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-rw-r--r--plugins/crypto-vala/src/srtp.c836
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diff --git a/plugins/crypto-vala/src/srtp.c b/plugins/crypto-vala/src/srtp.c
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+/*
+ * Secure RTP with libgcrypt
+ * Copyright (C) 2007 RĂ©mi Denis-Courmont
+ *
+ * This library is free software; you can redistribute it and/or
+ * modify it under the terms of the GNU Lesser General Public
+ * License as published by the Free Software Foundation; either
+ * version 2.1 of the License, or (at your option) any later version.
+ *
+ * This library is distributed in the hope that it will be useful,
+ * but WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU
+ * Lesser General Public License for more details.
+ *
+ * You should have received a copy of the GNU Lesser General Public
+ * License along with this library; if not, write to the Free Software
+ * Foundation, Inc., 51 Franklin St, Fifth Floor, Boston, MA 02110-1301 USA
+ */
+
+/* TODO:
+ * Useless stuff (because nothing depends on it):
+ * - non-nul key derivation rate
+ * - MKI payload
+ */
+
+#ifdef HAVE_CONFIG_H
+# include <config.h>
+#endif
+
+#include <stdint.h>
+#include <stddef.h>
+
+#include "srtp.h"
+
+#include <stdbool.h>
+#include <stdlib.h>
+#include <assert.h>
+#include <errno.h>
+
+#include <gcrypt.h>
+
+#ifdef _WIN32
+# include <winsock2.h>
+#else
+# include <netinet/in.h>
+#endif
+
+#define debug( ... ) (void)0
+
+typedef struct srtp_proto_t
+{
+ gcry_cipher_hd_t cipher;
+ gcry_md_hd_t mac;
+ uint64_t window;
+ uint32_t salt[4];
+} srtp_proto_t;
+
+struct srtp_session_t
+{
+ srtp_proto_t rtp;
+ srtp_proto_t rtcp;
+ unsigned flags;
+ unsigned kdr;
+ uint32_t rtcp_index;
+ uint32_t rtp_roc;
+ uint16_t rtp_seq;
+ uint16_t rtp_rcc;
+ uint8_t tag_len;
+};
+
+enum
+{
+ SRTP_CRYPT,
+ SRTP_AUTH,
+ SRTP_SALT,
+ SRTCP_CRYPT,
+ SRTCP_AUTH,
+ SRTCP_SALT
+};
+
+
+static inline unsigned rcc_mode (const srtp_session_t *s)
+{
+ return (s->flags >> 4) & 3;
+}
+
+
+static void proto_destroy (srtp_proto_t *p)
+{
+ gcry_md_close (p->mac);
+ gcry_cipher_close (p->cipher);
+}
+
+
+/**
+ * Releases all resources associated with a Secure RTP session.
+ */
+void srtp_destroy (srtp_session_t *s)
+{
+ assert (s != NULL);
+
+ proto_destroy (&s->rtcp);
+ proto_destroy (&s->rtp);
+ free (s);
+}
+
+
+static int proto_create (srtp_proto_t *p, int gcipher, int gmd)
+{
+ if (gcry_cipher_open (&p->cipher, gcipher, GCRY_CIPHER_MODE_CTR, 0) == 0)
+ {
+ if (gcry_md_open (&p->mac, gmd, GCRY_MD_FLAG_HMAC) == 0)
+ return 0;
+ gcry_cipher_close (p->cipher);
+ }
+ return -1;
+}
+
+
+/**
+ * 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 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 (int encr, int auth, unsigned tag_len, int prf, unsigned flags)
+{
+ if ((flags & ~SRTP_FLAGS_MASK))
+ return NULL;
+
+ int cipher, md;
+ switch (encr)
+ {
+ case SRTP_ENCR_NULL:
+ cipher = GCRY_CIPHER_NONE;
+ break;
+
+ case SRTP_ENCR_AES_CM:
+ cipher = GCRY_CIPHER_AES;
+ break;
+
+ 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 (md))
+ return NULL;
+
+ if (prf != SRTP_PRF_AES_CM)
+ return NULL;
+
+ srtp_session_t *s = malloc (sizeof (*s));
+ if (s == NULL)
+ return NULL;
+
+ memset (s, 0, sizeof (*s));
+ s->flags = flags;
+ s->tag_len = tag_len;
+ s->rtp_rcc = 1; /* Default RCC rate */
+ if (rcc_mode (s))
+ {
+ if (tag_len < 4)
+ goto error;
+ }
+
+ if (proto_create (&s->rtp, cipher, md) == 0)
+ {
+ if (proto_create (&s->rtcp, cipher, md) == 0)
+ return s;
+ proto_destroy (&s->rtp);
+ }
+
+ error:
+ free (s);
+ return NULL;
+}
+
+
+/**
+ * Counter Mode encryption/decryption (ctr length = 16 bytes)
+ * with non-padded (truncated) text
+ */
+static int
+do_ctr_crypt (gcry_cipher_hd_t hd, const void *ctr, uint8_t *data, size_t len)
+{
+ const size_t ctrlen = 16;
+ div_t d = div (len, ctrlen);
+
+ if (gcry_cipher_setctr (hd, ctr, ctrlen)
+ || gcry_cipher_encrypt (hd, data, d.quot * ctrlen, NULL, 0))
+ return -1;
+
+ if (d.rem)
+ {
+ /* Truncated last block */
+ uint8_t dummy[ctrlen];
+ data += d.quot * ctrlen;
+ memcpy (dummy, data, d.rem);
+ memset (dummy + d.rem, 0, ctrlen - d.rem);
+
+ if (gcry_cipher_encrypt (hd, dummy, ctrlen, data, ctrlen))
+ return -1;
+ memcpy (data, dummy, d.rem);
+ }
+
+ return 0;
+}
+
+
+/**
+ * AES-CM key derivation (saltlen = 14 bytes)
+ */
+static int
+do_derive (gcry_cipher_hd_t prf, const void *salt,
+ const uint8_t *r, size_t rlen, uint8_t label,
+ void *out, size_t outlen)
+{
+ uint8_t iv[16];
+
+ memcpy (iv, salt, 14);
+ iv[14] = iv[15] = 0;
+
+ assert (rlen < 14);
+ iv[13 - rlen] ^= label;
+ for (size_t i = 0; i < rlen; i++)
+ iv[sizeof (iv) - rlen + i] ^= r[i];
+
+ memset (out, 0, outlen);
+ return do_ctr_crypt (prf, iv, out, outlen);
+}
+
+
+/**
+ * Sets (or resets) the master key and master salt for a SRTP session.
+ * This must be done at least once before using srtp_send(), srtp_recv(),
+ * srtcp_send() or srtcp_recv(). Also, rekeying is required every
+ * 2^48 RTP packets or 2^31 RTCP packets (whichever comes first),
+ * otherwise the protocol security might be broken.
+ *
+ * @return 0 on success, in case of error:
+ * EINVAL invalid or unsupported key/salt sizes combination
+ */
+int
+srtp_setkey (srtp_session_t *s, const void *key, size_t keylen,
+ const void *salt, size_t saltlen)
+{
+ /* SRTP/SRTCP cipher/salt/MAC keys derivation */
+ gcry_cipher_hd_t prf;
+ uint8_t r[6], keybuf[20];
+
+ if (saltlen != 14)
+ return EINVAL;
+
+ if (gcry_cipher_open (&prf, GCRY_CIPHER_AES, GCRY_CIPHER_MODE_CTR, 0)
+ || gcry_cipher_setkey (prf, key, keylen))
+ return EINVAL;
+
+ /* SRTP key derivation */
+#if 0
+ if (s->kdr != 0)
+ {
+ uint64_t index = (((uint64_t)s->rtp_roc) << 16) | s->rtp_seq;
+ index /= s->kdr;
+
+ for (int i = sizeof (r) - 1; i >= 0; i--)
+ {
+ r[i] = index & 0xff;
+ index = index >> 8;
+ }
+ }
+ else
+#endif
+ memset (r, 0, sizeof (r));
+ if (do_derive (prf, salt, r, 6, SRTP_CRYPT, keybuf, 16)
+ || gcry_cipher_setkey (s->rtp.cipher, keybuf, 16)
+ || do_derive (prf, salt, r, 6, SRTP_AUTH, keybuf, 20)
+ || gcry_md_setkey (s->rtp.mac, keybuf, 20)
+ || do_derive (prf, salt, r, 6, SRTP_SALT, s->rtp.salt, 14))
+ return -1;
+
+ /* SRTCP key derivation */
+ memcpy (r, &(uint32_t){ htonl (s->rtcp_index) }, 4);
+ if (do_derive (prf, salt, r, 4, SRTCP_CRYPT, keybuf, 16)
+ || gcry_cipher_setkey (s->rtcp.cipher, keybuf, 16)
+ || do_derive (prf, salt, r, 4, SRTCP_AUTH, keybuf, 20)
+ || gcry_md_setkey (s->rtcp.mac, keybuf, 20)
+ || do_derive (prf, salt, r, 4, SRTCP_SALT, s->rtcp.salt, 14))
+ return -1;
+
+ (void)gcry_cipher_close (prf);
+ return 0;
+}
+
+static int hexdigit (char c)
+{
+ if ((c >= '0') && (c <= '9'))
+ return c - '0';
+ if ((c >= 'A') && (c <= 'F'))
+ return c - 'A' + 0xA;
+ if ((c >= 'a') && (c <= 'f'))
+ return c - 'a' + 0xa;
+ return -1;
+}
+
+static ssize_t hexstring (const char *in, uint8_t *out, size_t outlen)
+{
+ size_t inlen = strlen (in);
+
+ if ((inlen > (2 * outlen)) || (inlen & 1))
+ return -1;
+
+ for (size_t i = 0; i < inlen; i += 2)
+ {
+ int a = hexdigit (in[i]), b = hexdigit (in[i + 1]);
+ if ((a == -1) || (b == -1))
+ return -1;
+ out[i / 2] = (a << 4) | b;
+ }
+ return inlen / 2;
+}
+
+/**
+ * Sets (or resets) the master key and master salt for a SRTP session
+ * from hexadecimal strings. See also srtp_setkey().
+ *
+ * @return 0 on success, in case of error:
+ * EINVAL invalid or unsupported key/salt sizes combination
+ */
+int
+srtp_setkeystring (srtp_session_t *s, const char *key, const char *salt)
+{
+ uint8_t bkey[16]; /* TODO/NOTE: hard-coded for AES */
+ uint8_t bsalt[14]; /* TODO/NOTE: hard-coded for the PRF-AES-CM */
+ ssize_t bkeylen = hexstring (key, bkey, sizeof (bkey));
+ ssize_t bsaltlen = hexstring (salt, bsalt, sizeof (bsalt));
+
+ if ((bkeylen == -1) || (bsaltlen == -1))
+ return EINVAL;
+ return srtp_setkey (s, bkey, bkeylen, bsalt, bsaltlen) ? EINVAL : 0;
+}
+
+/**
+ * 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 for RTP (salt = 14 bytes + 2 nul bytes) */
+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[0] = salt[0];
+ counter[1] = salt[1] ^ ssrc;
+ counter[2] = salt[2] ^ htonl (roc);
+ counter[3] = salt[3] ^ htonl (seq << 16);
+
+ /* Encryption */
+ return do_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 (gcry_md_hd_t md, const uint8_t *data, size_t len,
+ uint32_t roc)
+{
+ gcry_md_reset (md);
+ gcry_md_write (md, data, len);
+ gcry_md_write (md, &(uint32_t){ htonl (roc) }, 4);
+ return gcry_md_read (md, 0);
+}
+
+
+/**
+ * Encrypts/decrypts a RTP packet and updates SRTP context
+ * (CTR block cypher mode of operation has identical encryption and
+ * decryption function).
+ *
+ * @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_crypt (srtp_session_t *s, uint8_t *buf, size_t len)
+{
+ assert (s != NULL);
+ assert (len >= 12u);
+
+ if ((buf[0] >> 6) != 2)
+ return EINVAL;
+
+ /* Computes encryption offset */
+ uint16_t offset = 12;
+ offset += (buf[0] & 0xf) * 4; // skips CSRC
+
+ if (buf[0] & 0x10)
+ {
+ uint16_t extlen;
+
+ offset += 4;
+ if (len < offset)
+ return EINVAL;
+
+ memcpy (&extlen, buf + offset - 2, 2);
+ offset += htons (extlen); // skips RTP extension header
+ }
+
+ if (len < offset)
+ return EINVAL;
+
+ /* Determines RTP 48-bits counter and SSRC */
+ uint16_t seq = rtp_seq (buf);
+ uint32_t roc = srtp_compute_roc (s, seq), ssrc;
+ memcpy (&ssrc, buf + 8, 4);
+
+ /* Updates ROC and sequence (it's safe now) */
+ int16_t diff = seq - s->rtp_seq;
+ if (diff > 0)
+ {
+ /* Sequence in the future, good */
+ s->rtp.window = s->rtp.window << diff;
+ s->rtp.window |= UINT64_C(1);
+ s->rtp_seq = seq, s->rtp_roc = roc;
+ }
+ else
+ {
+ /* Sequence in the past/present, bad */
+ diff = -diff;
+ if ((diff >= 64) || ((s->rtp.window >> diff) & 1))
+ return EACCES; /* Replay attack */
+ s->rtp.window |= UINT64_C(1) << diff;
+ }
+
+ /* Encrypt/Decrypt */
+ if (s->flags & SRTP_UNENCRYPTED)
+ return 0;
+
+ if (rtp_crypt (s->rtp.cipher, ssrc, roc, seq, s->rtp.salt,
+ buf + offset, len - offset))
+ return EINVAL;
+
+ 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.
+ *
+ * @param buf RTP packet to be encrypted/digested
+ * @param lenp pointer to the RTP packet length on entry,
+ * set to the SRTP length on exit (undefined on non-ENOSPC error)
+ * @param bufsize size (bytes) of the packet buffer
+ *
+ * @return 0 on success, in case of error:
+ * EINVAL malformatted RTP packet or internal error
+ * ENOSPC bufsize is too small to add authentication tag
+ * (<lenp> will hold the required byte size)
+ * 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;
+ size_t tag_len;
+ size_t roc_len = 0;
+
+ /* Compute required buffer size */
+ if (len < 12u)
+ return EINVAL;
+
+ if (!(s->flags & SRTP_UNAUTHENTICATED))
+ {
+ tag_len = s->tag_len;
+
+ if (rcc_mode (s))
+ {
+ assert (tag_len >= 4);
+ assert (s->rtp_rcc != 0);
+ if ((rtp_seq (buf) % s->rtp_rcc) == 0)
+ {
+ roc_len = 4;
+ if (rcc_mode (s) == 3)
+ tag_len = 0; /* RCC mode 3 -> no auth*/
+ else
+ tag_len -= 4; /* RCC mode 1 or 2 -> auth*/
+ }
+ else
+ {
+ if (rcc_mode (s) & 1)
+ tag_len = 0; /* RCC mode 1 or 3 -> no auth */
+ }
+ }
+
+ *lenp = len + roc_len + tag_len;
+ }
+ else
+ tag_len = 0;
+
+ if (bufsize < *lenp)
+ return ENOSPC;
+
+ /* Encrypt payload */
+ int val = srtp_crypt (s, buf, len);
+ if (val)
+ return val;
+
+ /* Authenticate payload */
+ if (!(s->flags & SRTP_UNAUTHENTICATED))
+ {
+ uint32_t roc = srtp_compute_roc (s, rtp_seq (buf));
+ const uint8_t *tag = rtp_digest (s->rtp.mac, buf, len, roc);
+
+ if (roc_len)
+ {
+ memcpy (buf + len, &(uint32_t){ htonl (s->rtp_roc) }, 4);
+ len += 4;
+ }
+ memcpy (buf + len, tag, tag_len);
+#if 0
+ printf ("Sent : 0x");
+ for (unsigned i = 0; i < tag_len; i++)
+ printf ("%02x", tag[i]);
+ puts ("");
+#endif
+ }
+
+ return 0;
+}
+
+
+/**
+ * Turns a SRTP packet into a RTP packet: authenticates the packet,
+ * then decrypts it.
+ *
+ * @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 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;
+ if (len < 12u)
+ return EINVAL;
+
+ if (!(s->flags & SRTP_UNAUTHENTICATED))
+ {
+ size_t tag_len = s->tag_len, roc_len = 0;
+ if (rcc_mode (s))
+ {
+ if ((rtp_seq (buf) % s->rtp_rcc) == 0)
+ {
+ roc_len = 4;
+ if (rcc_mode (s) == 3)
+ tag_len = 0;
+ else
+ tag_len -= 4;
+ }
+ else
+ {
+ if (rcc_mode (s) & 1)
+ tag_len = 0; // RCC mode 1 or 3: no auth
+ }
+ }
+
+ if (len < (12u + roc_len + tag_len))
+ return EINVAL;
+ len -= roc_len + tag_len;
+
+ uint32_t roc = srtp_compute_roc (s, rtp_seq (buf)), rcc;
+ if (roc_len)
+ {
+ assert (roc_len == 4);
+ memcpy (&rcc, buf + len, 4);
+ rcc = ntohl (rcc);
+ }
+ else
+ rcc = roc;
+
+ const uint8_t *tag = rtp_digest (s->rtp.mac, buf, len, rcc);
+#if 0
+ printf ("Computed: 0x");
+ for (unsigned i = 0; i < tag_len; i++)
+ printf ("%02x", tag[i]);
+ printf ("\nReceived: 0x");
+ for (unsigned i = 0; i < tag_len; i++)
+ printf ("%02x", buf[len + roc_len + i]);
+ puts ("");
+#endif
+ if (memcmp (buf + len + roc_len, tag, tag_len))
+ return EACCES;
+
+ if (roc_len)
+ {
+ /* Authenticated packet carried a Roll-Over-Counter */
+ s->rtp_roc += rcc - roc;
+ assert (srtp_compute_roc (s, rtp_seq (buf)) == rcc);
+ }
+ *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 |= UINT64_C(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 |= UINT64_C(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;
+
+ len += 4; /* Digests SRTCP index too */
+
+ const uint8_t *tag = rtcp_digest (s->rtcp.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->rtcp.mac, buf, len);
+ if (memcmp (buf + len, tag, s->tag_len))
+ return EACCES;
+
+ len -= 4; /* Remove SRTCP index before decryption */
+ *lenp = len;
+ return srtcp_crypt (s, buf, len);
+} \ No newline at end of file