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;
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 flags OR'ed optional flags.
166 * @return NULL in case of error
169 srtp_create (const char *name, unsigned flags, unsigned kdr)
171 assert (name != NULL);
174 return NULL; // FIXME: KDR not implemented yet
177 int cipher = GCRY_CIPHER_AES, md = GCRY_MD_SHA1;
179 if (strcmp (name, "AES_CM_128_HMAC_SHA1_80") == 0)
182 if (strcmp (name, "AES_CM_128_HMAC_SHA1_32") == 0)
185 // F8_128_HMAC_SHA1_80 is not implemented
188 if ((flags & ~SRTP_FLAGS_MASK) || init_libgcrypt ())
191 srtp_session_t *s = malloc (sizeof (*s));
195 memset (s, 0, sizeof (*s));
198 s->tag_len = tag_len;
200 if (proto_create (&s->rtp, cipher, md) == 0)
202 if (proto_create (&s->rtcp, cipher, md) == 0)
204 proto_destroy (&s->rtp);
213 * AES-CM key derivation (saltlen = 14 bytes)
216 derive (gcry_cipher_hd_t prf, const void *salt,
217 const uint8_t *r, size_t rlen, uint8_t label,
218 void *out, size_t outlen)
222 memcpy (iv, salt, 14);
226 iv[13 - rlen] ^= label;
227 for (size_t i = 0; i < rlen; i++)
228 iv[sizeof (iv) - rlen + i] ^= r[i];
230 /* TODO: retry with CTR mode */
231 while (outlen >= sizeof (iv))
234 if (gcry_cipher_encrypt (prf, out, sizeof (iv), iv, sizeof (iv)))
236 outlen -= sizeof (iv);
237 out = ((uint8_t *)out) + sizeof (iv);
239 /* Increment IV in network byte order */
240 if (++iv[sizeof (iv) - 1] == 0)
241 ++iv[sizeof (iv) -2];
246 /* Truncated last AES output block */
247 if (gcry_cipher_encrypt (prf, iv, sizeof (iv), NULL, 0))
249 memcpy (out, iv, outlen);
257 proto_derive (srtp_proto_t *p, gcry_cipher_hd_t prf,
258 const void *salt, size_t saltlen,
259 const uint8_t *r, size_t rlen, bool rtcp)
265 uint8_t label = rtcp ? SRTCP_CRYPT : SRTP_CRYPT;
267 if (derive (prf, salt, r, rlen, label++, keybuf, 16)
268 || gcry_cipher_setkey (p->cipher, keybuf, 16)
269 || derive (prf, salt, r, rlen, label++, keybuf, 20)
270 || gcry_md_setkey (p->mac, keybuf, 20)
271 || derive (prf, salt, r, rlen, label, p->salt, 14))
279 * SRTP/SRTCP cipher/salt/MAC keys derivation.
282 srtp_derive (srtp_session_t *s, const void *key, size_t keylen,
283 const void *salt, size_t saltlen)
285 gcry_cipher_hd_t prf;
288 /* TODO: retry with CTR mode */
289 if (gcry_cipher_open (&prf, GCRY_CIPHER_AES, GCRY_CIPHER_MODE_ECB, 0)
290 || gcry_cipher_setkey (prf, key, keylen))
293 /* RTP key derivation */
296 uint64_t index = (((uint64_t)s->rtp_roc) << 16) | s->rtp_seq;
299 for (int i = sizeof (r) - 1; i >= 0; i--)
306 memset (r, 0, sizeof (r));
308 if (proto_derive (&s->rtp, prf, salt, saltlen, r, 6, false))
311 /* RTCP key derivation */
312 memcpy (r, &(uint32_t){ htonl (s->rtcp_index) }, 4);
313 if (proto_derive (&s->rtcp, prf, salt, saltlen, r, 4, true))
316 (void)gcry_cipher_close (prf);
322 * Sets (or resets) the master key and master salt for a SRTP session.
323 * This must be done at least once before using rtp_send(), rtp_recv(),
324 * rtcp_send() or rtcp_recv(). Also, rekeying is required every
325 * 2^48 RTP packets or 2^31 RTCP packets (whichever comes first),
326 * otherwise the protocol security might be broken.
328 * @return 0 on success, in case of error:
329 * EINVAL invalid or unsupported key/salt sizes combination
332 srtp_setkey (srtp_session_t *s, const void *key, size_t keylen,
333 const void *salt, size_t saltlen)
335 return srtp_derive (s, key, keylen, salt, saltlen) ? EINVAL : 0;
339 /** AES-CM encryption/decryption (ctr length = 16 bytes) */
341 ctr_crypt (gcry_cipher_hd_t hd, uint32_t *ctr, uint8_t *data, size_t len)
343 const size_t ctrlen = 16;
344 while (len >= ctrlen)
346 if (gcry_cipher_setctr (hd, ctr, ctrlen)
347 || gcry_cipher_encrypt (hd, data, ctrlen, NULL, 0))
352 ctr[3] = htonl (ntohl (ctr[3]) + 1);
357 /* Truncated last block */
358 uint8_t dummy[ctrlen];
359 memcpy (dummy, data, len);
360 memset (dummy + len, 0, ctrlen - len);
362 if (gcry_cipher_setctr (hd, ctr, ctrlen)
363 || gcry_cipher_encrypt (hd, dummy, ctrlen, data, ctrlen))
365 memcpy (data, dummy, len);
372 /** AES-CM for RTP (salt = 14 bytes + 2 nul bytes) */
374 rtp_crypt (gcry_cipher_hd_t hd, uint32_t ssrc, uint32_t roc, uint16_t seq,
375 const uint32_t *salt, uint8_t *data, size_t len)
377 /* Determines cryptographic counter (IV) */
379 counter[0] = salt[0];
380 counter[1] = salt[1] ^ ssrc;
381 counter[2] = salt[2] ^ htonl (roc);
382 counter[3] = salt[3] ^ htonl (seq << 16);
385 return ctr_crypt (hd, counter, data, len);
389 /** Determines SRTP Roll-Over-Counter (in host-byte order) */
391 srtp_compute_roc (const srtp_session_t *s, uint16_t seq)
393 uint32_t roc = s->rtp_roc;
395 if (((seq - s->rtp_seq) & 0xffff) < 0x8000)
397 /* Sequence is ahead, good */
398 if (seq < s->rtp_seq)
399 roc++; /* Sequence number wrap */
403 /* Sequence is late, bad */
404 if (seq > s->rtp_seq)
405 roc--; /* Wrap back */
411 /** Returns RTP sequence (in host-byte order) */
412 static inline uint16_t rtp_seq (const uint8_t *buf)
414 return (buf[2] << 8) | buf[3];
418 /** Message Authentication and Integrity for RTP */
419 static const uint8_t *
420 rtp_digest (srtp_session_t *s, const uint8_t *data, size_t len)
422 const gcry_md_hd_t md = s->rtp.mac;
423 uint32_t roc = htonl (srtp_compute_roc (s, rtp_seq (data)));
426 gcry_md_write (md, data, len);
427 gcry_md_write (md, &roc, 4);
428 return gcry_md_read (md, 0);
433 * Encrypts/decrypts a RTP packet and updates SRTP context
434 * (CTR block cypher mode of operation has identical encryption and
435 * decryption function).
437 * @param buf RTP packet to be en-/decrypted
438 * @param len RTP packet length
440 * @return 0 on success, in case of error:
441 * EINVAL malformatted RTP packet
442 * EACCES replayed packet or out-of-window or sync lost
444 static int srtp_crypt (srtp_session_t *s, uint8_t *buf, size_t len)
448 if ((len < 12) || ((buf[0] >> 6) != 2))
451 /* Computes encryption offset */
452 uint16_t offset = 12;
453 offset += (buf[0] & 0xf) * 4; // skips CSRC
463 memcpy (&extlen, buf + offset - 2, 2);
464 offset += htons (extlen); // skips RTP extension header
470 /* Determines RTP 48-bits counter and SSRC */
471 uint16_t seq = rtp_seq (buf);
472 uint32_t roc = srtp_compute_roc (s, seq), ssrc;
473 memcpy (&ssrc, buf + 8, 4);
475 /* Updates ROC and sequence (it's safe now) */
476 int16_t diff = seq - s->rtp_seq;
479 /* Sequence in the future, good */
480 s->rtp.window = s->rtp.window << diff;
482 s->rtp_seq = seq, s->rtp_roc = roc;
486 /* Sequence in the past/present, bad */
488 if ((diff >= 64) || ((s->rtp.window >> diff) & 1))
489 return EACCES; /* Replay attack */
490 s->rtp.window |= 1 << diff;
493 /* Encrypt/Decrypt */
494 if (s->flags & SRTP_UNENCRYPTED)
497 if (rtp_crypt (s->rtp.cipher, ssrc, roc, seq, s->rtp.salt,
498 buf + offset, len - offset))
506 * Turns a RTP packet into a SRTP packet: encrypt it, then computes
507 * the authentication tag and appends it.
508 * Note that you can encrypt packet in disorder.
510 * @param buf RTP packet to be encrypted/digested
511 * @param lenp pointer to the RTP packet length on entry,
512 * set to the SRTP length on exit (undefined in case of error)
513 * @param bufsize size (bytes) of the packet buffer
515 * @return 0 on success, in case of error:
516 * EINVAL malformatted RTP packet or internal error
517 * ENOSPC bufsize is too small (to add authentication tag)
518 * EACCES packet would trigger a replay error on receiver
521 srtp_send (srtp_session_t *s, uint8_t *buf, size_t *lenp, size_t bufsize)
524 int val = srtp_crypt (s, buf, len);
528 if (!(s->flags & SRTP_UNAUTHENTICATED))
530 if (bufsize < (len + s->tag_len))
533 const uint8_t *tag = rtp_digest (s, buf, len);
534 memcpy (buf + len, tag, s->tag_len);
535 *lenp = len + s->tag_len;
543 * Turns a SRTP packet into a RTP packet: authenticates the packet,
546 * @param buf RTP packet to be digested/decrypted
547 * @param lenp pointer to the SRTP packet length on entry,
548 * set to the RTP length on exit (undefined in case of error)
550 * @return 0 on success, in case of error:
551 * EINVAL malformatted SRTP packet
552 * EACCES authentication failed (spoofed packet or out-of-sync)
555 srtp_recv (srtp_session_t *s, uint8_t *buf, size_t *lenp)
559 if (!(s->flags & SRTP_UNAUTHENTICATED))
561 if (len < (12u + s->tag_len))
565 const uint8_t *tag = rtp_digest (s, buf, len);
566 if (memcmp (buf + len, tag, s->tag_len))
572 return srtp_crypt (s, buf, len);
576 /** AES-CM for RTCP (salt = 14 bytes + 2 nul bytes) */
578 rtcp_crypt (gcry_cipher_hd_t hd, uint32_t ssrc, uint32_t index,
579 const uint32_t *salt, uint8_t *data, size_t len)
581 return rtp_crypt (hd, ssrc, index >> 16, index & 0xffff, salt, data, len);
585 /** Message Authentication and Integrity for RTCP */
586 static const uint8_t *
587 rtcp_digest (gcry_md_hd_t md, const void *data, size_t len)
590 gcry_md_write (md, data, len);
591 return gcry_md_read (md, 0);
596 * Encrypts/decrypts a RTCP packet and updates SRTCP context
597 * (CTR block cypher mode of operation has identical encryption and
598 * decryption function).
600 * @param buf RTCP packet to be en-/decrypted
601 * @param len RTCP packet length
603 * @return 0 on success, in case of error:
604 * EINVAL malformatted RTCP packet
606 static int srtcp_crypt (srtp_session_t *s, uint8_t *buf, size_t len)
610 /* 8-bytes unencrypted header, and 4-bytes unencrypted footer */
611 if ((len < 12) || ((buf[0] >> 6) != 2))
615 memcpy (&index, buf + len, 4);
616 index = ntohl (index);
617 if (((index >> 31) != 0) != ((s->flags & SRTCP_UNENCRYPTED) == 0))
618 return EINVAL; // E-bit mismatch
620 index &= ~(1 << 31); // clear E-bit for counter
622 /* Updates SRTCP index (safe here) */
623 int32_t diff = index - s->rtcp_index;
626 /* Packet in the future, good */
627 s->rtcp.window = s->rtcp.window << diff;
629 s->rtcp_index = index;
633 /* Packet in the past/present, bad */
635 if ((diff >= 64) || ((s->rtcp.window >> diff) & 1))
636 return EACCES; // replay attack!
637 s->rtp.window |= 1 << diff;
641 if (s->flags & SRTCP_UNENCRYPTED)
645 memcpy (&ssrc, buf + 4, 4);
647 if (rtcp_crypt (s->rtcp.cipher, ssrc, index, s->rtp.salt,
655 * Turns a RTCP packet into a SRTCP packet: encrypt it, then computes
656 * the authentication tag and appends it.
658 * @param buf RTCP packet to be encrypted/digested
659 * @param lenp pointer to the RTCP packet length on entry,
660 * set to the SRTCP length on exit (undefined in case of error)
661 * @param bufsize size (bytes) of the packet buffer
663 * @return 0 on success, in case of error:
664 * EINVAL malformatted RTCP packet or internal error
665 * ENOSPC bufsize is too small (to add index and authentication tag)
668 srtcp_send (srtp_session_t *s, uint8_t *buf, size_t *lenp, size_t bufsize)
671 if (bufsize < (len + 4 + s->tag_len))
674 uint32_t index = ++s->rtcp_index;
676 s->rtcp_index = index = 0; /* 31-bit wrap */
678 if ((s->flags & SRTCP_UNENCRYPTED) == 0)
679 index |= 0x80000000; /* Set Encrypted bit */
680 memcpy (buf + len, &(uint32_t){ htonl (index) }, 4);
682 int val = srtcp_crypt (s, buf, len);
686 len += 4; /* Digests SRTCP index too */
688 const uint8_t *tag = rtcp_digest (s->rtp.mac, buf, len);
689 memcpy (buf + len, tag, s->tag_len);
690 *lenp = len + s->tag_len;
696 * Turns a SRTCP packet into a RTCP packet: authenticates the packet,
699 * @param buf RTCP packet to be digested/decrypted
700 * @param lenp pointer to the SRTCP packet length on entry,
701 * set to the RTCP length on exit (undefined in case of error)
703 * @return 0 on success, in case of error:
704 * EINVAL malformatted SRTCP packet
705 * EACCES authentication failed (spoofed packet or out-of-sync)
708 srtcp_recv (srtp_session_t *s, uint8_t *buf, size_t *lenp)
712 if (len < (4u + s->tag_len))
716 const uint8_t *tag = rtcp_digest (s->rtp.mac, buf, len);
717 if (memcmp (buf + len, tag, s->tag_len))
720 len -= 4; /* Remove SRTCP index before decryption */
722 return srtp_crypt (s, buf, len);