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 (md))
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))
335 /* RTP key derivation */
338 uint64_t index = (((uint64_t)s->rtp_roc) << 16) | s->rtp_seq;
341 for (int i = sizeof (r) - 1; i >= 0; i--)
349 memset (r, 0, sizeof (r));
351 if (proto_derive (&s->rtp, prf, salt, saltlen, r, 6, false))
354 /* RTCP key derivation */
355 memcpy (r, &(uint32_t){ htonl (s->rtcp_index) }, 4);
356 if (proto_derive (&s->rtcp, prf, salt, saltlen, r, 4, true))
359 (void)gcry_cipher_close (prf);
365 * Sets (or resets) the master key and master salt for a SRTP session.
366 * This must be done at least once before using rtp_send(), rtp_recv(),
367 * rtcp_send() or rtcp_recv(). Also, rekeying is required every
368 * 2^48 RTP packets or 2^31 RTCP packets (whichever comes first),
369 * otherwise the protocol security might be broken.
371 * @return 0 on success, in case of error:
372 * EINVAL invalid or unsupported key/salt sizes combination
375 srtp_setkey (srtp_session_t *s, const void *key, size_t keylen,
376 const void *salt, size_t saltlen)
378 return srtp_derive (s, key, keylen, salt, saltlen) ? EINVAL : 0;
383 * Sets Roll-over-Counter Carry (RCC) rate for the SRTP session. If not
384 * specified (through this function), the default rate of ONE is assumed
385 * (i.e. every RTP packets will carry the RoC). RCC rate is ignored if none
386 * of the RCC mode has been selected.
388 * The RCC mode is selected through one of these flags for srtp_create():
389 * SRTP_RCC_MODE1: integrity protection only for RoC carrying packets
390 * SRTP_RCC_MODE2: integrity protection for all packets
391 * SRTP_RCC_MODE3: no integrity protection
393 * RCC mode 3 is insecure. Compared to plain RTP, it provides confidentiality
394 * (through encryption) but is much more prone to DoS. It can only be used if
395 * anti-spoofing protection is provided by lower network layers (e.g. IPsec,
396 * or trusted routers and proper source address filtering).
398 * If RCC rate is 1, RCC mode 1 and 2 are functionally identical.
400 * @param rate RoC Carry rate (MUST NOT be zero)
402 void srtp_setrcc_rate (srtp_session_t *s, uint16_t rate)
409 /** AES-CM for RTP (salt = 14 bytes + 2 nul bytes) */
411 rtp_crypt (gcry_cipher_hd_t hd, uint32_t ssrc, uint32_t roc, uint16_t seq,
412 const uint32_t *salt, uint8_t *data, size_t len)
414 /* Determines cryptographic counter (IV) */
416 counter[0] = salt[0];
417 counter[1] = salt[1] ^ ssrc;
418 counter[2] = salt[2] ^ htonl (roc);
419 counter[3] = salt[3] ^ htonl (seq << 16);
422 return ctr_crypt (hd, counter, data, len);
426 /** Determines SRTP Roll-Over-Counter (in host-byte order) */
428 srtp_compute_roc (const srtp_session_t *s, uint16_t seq)
430 uint32_t roc = s->rtp_roc;
432 if (((seq - s->rtp_seq) & 0xffff) < 0x8000)
434 /* Sequence is ahead, good */
435 if (seq < s->rtp_seq)
436 roc++; /* Sequence number wrap */
440 /* Sequence is late, bad */
441 if (seq > s->rtp_seq)
442 roc--; /* Wrap back */
448 /** Returns RTP sequence (in host-byte order) */
449 static inline uint16_t rtp_seq (const uint8_t *buf)
451 return (buf[2] << 8) | buf[3];
455 /** Message Authentication and Integrity for RTP */
456 static const uint8_t *
457 rtp_digest (srtp_session_t *s, const uint8_t *data, size_t len,
460 const gcry_md_hd_t md = s->rtp.mac;
463 gcry_md_write (md, data, len);
464 gcry_md_write (md, &(uint32_t){ htonl (roc) }, 4);
465 return gcry_md_read (md, 0);
470 * Encrypts/decrypts a RTP packet and updates SRTP context
471 * (CTR block cypher mode of operation has identical encryption and
472 * decryption function).
474 * @param buf RTP packet to be en-/decrypted
475 * @param len RTP packet length
477 * @return 0 on success, in case of error:
478 * EINVAL malformatted RTP packet
479 * EACCES replayed packet or out-of-window or sync lost
481 static int srtp_crypt (srtp_session_t *s, uint8_t *buf, size_t len)
485 if ((len < 12) || ((buf[0] >> 6) != 2))
488 /* Computes encryption offset */
489 uint16_t offset = 12;
490 offset += (buf[0] & 0xf) * 4; // skips CSRC
500 memcpy (&extlen, buf + offset - 2, 2);
501 offset += htons (extlen); // skips RTP extension header
507 /* Determines RTP 48-bits counter and SSRC */
508 uint16_t seq = rtp_seq (buf);
509 uint32_t roc = srtp_compute_roc (s, seq), ssrc;
510 memcpy (&ssrc, buf + 8, 4);
512 /* Updates ROC and sequence (it's safe now) */
513 int16_t diff = seq - s->rtp_seq;
516 /* Sequence in the future, good */
517 s->rtp.window = s->rtp.window << diff;
519 s->rtp_seq = seq, s->rtp_roc = roc;
523 /* Sequence in the past/present, bad */
525 if ((diff >= 64) || ((s->rtp.window >> diff) & 1))
526 return EACCES; /* Replay attack */
527 s->rtp.window |= 1 << diff;
530 /* Encrypt/Decrypt */
531 if (s->flags & SRTP_UNENCRYPTED)
534 if (rtp_crypt (s->rtp.cipher, ssrc, roc, seq, s->rtp.salt,
535 buf + offset, len - offset))
543 * Turns a RTP packet into a SRTP packet: encrypt it, then computes
544 * the authentication tag and appends it.
545 * Note that you can encrypt packet in disorder.
547 * @param buf RTP packet to be encrypted/digested
548 * @param lenp pointer to the RTP packet length on entry,
549 * set to the SRTP length on exit (undefined on non-ENOSPC error)
550 * @param bufsize size (bytes) of the packet buffer
552 * @return 0 on success, in case of error:
553 * EINVAL malformatted RTP packet or internal error
554 * ENOSPC bufsize is too small to add authentication tag
555 * (<lenp> will hold the required byte size)
556 * EACCES packet would trigger a replay error on receiver
559 srtp_send (srtp_session_t *s, uint8_t *buf, size_t *lenp, size_t bufsize)
562 int val = srtp_crypt (s, buf, len);
566 if (!(s->flags & SRTP_UNAUTHENTICATED))
568 size_t tag_len = s->tag_len;
569 *lenp = len + tag_len;
570 if (bufsize < (len + tag_len))
573 uint32_t roc = srtp_compute_roc (s, rtp_seq (buf));
574 const uint8_t *tag = rtp_digest (s, buf, len, roc);
578 if ((rtp_seq (buf) % s->rtp_rcc) == 0)
580 memcpy (buf + len, &(uint32_t){ htonl (s->rtp_roc) }, 4);
582 if (rcc_mode (s) == 3)
589 if (rcc_mode (s) & 1)
593 memcpy (buf + len, tag, tag_len);
601 * Turns a SRTP packet into a RTP packet: authenticates the packet,
604 * @param buf RTP packet to be digested/decrypted
605 * @param lenp pointer to the SRTP packet length on entry,
606 * set to the RTP length on exit (undefined in case of error)
608 * @return 0 on success, in case of error:
609 * EINVAL malformatted SRTP packet
610 * EACCES authentication failed (spoofed packet or out-of-sync)
613 srtp_recv (srtp_session_t *s, uint8_t *buf, size_t *lenp)
619 if (!(s->flags & SRTP_UNAUTHENTICATED))
621 size_t tag_len = s->tag_len, roc_len = 0;
624 if ((rtp_seq (buf) % s->rtp_rcc) == 0)
627 if (rcc_mode (s) == 3)
634 if (rcc_mode (s) & 1)
635 tag_len = 0; // RCC mode 1 or 3: no auth
639 if (len < (12u + roc_len + tag_len))
641 len -= roc_len + tag_len;
643 uint32_t roc = srtp_compute_roc (s, rtp_seq (buf)), rcc;
646 assert (roc_len == 4);
647 memcpy (&rcc, buf + len, 4);
653 const uint8_t *tag = rtp_digest (s, buf, len, rcc);
654 if (memcmp (buf + len + roc_len, tag, s->tag_len))
659 /* Authenticated packet carried a Roll-Over-Counter */
660 s->rtp_roc += rcc - roc;
661 assert (srtp_compute_roc (s, rtp_seq (buf)) == rcc);
666 return srtp_crypt (s, buf, len);
670 /** AES-CM for RTCP (salt = 14 bytes + 2 nul bytes) */
672 rtcp_crypt (gcry_cipher_hd_t hd, uint32_t ssrc, uint32_t index,
673 const uint32_t *salt, uint8_t *data, size_t len)
675 return rtp_crypt (hd, ssrc, index >> 16, index & 0xffff, salt, data, len);
679 /** Message Authentication and Integrity for RTCP */
680 static const uint8_t *
681 rtcp_digest (gcry_md_hd_t md, const void *data, size_t len)
684 gcry_md_write (md, data, len);
685 return gcry_md_read (md, 0);
690 * Encrypts/decrypts a RTCP packet and updates SRTCP context
691 * (CTR block cypher mode of operation has identical encryption and
692 * decryption function).
694 * @param buf RTCP packet to be en-/decrypted
695 * @param len RTCP packet length
697 * @return 0 on success, in case of error:
698 * EINVAL malformatted RTCP packet
700 static int srtcp_crypt (srtp_session_t *s, uint8_t *buf, size_t len)
704 /* 8-bytes unencrypted header, and 4-bytes unencrypted footer */
705 if ((len < 12) || ((buf[0] >> 6) != 2))
709 memcpy (&index, buf + len, 4);
710 index = ntohl (index);
711 if (((index >> 31) != 0) != ((s->flags & SRTCP_UNENCRYPTED) == 0))
712 return EINVAL; // E-bit mismatch
714 index &= ~(1 << 31); // clear E-bit for counter
716 /* Updates SRTCP index (safe here) */
717 int32_t diff = index - s->rtcp_index;
720 /* Packet in the future, good */
721 s->rtcp.window = s->rtcp.window << diff;
723 s->rtcp_index = index;
727 /* Packet in the past/present, bad */
729 if ((diff >= 64) || ((s->rtcp.window >> diff) & 1))
730 return EACCES; // replay attack!
731 s->rtp.window |= 1 << diff;
735 if (s->flags & SRTCP_UNENCRYPTED)
739 memcpy (&ssrc, buf + 4, 4);
741 if (rtcp_crypt (s->rtcp.cipher, ssrc, index, s->rtp.salt,
749 * Turns a RTCP packet into a SRTCP packet: encrypt it, then computes
750 * the authentication tag and appends it.
752 * @param buf RTCP packet to be encrypted/digested
753 * @param lenp pointer to the RTCP packet length on entry,
754 * set to the SRTCP length on exit (undefined in case of error)
755 * @param bufsize size (bytes) of the packet buffer
757 * @return 0 on success, in case of error:
758 * EINVAL malformatted RTCP packet or internal error
759 * ENOSPC bufsize is too small (to add index and authentication tag)
762 srtcp_send (srtp_session_t *s, uint8_t *buf, size_t *lenp, size_t bufsize)
765 if (bufsize < (len + 4 + s->tag_len))
768 uint32_t index = ++s->rtcp_index;
770 s->rtcp_index = index = 0; /* 31-bit wrap */
772 if ((s->flags & SRTCP_UNENCRYPTED) == 0)
773 index |= 0x80000000; /* Set Encrypted bit */
774 memcpy (buf + len, &(uint32_t){ htonl (index) }, 4);
776 int val = srtcp_crypt (s, buf, len);
780 len += 4; /* Digests SRTCP index too */
782 const uint8_t *tag = rtcp_digest (s->rtp.mac, buf, len);
783 memcpy (buf + len, tag, s->tag_len);
784 *lenp = len + s->tag_len;
790 * Turns a SRTCP packet into a RTCP packet: authenticates the packet,
793 * @param buf RTCP packet to be digested/decrypted
794 * @param lenp pointer to the SRTCP packet length on entry,
795 * set to the RTCP length on exit (undefined in case of error)
797 * @return 0 on success, in case of error:
798 * EINVAL malformatted SRTCP packet
799 * EACCES authentication failed (spoofed packet or out-of-sync)
802 srtcp_recv (srtp_session_t *s, uint8_t *buf, size_t *lenp)
806 if (len < (4u + s->tag_len))
810 const uint8_t *tag = rtcp_digest (s->rtp.mac, buf, len);
811 if (memcmp (buf + len, tag, s->tag_len))
814 len -= 4; /* Remove SRTCP index before decryption */
816 return srtp_crypt (s, buf, len);