const void *salt, size_t saltlen)
373 return srtp_derive (s, key, keylen, salt, saltlen) ? EINVAL : 0;
378 * Sets Roll-over-Counter Carry (RCC) rate for the SRTP session. If not
379 * specified (through this function), the default rate of ONE is assumed
380 * (i.e. every RTP packets will carry the RoC). RCC rate is ignored if none
381 * of the RCC mode has been selected.
383 * The RCC mode is selected through one of these flags for srtp_create():
384 * SRTP_RCC_MODE1: integrity protection only for RoC carrying packets
385 * SRTP_RCC_MODE2: integrity protection for all packets
386 * SRTP_RCC_MODE3: no integrity protection
388 * RCC mode 3 is insecure. Compared to plain RTP, it provides confidentiality
389 * (through encryption) but is much more prone to DoS. It can only be used if
390 * anti-spoofing protection is provided by lower network layers (e.g. IPsec,
391 * or trusted routers and proper source address filtering).
393 * If RCC rate is 1, RCC mode 1 and 2 are functionally identical.
395 * @param rate RoC Carry rate (MUST NOT be zero)
397 void srtp_setrcc_rate (srtp_session_t *s, uint16_t rate)
404 /** AES-CM for RTP (salt = 14 bytes + 2 nul bytes) */
406 rtp_crypt (gcry_cipher_hd_t hd, uint32_t ssrc, uint32_t roc, uint16_t seq,
407 const uint32_t *salt, uint8_t *data, size_t len)
409 /* Determines cryptographic counter (IV) */
411 counter[0] = salt[0];
412 counter[1] = salt[1] ^ ssrc;
413 counter[2] = salt[2] ^ htonl (roc);
414 counter[3] = salt[3] ^ htonl (seq << 16);
417 return ctr_crypt (hd, counter, data, len);
421 /** Determines SRTP Roll-Over-Counter (in host-byte order) */
423 srtp_compute_roc (const srtp_session_t *s, uint16_t seq)
425 uint32_t roc = s->rtp_roc;
427 if (((seq - s->rtp_seq) & 0xffff) < 0x8000)
429 /* Sequence is ahead, good */
430 if (seq < s->rtp_seq)
431 roc++; /* Sequence number wrap */
435 /* Sequence is late, bad */
436 if (seq > s->rtp_seq)
437 roc--; /* Wrap back */
443 /** Returns RTP sequence (in host-byte order) */
444 static inline uint16_t rtp_seq (const uint8_t *buf)
446 return (buf[2] << 8) | buf[3];
450 /** Message Authentication and Integrity for RTP */
451 static const uint8_t *
452 rtp_digest (srtp_session_t *s, const uint8_t *data, size_t len,
455 const gcry_md_hd_t md = s->rtp.mac;
458 gcry_md_write (md, data, len);
459 gcry_md_write (md, &(uint32_t){ htonl (roc) }, 4);
460 return gcry_md_read (md, 0);
465 * Encrypts/decrypts a RTP packet and updates SRTP context
466 * (CTR block cypher mode of operation has identical encryption and
467 * decryption function).
469 * @param buf RTP packet to be en-/decrypted
470 * @param len RTP packet length
472 * @return 0 on success, in case of error:
473 * EINVAL malformatted RTP packet
474 * EACCES replayed packet or out-of-window or sync lost
476 static int srtp_crypt (srtp_session_t *s, uint8_t *buf, size_t len)
480 if ((len < 12) || ((buf[0] >> 6) != 2))
483 /* Computes encryption offset */
484 uint16_t offset = 12;
485 offset += (buf[0] & 0xf) * 4; // skips CSRC
495 memcpy (&extlen, buf + offset - 2, 2);
496 offset += htons (extlen); // skips RTP extension header
502 /* Determines RTP 48-bits counter and SSRC */
503 uint16_t seq = rtp_seq (buf);
504 uint32_t roc = srtp_compute_roc (s, seq), ssrc;
505 memcpy (&ssrc, buf + 8, 4);
507 /* Updates ROC and sequence (it's safe now) */
508 int16_t diff = seq - s->rtp_seq;
511 /* Sequence in the future, good */
512 s->rtp.window = s->rtp.window << diff;
514 s->rtp_seq = seq, s->rtp_roc = roc;
518 /* Sequence in the past/present, bad */
520 if ((diff >= 64) || ((s->rtp.window >> diff) & 1))
521 return EACCES; /* Replay attack */
522 s->rtp.window |= 1 << diff;
525 /* Encrypt/Decrypt */
526 if (s->flags & SRTP_UNENCRYPTED)
529 if (rtp_crypt (s->rtp.cipher, ssrc, roc, seq, s->rtp.salt,
530 buf + offset, len - offset))
538 * Turns a RTP packet into a SRTP packet: encrypt it, then computes
539 * the authentication tag and appends it.
540 * Note that you can encrypt packet in disorder.
542 * @param buf RTP packet to be encrypted/digested
543 * @param lenp pointer to the RTP packet length on entry,
544 * set to the SRTP length on exit (undefined on non-ENOSPC error)
545 * @param bufsize size (bytes) of the packet buffer
547 * @return 0 on success, in case of error:
548 * EINVAL malformatted RTP packet or internal error
549 * ENOSPC bufsize is too small to add authentication tag
550 * (<lenp> will hold the required byte size)
551 * EACCES packet would trigger a replay error on receiver
554 srtp_send (srtp_session_t *s, uint8_t *buf, size_t *lenp, size_t bufsize)
557 int val = srtp_crypt (s, buf, len);
561 if (!(s->flags & SRTP_UNAUTHENTICATED))
563 size_t tag_len = s->tag_len;
564 *lenp = len + tag_len;
565 if (bufsize < (len + tag_len))
568 uint32_t roc = srtp_compute_roc (s, rtp_seq (buf));
569 const uint8_t *tag = rtp_digest (s, buf, len, roc);
573 if ((rtp_seq (buf) % s->rtp_rcc) == 0)
575 memcpy (buf + len, &(uint32_t){ htonl (s->rtp_roc) }, 4);
577 if (rcc_mode (s) == 3)
584 if (rcc_mode (s) & 1)
588 memcpy (buf + len, tag, tag_len);
596 * Turns a SRTP packet into a RTP packet: authenticates the packet,
599 * @param buf RTP packet to be digested/decrypted
600 * @param lenp pointer to the SRTP packet length on entry,
601 * set to the RTP length on exit (undefined in case of error)
603 * @return 0 on success, in case of error:
604 * EINVAL malformatted SRTP packet
605 * EACCES authentication failed (spoofed packet or out-of-sync)
608 srtp_recv (srtp_session_t *s, uint8_t *buf, size_t *lenp)
614 if (!(s->flags & SRTP_UNAUTHENTICATED))
616 size_t tag_len = s->tag_len, roc_len = 0;
619 if ((rtp_seq (buf) % s->rtp_rcc) == 0)
622 if (rcc_mode (s) == 3)
629 if (rcc_mode (s) & 1)
630 tag_len = 0; // RCC mode 1 or 3: no auth
634 if (len < (12u + roc_len + tag_len))
636 len -= roc_len + tag_len;
638 uint32_t roc = srtp_compute_roc (s, rtp_seq (buf)), rcc;
641 assert (roc_len == 4);
642 memcpy (&rcc, buf + len, 4);
648 const uint8_t *tag = rtp_digest (s, buf, len, rcc);
649 if (memcmp (buf + len + roc_len, tag, s->tag_len))
654 /* Authenticated packet carried a Roll-Over-Counter */
655 s->rtp_roc += rcc - roc;
656 assert (srtp_compute_roc (s, rtp_seq (buf)) == rcc);
661 return srtp_crypt (s, buf, len);
665 /** AES-CM for RTCP (salt = 14 bytes + 2 nul bytes) */
667 rtcp_crypt (gcry_cipher_hd_t hd, uint32_t ssrc, uint32_t index,
668 const uint32_t *salt, uint8_t *data, size_t len)
670 return rtp_crypt (hd, ssrc, index >> 16, index & 0xffff, salt, data, len);
674 /** Message Authentication and Integrity for RTCP */
675 static const uint8_t *
676 rtcp_digest (gcry_md_hd_t md, const void *data, size_t len)
679 gcry_md_write (md, data, len);
680 return gcry_md_read (md, 0);
685 * Encrypts/decrypts a RTCP packet and updates SRTCP context
686 * (CTR block cypher mode of operation has identical encryption and
687 * decryption function).
689 * @param buf RTCP packet to be en-/decrypted
690 * @param len RTCP packet length
692 * @return 0 on success, in case of error:
693 * EINVAL malformatted RTCP packet
695 static int srtcp_crypt (srtp_session_t *s, uint8_t *buf, size_t len)
699 /* 8-bytes unencrypted header, and 4-bytes unencrypted footer */
700 if ((len < 12) || ((buf[0] >> 6) != 2))
704 memcpy (&index, buf + len, 4);
705 index = ntohl (index);
706 if (((index >> 31) != 0) != ((s->flags & SRTCP_UNENCRYPTED) == 0))
707 return EINVAL; // E-bit mismatch
709 index &= ~(1 << 31); // clear E-bit for counter
711 /* Updates SRTCP index (safe here) */
712 int32_t diff = index - s->rtcp_index;
715 /* Packet in the future, good */
716 s->rtcp.window = s->rtcp.window << diff;
718 s->rtcp_index = index;
722 /* Packet in the past/present, bad */
724 if ((diff >= 64) || ((s->rtcp.window >> diff) & 1))
725 return EACCES; // replay attack!
726 s->rtp.window |= 1 << diff;
730 if (s->flags & SRTCP_UNENCRYPTED)
734 memcpy (&ssrc, buf + 4, 4);
736 if (rtcp_crypt (s->rtcp.cipher, ssrc, index, s->rtp.salt,
744 * Turns a RTCP packet into a SRTCP packet: encrypt it, then computes
745 * the authentication tag and appends it.
747 * @param buf RTCP packet to be encrypted/digested
748 * @param lenp pointer to the RTCP packet length on entry,
749 * set to the SRTCP length on exit (undefined in case of error)
750 * @param bufsize size (bytes) of the packet buffer
752 * @return 0 on success, in case of error:
753 * EINVAL malformatted RTCP packet or internal error
754 * ENOSPC bufsize is too small (to add index and authentication tag)
757 srtcp_send (srtp_session_t *s, uint8_t *buf, size_t *lenp, size_t bufsize)
760 if (bufsize < (len + 4 + s->tag_len))
763 uint32_t index = ++s->rtcp_index;
765 s->rtcp_index = index = 0; /* 31-bit wrap */
767 if ((s->flags & SRTCP_UNENCRYPTED) == 0)
768 index |= 0x80000000; /* Set Encrypted bit */
769 memcpy (buf + len, &(uint32_t){ htonl (index) }, 4);
771 int val = srtcp_crypt (s, buf, len);
775 len += 4; /* Digests SRTCP index too */
777 const uint8_t *tag = rtcp_digest (s->rtp.mac, buf, len);
778 memcpy (buf + len, tag, s->tag_len);
779 *lenp = len + s->tag_len;
785 * Turns a SRTCP packet into a RTCP packet: authenticates the packet,
788 * @param buf RTCP packet to be digested/decrypted
789 * @param lenp pointer to the SRTCP packet length on entry,
790 * set to the RTCP length on exit (undefined in case of error)
792 * @return 0 on success, in case of error:
793 * EINVAL malformatted SRTCP packet
794 * EACCES authentication failed (spoofed packet or out-of-sync)
797 srtcp_recv (srtp_session_t *s, uint8_t *buf, size_t *lenp)
801 if (len < (4u + s->tag_len))
805 const uint8_t *tag = rtcp_digest (s->rtp.mac, buf, len);
806 if (memcmp (buf + len, tag, s->tag_len))
809 len -= 4; /* Remove SRTCP index before decryption */
811 return srtp_crypt (s, buf, len);