#include <gcrypt.h>
+#define debug( ... ) (void)0
+
/* TODO:
* Useful stuff:
- * - ROC profil thingy (multicast really needs this)
- * - replay protection
- *
- * Requirements for conformance:
- * - suites with NULL cipher
- * - SRTCP
+ * - ROC profile thingy (multicast really needs this)
*
* Useless stuff (because nothing depends on it):
* - non-nul key derivation rate
{
gcry_cipher_hd_t cipher;
gcry_md_hd_t mac;
+ uint64_t window;
uint32_t salt[4];
- uint8_t mac_len;
} srtp_proto_t;
struct srtp_session_t
uint32_t rtcp_index;
uint32_t rtp_roc;
uint16_t rtp_seq;
+ uint16_t rtp_rcc;
+ uint8_t tag_len;
};
enum
SRTCP_SALT
};
+
#ifdef WIN32
# include <winsock2.h>
#else
/**
- * Allocates a Secure RTP session.
+ * Allocates a Secure RTP one-way session.
+ * The same session cannot be used both ways because this would confuse
+ * internal cryptographic counters; it is however of course feasible to open
+ * multiple simultaneous sessions with the same master key.
*
- * @param name cipher-suite name
- * @param kdr key derivation rate
- * @param winsize anti-replay windows size (between 64 and 32767 inclusive)
- * 0 disable replay attack protection (OK for send only)
+ * @param encr encryption algorithm number
+ * @param auth authentication algortihm number
+ * @param tag_len authentication tag byte length (NOT including RCC)
* @param flags OR'ed optional flags.
*
* @return NULL in case of error
*/
srtp_session_t *
-srtp_create (const char *name, unsigned flags, unsigned kdr, uint16_t winsize)
+srtp_create (int encr, int auth, unsigned tag_len, int prf, unsigned flags)
{
- assert (name != NULL);
+ if ((flags & ~SRTP_FLAGS_MASK) || init_libgcrypt ())
+ return NULL;
- if (kdr != 0)
- return NULL; // FIXME: KDR not implemented yet
- if (winsize != 0)
- return NULL; // FIXME: replay protection not implemented yet
+ int cipher, md;
+ switch (encr)
+ {
+ case SRTP_ENCR_NULL:
+ cipher = GCRY_CIPHER_NONE;
+ break;
- uint8_t mac_len;
- int cipher = GCRY_CIPHER_AES, md = GCRY_MD_SHA1;
+ case SRTP_ENCR_AES_CM:
+ cipher = GCRY_CIPHER_AES;
+ break;
- if (strcmp (name, "AES_CM_128_HMAC_SHA1_80") == 0)
- mac_len = 80;
- else
- if (strcmp (name, "AES_CM_128_HMAC_SHA1_32") == 0)
- mac_len = 32;
- else
- // F8_128_HMAC_SHA1_80 is not implemented
+ default:
+ return NULL;
+ }
+
+ switch (auth)
+ {
+ case SRTP_AUTH_NULL:
+ md = GCRY_MD_NONE;
+ break;
+
+ case SRTP_AUTH_HMAC_SHA1:
+ md = GCRY_MD_SHA1;
+ break;
+
+ default:
+ return NULL;
+ }
+
+ if (tag_len > gcry_md_get_algo_dlen (auth))
return NULL;
- if ((flags & ~SRTP_FLAGS_MASK) || (winsize > 32767) || init_libgcrypt ())
+ if (prf != SRTP_PRF_AES_CM)
return NULL;
srtp_session_t *s = malloc (sizeof (*s));
memset (s, 0, sizeof (*s));
s->flags = flags;
- s->kdr = kdr;
+ s->tag_len = tag_len;
if (proto_create (&s->rtp, cipher, md) == 0)
{
if (saltlen != 14)
return -1;
- uint8_t cipherkey[16];
+ uint8_t keybuf[20];
uint8_t label = rtcp ? SRTCP_CRYPT : SRTP_CRYPT;
- if (derive (prf, salt, r, rlen, label++, cipherkey, 16)
- || gcry_cipher_setkey (p->cipher, cipherkey, 16)
- || derive (prf, salt, r, rlen, label++, NULL, 0) /* FIXME HMAC */
- || derive (prf, salt, r, rlen, label++, p->salt, 14))
+ if (derive (prf, salt, r, rlen, label++, keybuf, 16)
+ || gcry_cipher_setkey (p->cipher, keybuf, 16)
+ || derive (prf, salt, r, rlen, label++, keybuf, 20)
+ || gcry_md_setkey (p->mac, keybuf, 20)
+ || derive (prf, salt, r, rlen, label, p->salt, 14))
return -1;
return 0;
}
-
/**
* Sets (or resets) the master key and master salt for a SRTP session.
* This must be done at least once before using rtp_send(), rtp_recv(),
}
+/**
+ * Sets Roll-over-Counter Carry (RCC) rate for the SRTP session. If not
+ * specified (through this function), the default rate of ONE is assumed
+ * (i.e. every RTP packets will carry the RoC). RCC rate is ignored if none
+ * of the RCC mode has been selected.
+ *
+ * The RCC mode is selected through one of these flags for srtp_create():
+ * SRTP_RCC_MODE1: integrity protection only for RoC carrying packets
+ * SRTP_RCC_MODE2: integrity protection for all packets
+ * SRTP_RCC_MODE3: no integrity protection
+ *
+ * RCC mode 3 is insecure. Compared to plain RTP, it provides confidentiality
+ * (through encryption) but is much more prone to DoS. It can only be used if
+ * anti-spoofing protection is provided by lower network layers (e.g. IPsec,
+ * or trusted routers and proper source address filtering).
+ *
+ * If RCC rate is 1, RCC mode 1 and 2 are functionally identical.
+ *
+ * @param rate RoC Carry rate (MUST NOT be zero)
+ */
+void srtp_setrcc_rate (srtp_session_t *s, uint16_t rate)
+{
+ assert (rate != 0);
+ s->rtp_rcc = rate;
+}
+
+
/** AES-CM encryption/decryption (ctr length = 16 bytes) */
static int
-encrypt (gcry_cipher_hd_t hd, uint32_t *ctr, uint8_t *data, size_t len)
+ctr_crypt (gcry_cipher_hd_t hd, uint32_t *ctr, uint8_t *data, size_t len)
{
const size_t ctrlen = 16;
while (len >= ctrlen)
/** AES-CM for RTP (salt = 14 bytes + 2 nul bytes) */
-static inline int
-rtp_encrypt (gcry_cipher_hd_t hd, uint32_t ssrc, uint32_t roc, uint16_t seq,
- const uint32_t *salt, uint8_t *data, size_t len)
+static int
+rtp_crypt (gcry_cipher_hd_t hd, uint32_t ssrc, uint32_t roc, uint16_t seq,
+ const uint32_t *salt, uint8_t *data, size_t len)
{
/* Determines cryptographic counter (IV) */
uint32_t counter[4];
counter[3] = salt[3] ^ htonl (seq << 16);
/* Encryption */
- return encrypt (hd, counter, data, len);
+ return ctr_crypt (hd, counter, data, len);
+}
+
+
+/** Determines SRTP Roll-Over-Counter (in host-byte order) */
+static uint32_t
+srtp_compute_roc (const srtp_session_t *s, uint16_t seq)
+{
+ uint32_t roc = s->rtp_roc;
+
+ if (((seq - s->rtp_seq) & 0xffff) < 0x8000)
+ {
+ /* Sequence is ahead, good */
+ if (seq < s->rtp_seq)
+ roc++; /* Sequence number wrap */
+ }
+ else
+ {
+ /* Sequence is late, bad */
+ if (seq > s->rtp_seq)
+ roc--; /* Wrap back */
+ }
+ return roc;
+}
+
+
+/** Returns RTP sequence (in host-byte order) */
+static inline uint16_t rtp_seq (const uint8_t *buf)
+{
+ return (buf[2] << 8) | buf[3];
+}
+
+
+/** Message Authentication and Integrity for RTP */
+static const uint8_t *
+rtp_digest (srtp_session_t *s, const uint8_t *data, size_t len)
+{
+ const gcry_md_hd_t md = s->rtp.mac;
+ uint32_t roc = htonl (srtp_compute_roc (s, rtp_seq (data)));
+
+ gcry_md_reset (md);
+ gcry_md_write (md, data, len);
+ gcry_md_write (md, &roc, 4);
+ return gcry_md_read (md, 0);
}
* (CTR block cypher mode of operation has identical encryption and
* decryption function).
*
- * @param buf RTP packet to be encrypted/digested
+ * @param buf RTP packet to be en-/decrypted
* @param len RTP packet length
*
* @return 0 on success, in case of error:
* EINVAL malformatted RTP packet
+ * EACCES replayed packet or out-of-window or sync lost
*/
-static int srtp_encrypt (srtp_session_t *s, uint8_t *buf, size_t len)
+static int srtp_crypt (srtp_session_t *s, uint8_t *buf, size_t len)
{
assert (s != NULL);
return EINVAL;
memcpy (&extlen, buf + offset - 2, 2);
- offset += htons (extlen);
+ offset += htons (extlen); // skips RTP extension header
}
if (len < offset)
return EINVAL;
/* Determines RTP 48-bits counter and SSRC */
- uint32_t ssrc;
+ uint16_t seq = rtp_seq (buf);
+ uint32_t roc = srtp_compute_roc (s, seq), ssrc;
memcpy (&ssrc, buf + 8, 4);
- uint16_t seq = (buf[2] << 8) | buf[3];
- if (((seq - s->rtp_seq) & 0xffff) < 32768)
+ /* Updates ROC and sequence (it's safe now) */
+ int16_t diff = seq - s->rtp_seq;
+ if (diff > 0)
{
- if (seq < s->rtp_seq)
- s->rtp_roc++; /* Sequence number wrap */
+ /* Sequence in the future, good */
+ s->rtp.window = s->rtp.window << diff;
+ s->rtp.window |= 1;
+ s->rtp_seq = seq, s->rtp_roc = roc;
}
else
{
- if (seq > s->rtp_seq)
- s->rtp_roc--;
+ /* Sequence in the past/present, bad */
+ diff = -diff;
+ if ((diff >= 64) || ((s->rtp.window >> diff) & 1))
+ return EACCES; /* Replay attack */
+ s->rtp.window |= 1 << diff;
}
- s->rtp_seq = seq;
-
+ /* Encrypt/Decrypt */
if (s->flags & SRTP_UNENCRYPTED)
return 0;
- if (rtp_encrypt (s->rtp.cipher, ssrc, s->rtp_roc, seq, s->rtp.salt,
- buf + offset, len - offset))
+ if (rtp_crypt (s->rtp.cipher, ssrc, roc, seq, s->rtp.salt,
+ buf + offset, len - offset))
return EINVAL;
return 0;
* @param bufsize size (bytes) of the packet buffer
*
* @return 0 on success, in case of error:
- * EINVAL malformatted RTP packet
+ * EINVAL malformatted RTP packet or internal error
* ENOSPC bufsize is too small (to add authentication tag)
+ * EACCES packet would trigger a replay error on receiver
*/
int
srtp_send (srtp_session_t *s, uint8_t *buf, size_t *lenp, size_t bufsize)
{
size_t len = *lenp;
- int val = srtp_encrypt (s, buf, len);
+ int val = srtp_crypt (s, buf, len);
if (val)
return val;
- if (bufsize < (len + s->rtp.mac_len))
- return ENOSPC;
+ if (!(s->flags & SRTP_UNAUTHENTICATED))
+ {
+ if (bufsize < (len + s->tag_len))
+ return ENOSPC;
+
+ const uint8_t *tag = rtp_digest (s, buf, len);
+ memcpy (buf + len, tag, s->tag_len);
+ *lenp = len + s->tag_len;
+ }
- /* FIXME: HMAC and anti-replay */
return 0;
}
/**
- * Turns a RTP packet into a SRTP packet: encrypt it, then computes
- * the authentication tag and appends it.
- * Note that you can encrypt packet in disorder.
+ * Turns a SRTP packet into a RTP packet: authenticates the packet,
+ * then decrypts it.
*
- * @param buf RTP packet to be decrypted/digested
- * @param lenp pointer to the RTP packet length on entry,
- * set to the SRTP length on exit (undefined in case of error)
+ * @param buf RTP packet to be digested/decrypted
+ * @param lenp pointer to the SRTP packet length on entry,
+ * set to the RTP length on exit (undefined in case of error)
*
* @return 0 on success, in case of error:
- * EINVAL malformatted RTP packet
+ * EINVAL malformatted SRTP packet
* EACCES authentication failed (spoofed packet or out-of-sync)
*/
int
srtp_recv (srtp_session_t *s, uint8_t *buf, size_t *lenp)
{
size_t len = *lenp;
- int val = srtp_encrypt (s, buf, len);
+
+ if (!(s->flags & SRTP_UNAUTHENTICATED))
+ {
+ if (len < (12u + s->tag_len))
+ return EINVAL;
+ len -= s->tag_len;
+
+ const uint8_t *tag = rtp_digest (s, buf, len);
+ if (memcmp (buf + len, tag, s->tag_len))
+ return EACCES;
+
+ *lenp = len;
+ }
+
+ return srtp_crypt (s, buf, len);
+}
+
+
+/** AES-CM for RTCP (salt = 14 bytes + 2 nul bytes) */
+static int
+rtcp_crypt (gcry_cipher_hd_t hd, uint32_t ssrc, uint32_t index,
+ const uint32_t *salt, uint8_t *data, size_t len)
+{
+ return rtp_crypt (hd, ssrc, index >> 16, index & 0xffff, salt, data, len);
+}
+
+
+/** Message Authentication and Integrity for RTCP */
+static const uint8_t *
+rtcp_digest (gcry_md_hd_t md, const void *data, size_t len)
+{
+ gcry_md_reset (md);
+ gcry_md_write (md, data, len);
+ return gcry_md_read (md, 0);
+}
+
+
+/**
+ * Encrypts/decrypts a RTCP packet and updates SRTCP context
+ * (CTR block cypher mode of operation has identical encryption and
+ * decryption function).
+ *
+ * @param buf RTCP packet to be en-/decrypted
+ * @param len RTCP packet length
+ *
+ * @return 0 on success, in case of error:
+ * EINVAL malformatted RTCP packet
+ */
+static int srtcp_crypt (srtp_session_t *s, uint8_t *buf, size_t len)
+{
+ assert (s != NULL);
+
+ /* 8-bytes unencrypted header, and 4-bytes unencrypted footer */
+ if ((len < 12) || ((buf[0] >> 6) != 2))
+ return EINVAL;
+
+ uint32_t index;
+ memcpy (&index, buf + len, 4);
+ index = ntohl (index);
+ if (((index >> 31) != 0) != ((s->flags & SRTCP_UNENCRYPTED) == 0))
+ return EINVAL; // E-bit mismatch
+
+ index &= ~(1 << 31); // clear E-bit for counter
+
+ /* Updates SRTCP index (safe here) */
+ int32_t diff = index - s->rtcp_index;
+ if (diff > 0)
+ {
+ /* Packet in the future, good */
+ s->rtcp.window = s->rtcp.window << diff;
+ s->rtcp.window |= 1;
+ s->rtcp_index = index;
+ }
+ else
+ {
+ /* Packet in the past/present, bad */
+ diff = -diff;
+ if ((diff >= 64) || ((s->rtcp.window >> diff) & 1))
+ return EACCES; // replay attack!
+ s->rtp.window |= 1 << diff;
+ }
+
+ /* Crypts SRTCP */
+ if (s->flags & SRTCP_UNENCRYPTED)
+ return 0;
+
+ uint32_t ssrc;
+ memcpy (&ssrc, buf + 4, 4);
+
+ if (rtcp_crypt (s->rtcp.cipher, ssrc, index, s->rtp.salt,
+ buf + 8, len - 8))
+ return EINVAL;
+ return 0;
+}
+
+
+/**
+ * Turns a RTCP packet into a SRTCP packet: encrypt it, then computes
+ * the authentication tag and appends it.
+ *
+ * @param buf RTCP packet to be encrypted/digested
+ * @param lenp pointer to the RTCP packet length on entry,
+ * set to the SRTCP length on exit (undefined in case of error)
+ * @param bufsize size (bytes) of the packet buffer
+ *
+ * @return 0 on success, in case of error:
+ * EINVAL malformatted RTCP packet or internal error
+ * ENOSPC bufsize is too small (to add index and authentication tag)
+ */
+int
+srtcp_send (srtp_session_t *s, uint8_t *buf, size_t *lenp, size_t bufsize)
+{
+ size_t len = *lenp;
+ if (bufsize < (len + 4 + s->tag_len))
+ return ENOSPC;
+
+ uint32_t index = ++s->rtcp_index;
+ if (index >> 31)
+ s->rtcp_index = index = 0; /* 31-bit wrap */
+
+ if ((s->flags & SRTCP_UNENCRYPTED) == 0)
+ index |= 0x80000000; /* Set Encrypted bit */
+ memcpy (buf + len, &(uint32_t){ htonl (index) }, 4);
+
+ int val = srtcp_crypt (s, buf, len);
if (val)
return val;
- /* FIXME: HMAC and anti-replay */
+ len += 4; /* Digests SRTCP index too */
+
+ const uint8_t *tag = rtcp_digest (s->rtp.mac, buf, len);
+ memcpy (buf + len, tag, s->tag_len);
+ *lenp = len + s->tag_len;
return 0;
}
+
+/**
+ * Turns a SRTCP packet into a RTCP packet: authenticates the packet,
+ * then decrypts it.
+ *
+ * @param buf RTCP packet to be digested/decrypted
+ * @param lenp pointer to the SRTCP packet length on entry,
+ * set to the RTCP length on exit (undefined in case of error)
+ *
+ * @return 0 on success, in case of error:
+ * EINVAL malformatted SRTCP packet
+ * EACCES authentication failed (spoofed packet or out-of-sync)
+ */
+int
+srtcp_recv (srtp_session_t *s, uint8_t *buf, size_t *lenp)
+{
+ size_t len = *lenp;
+
+ if (len < (4u + s->tag_len))
+ return EINVAL;
+ len -= s->tag_len;
+
+ const uint8_t *tag = rtcp_digest (s->rtp.mac, buf, len);
+ if (memcmp (buf + len, tag, s->tag_len))
+ return EACCES;
+
+ len -= 4; /* Remove SRTCP index before decryption */
+ *lenp = len;
+ return srtp_crypt (s, buf, len);
+}
+
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