Add current version of libSRTP from CVS.

libsrtp/include/srtp.h

+/*
+ * srtp.h
+ *
+ * interface to libsrtp
+ *
+ * David A. McGrew
+ * Cisco Systems, Inc.
+ */
+/*
+ *      
+ * Copyright (c) 2001-2006, Cisco Systems, Inc.
+ * All rights reserved.
+ * 
+ * Redistribution and use in source and binary forms, with or without
+ * modification, are permitted provided that the following conditions
+ * are met:
+ * 
+ *   Redistributions of source code must retain the above copyright
+ *   notice, this list of conditions and the following disclaimer.
+ * 
+ *   Redistributions in binary form must reproduce the above
+ *   copyright notice, this list of conditions and the following
+ *   disclaimer in the documentation and/or other materials provided
+ *   with the distribution.
+ * 
+ *   Neither the name of the Cisco Systems, Inc. nor the names of its
+ *   contributors may be used to endorse or promote products derived
+ *   from this software without specific prior written permission.
+ * 
+ * THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS
+ * "AS IS" AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT
+ * LIMITED TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS
+ * FOR A PARTICULAR PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL THE
+ * COPYRIGHT HOLDERS OR CONTRIBUTORS BE LIABLE FOR ANY DIRECT,
+ * INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES
+ * (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR
+ * SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION)
+ * HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT,
+ * STRICT LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE)
+ * ARISING IN ANY WAY OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED
+ * OF THE POSSIBILITY OF SUCH DAMAGE.
+ *
+ */
+
+
+#ifndef SRTP_H
+#define SRTP_H
+
+#ifdef __cplusplus
+extern "C" {
+#endif
+
+#include "crypto_kernel.h" 
+
+/**
+ * @defgroup SRTP Secure RTP
+ *
+ * @brief libSRTP provides functions for protecting RTP and RTCP.  See
+ * Section @ref Overview for an introduction to the use of the library.
+ *
+ * @{
+ */
+
+/*
+ * SRTP_MASTER_KEY_LEN is the nominal master key length supported by libSRTP
+ */
+
+#define SRTP_MASTER_KEY_LEN 30
+
+/*
+ * SRTP_MAX_KEY_LEN is the maximum key length supported by libSRTP
+ */
+#define SRTP_MAX_KEY_LEN      64
+
+/*
+ * SRTP_MAX_TAG_LEN is the maximum tag length supported by libSRTP
+ */
+
+#define SRTP_MAX_TAG_LEN 12 
+
+/**
+ * SRTP_MAX_TRAILER_LEN is the maximum length of the SRTP trailer
+ * (authentication tag and MKI) supported by libSRTP.  This value is
+ * the maximum number of octets that will be added to an RTP packet by
+ * srtp_protect().
+ *
+ * @brief the maximum number of octets added by srtp_protect().
+ */
+#define SRTP_MAX_TRAILER_LEN SRTP_MAX_TAG_LEN 
+
+/* 
+ * nota bene: since libSRTP doesn't support the use of the MKI, the
+ * SRTP_MAX_TRAILER_LEN value is just the maximum tag length
+ */
+
+/**
+ * @brief sec_serv_t describes a set of security services. 
+ *
+ * A sec_serv_t enumeration is used to describe the particular
+ * security services that will be applied by a particular crypto
+ * policy (or other mechanism).  
+ */
+
+typedef enum {
+  sec_serv_none          = 0, /**< no services                        */
+  sec_serv_conf          = 1, /**< confidentiality                    */
+  sec_serv_auth          = 2, /**< authentication                     */
+  sec_serv_conf_and_auth = 3  /**< confidentiality and authentication */
+} sec_serv_t;
+
+/** 
+ * @brief crypto_policy_t describes a particular crypto policy that
+ * can be applied to an SRTP stream.
+ *
+ * A crypto_policy_t describes a particular cryptographic policy that
+ * can be applied to an SRTP or SRTCP stream.  An SRTP session policy
+ * consists of a list of these policies, one for each SRTP stream 
+ * in the session.
+ */
+
+typedef struct crypto_policy_t {
+  cipher_type_id_t cipher_type;    /**< An integer representing
+                                    *   the type of cipher.  */
+  int              cipher_key_len; /**< The length of the cipher key
+                                    *   in octets.                       */
+  auth_type_id_t   auth_type;      /**< An integer representing the
+                                    *   authentication function.         */
+  int              auth_key_len;   /**< The length of the authentication 
+                                    *   function key in octets.          */
+  int              auth_tag_len;   /**< The length of the authentication 
+                                    *   tag in octets.                   */
+  sec_serv_t       sec_serv;       /**< The flag indicating the security
+                                    *   services to be applied.          */
+} crypto_policy_t;
+
+
+/** 
+ * @brief ssrc_type_t describes the type of an SSRC.
+ * 
+ * An ssrc_type_t enumeration is used to indicate a type of SSRC.  See
+ * @ref srtp_policy_t for more informataion.
+ */
+
+typedef enum { 
+  ssrc_undefined    = 0,  /**< Indicates an undefined SSRC type. */
+  ssrc_specific     = 1,  /**< Indicates a specific SSRC value   */
+  ssrc_any_inbound  = 2, /**< Indicates any inbound SSRC value 
+                            (i.e. a value that is used in the
+                            function srtp_unprotect())              */
+  ssrc_any_outbound = 3  /**< Indicates any outbound SSRC value 
+                            (i.e. a value that is used in the 
+                            function srtp_protect())              */
+} ssrc_type_t;
+
+/**
+ * @brief An ssrc_t represents a particular SSRC value, or a `wildcard' SSRC.
+ * 
+ * An ssrc_t represents a particular SSRC value (if its type is
+ * ssrc_specific), or a wildcard SSRC value that will match all
+ * outbound SSRCs (if its type is ssrc_any_outbound) or all inbound
+ * SSRCs (if its type is ssrc_any_inbound).  
+ *
+ */
+
+typedef struct { 
+  ssrc_type_t type;   /**< The type of this particular SSRC */
+  unsigned int value; /**< The value of this SSRC, if it is not a wildcard */
+} ssrc_t;
+
+
+/**
+ * @brief points to an EKT policy
+ */
+typedef struct ekt_policy_ctx_t *ekt_policy_t;
+
+
+/**
+ * @brief points to EKT stream data
+ */
+typedef struct ekt_stream_ctx_t *ekt_stream_t;
+
+
+/** 
+ * @brief represents the policy for an SRTP session.  
+ *
+ * A single srtp_policy_t struct represents the policy for a single
+ * SRTP stream, and a linked list of these elements represents the
+ * policy for an entire SRTP session.  Each element contains the SRTP
+ * and SRTCP crypto policies for that stream, a pointer to the SRTP
+ * master key for that stream, the SSRC describing that stream, or a
+ * flag indicating a `wildcard' SSRC value, and a `next' field that
+ * holds a pointer to the next element in the list of policy elements,
+ * or NULL if it is the last element. 
+ *
+ * The wildcard value SSRC_ANY_INBOUND matches any SSRC from an
+ * inbound stream that for which there is no explicit SSRC entry in
+ * another policy element.  Similarly, the value SSRC_ANY_OUTBOUND
+ * will matches any SSRC from an outbound stream that does not appear
+ * in another policy element.  Note that wildcard SSRCs &b cannot be
+ * used to match both inbound and outbound traffic.  This restriction
+ * is intentional, and it allows libSRTP to ensure that no security
+ * lapses result from accidental re-use of SSRC values during key
+ * sharing.
+ * 
+ * 
+ * @warning The final element of the list @b must have its `next' pointer
+ *          set to NULL.
+ */
+
+typedef struct srtp_policy_t {
+  ssrc_t        ssrc;        /**< The SSRC value of stream, or the 
+                              *   flags SSRC_ANY_INBOUND or 
+                              *   SSRC_ANY_OUTBOUND if key sharing
+                              *   is used for this policy element.
+                              */
+  crypto_policy_t rtp;         /**< SRTP crypto policy.                  */
+  crypto_policy_t rtcp;        /**< SRTCP crypto policy.                 */
+  unsigned char *key;          /**< Pointer to the SRTP master key for
+                                *    this stream.                        */
+  ekt_policy_t ekt;            /**< Pointer to the EKT policy structure
+                                *   for this stream (if any)             */ 
+  unsigned long window_size;   /**< The window size to use for replay
+                                *   protection. */
+  int        allow_repeat_tx;  /**< Whether retransmissions of
+                                *   packets with the same sequence number
+                                *   are allowed.  (Note that such repeated
+                                *   transmissions must have the same RTP
+                                *   payload, or a severe security weakness
+                                *   is introduced!)                      */
+  struct srtp_policy_t *next;  /**< Pointer to next stream policy.       */
+} srtp_policy_t;
+
+
+
+
+/**
+ * @brief An srtp_t points to an SRTP session structure.
+ *
+ * The typedef srtp_t is a pointer to a structure that represents
+ * an SRTP session.  This datatype is intentially opaque in 
+ * order to separate the interface from the implementation.
+ *
+ * An SRTP session consists of all of the traffic sent to the RTP and
+ * RTCP destination transport addresses, using the RTP/SAVP (Secure
+ * Audio/Video Profile).  A session can be viewed as a set of SRTP
+ * streams, each of which originates with a different participant.
+ */
+
+typedef struct srtp_ctx_t *srtp_t;
+
+
+/**
+ * @brief An srtp_stream_t points to an SRTP stream structure.
+ *
+ * The typedef srtp_stream_t is a pointer to a structure that
+ * represents an SRTP stream.  This datatype is intentionally
+ * opaque in order to separate the interface from the implementation. 
+ * 
+ * An SRTP stream consists of all of the traffic sent to an SRTP
+ * session by a single participant.  A session can be viewed as
+ * a set of streams.  
+ *
+ */
+typedef struct srtp_stream_ctx_t *srtp_stream_t;
+
+
+
+/**
+ * @brief srtp_init() initializes the srtp library.  
+ *
+ * @warning This function @b must be called before any other srtp
+ * functions.
+ */
+
+err_status_t
+srtp_init(void);
+
+/**
+ * @brief srtp_shutdown() de-initializes the srtp library.
+ *
+ * @warning No srtp functions may be called after calling this function.
+ */
+
+err_status_t
+srtp_shutdown(void);
+
+/**
+ * @brief srtp_protect() is the Secure RTP sender-side packet processing
+ * function.
+ * 
+ * The function call srtp_protect(ctx, rtp_hdr, len_ptr) applies SRTP
+ * protection to the RTP packet rtp_hdr (which has length *len_ptr) using
+ * the SRTP context ctx.  If err_status_ok is returned, then rtp_hdr
+ * points to the resulting SRTP packet and *len_ptr is the number of
+ * octets in that packet; otherwise, no assumptions should be made
+ * about the value of either data elements.
+ * 
+ * The sequence numbers of the RTP packets presented to this function
+ * need not be consecutive, but they @b must be out of order by less
+ * than 2^15 = 32,768 packets.
+ *
+ * @warning This function assumes that it can write the authentication
+ * tag into the location in memory immediately following the RTP
+ * packet, and assumes that the RTP packet is aligned on a 32-bit
+ * boundary.
+ *
+ * @param ctx is the SRTP context to use in processing the packet.
+ *
+ * @param rtp_hdr is a pointer to the RTP packet (before the call); after
+ * the function returns, it points to the srtp packet.
+ *
+ * @param len_ptr is a pointer to the length in octets of the complete
+ * RTP packet (header and body) before the function call, and of the
+ * complete SRTP packet after the call, if err_status_ok was returned.
+ * Otherwise, the value of the data to which it points is undefined.
+ *
+ * @return 
+ *    - err_status_ok            no problems
+ *    - err_status_replay_fail   rtp sequence number was non-increasing
+ *    - @e other                 failure in cryptographic mechanisms
+ */
+
+err_status_t
+srtp_protect(srtp_t ctx, void *rtp_hdr, int *len_ptr);
+             
+/**
+ * @brief srtp_unprotect() is the Secure RTP receiver-side packet
+ * processing function.
+ *
+ * The function call srtp_unprotect(ctx, srtp_hdr, len_ptr) verifies
+ * the Secure RTP protection of the SRTP packet pointed to by srtp_hdr
+ * (which has length *len_ptr), using the SRTP context ctx.  If
+ * err_status_ok is returned, then srtp_hdr points to the resulting
+ * RTP packet and *len_ptr is the number of octets in that packet;
+ * otherwise, no assumptions should be made about the value of either
+ * data elements.  
+ * 
+ * The sequence numbers of the RTP packets presented to this function
+ * need not be consecutive, but they @b must be out of order by less
+ * than 2^15 = 32,768 packets.
+ * 
+ * @warning This function assumes that the SRTP packet is aligned on a
+ * 32-bit boundary.
+ *
+ * @param ctx is a pointer to the srtp_t which applies to the
+ * particular packet.
+ *
+ * @param srtp_hdr is a pointer to the header of the SRTP packet
+ * (before the call).  after the function returns, it points to the
+ * rtp packet if err_status_ok was returned; otherwise, the value of
+ * the data to which it points is undefined.
+ *
+ * @param len_ptr is a pointer to the length in octets of the complete
+ * srtp packet (header and body) before the function call, and of the
+ * complete rtp packet after the call, if err_status_ok was returned.
+ * Otherwise, the value of the data to which it points is undefined.
+ *
+ * @return 
+ *    - err_status_ok          if the RTP packet is valid.
+ *    - err_status_auth_fail   if the SRTP packet failed the message 
+ *                             authentication check.
+ *    - err_status_replay_fail if the SRTP packet is a replay (e.g. packet has
+ *                             already been processed and accepted).
+ *    - [other]  if there has been an error in the cryptographic mechanisms.
+ *
+ */
+
+err_status_t
+srtp_unprotect(srtp_t ctx, void *srtp_hdr, int *len_ptr);
+
+
+/**
+ * @brief srtp_create() allocates and initializes an SRTP session.
+
+ * The function call srtp_create(session, policy, key) allocates and
+ * initializes an SRTP session context, applying the given policy and
+ * key.
+ *
+ * @param session is the SRTP session to which the policy is to be added.
+ * 
+ * @param policy is the srtp_policy_t struct that describes the policy
+ * for the session.  The struct may be a single element, or it may be
+ * the head of a list, in which case each element of the list is
+ * processed.  It may also be NULL, in which case streams should be added
+ * later using srtp_add_stream().  The final element of the list @b must
+ * have its `next' field set to NULL.
+ * 
+ * @return
+ *    - err_status_ok           if creation succeded.
+ *    - err_status_alloc_fail   if allocation failed.
+ *    - err_status_init_fail    if initialization failed.
+ */
+
+err_status_t
+srtp_create(srtp_t *session, const srtp_policy_t *policy);
+
+
+/**
+ * @brief srtp_add_stream() allocates and initializes an SRTP stream
+ * within a given SRTP session.
+ * 
+ * The function call srtp_add_stream(session, policy) allocates and
+ * initializes a new SRTP stream within a given, previously created
+ * session, applying the policy given as the other argument to that
+ * stream.
+ *
+ * @return values:
+ *    - err_status_ok           if stream creation succeded.
+ *    - err_status_alloc_fail   if stream allocation failed
+ *    - err_status_init_fail    if stream initialization failed.
+ */
+
+err_status_t
+srtp_add_stream(srtp_t session, 
+                const srtp_policy_t *policy);
+
+
+/**
+ * @brief srtp_remove_stream() deallocates an SRTP stream.
+ * 
+ * The function call srtp_remove_stream(session, ssrc) removes
+ * the SRTP stream with the SSRC value ssrc from the SRTP session
+ * context given by the argument session.
+ *
+ * @param session is the SRTP session from which the stream
+ *        will be removed.
+ *
+ * @param ssrc is the SSRC value of the stream to be removed.
+ *
+ * @warning Wildcard SSRC values cannot be removed from a
+ *          session.
+ * 
+ * @return
+ *    - err_status_ok     if the stream deallocation succeded.
+ *    - [other]           otherwise.
+ *
+ */
+
+err_status_t
+srtp_remove_stream(srtp_t session, unsigned int ssrc);
+
+/**
+ * @brief crypto_policy_set_rtp_default() sets a crypto policy
+ * structure to the SRTP default policy for RTP protection.
+ *
+ * @param p is a pointer to the policy structure to be set 
+ * 
+ * The function call crypto_policy_set_rtp_default(&p) sets the
+ * crypto_policy_t at location p to the SRTP default policy for RTP
+ * protection, as defined in the specification.  This function is a
+ * convenience that helps to avoid dealing directly with the policy
+ * data structure.  You are encouraged to initialize policy elements
+ * with this function call.  Doing so may allow your code to be
+ * forward compatible with later versions of libSRTP that include more
+ * elements in the crypto_policy_t datatype.
+ * 
+ * @return void.
+ * 
+ */
+
+void
+crypto_policy_set_rtp_default(crypto_policy_t *p);
+
+/**
+ * @brief crypto_policy_set_rtcp_default() sets a crypto policy
+ * structure to the SRTP default policy for RTCP protection.
+ *
+ * @param p is a pointer to the policy structure to be set 
+ * 
+ * The function call crypto_policy_set_rtcp_default(&p) sets the
+ * crypto_policy_t at location p to the SRTP default policy for RTCP
+ * protection, as defined in the specification.  This function is a
+ * convenience that helps to avoid dealing directly with the policy
+ * data structure.  You are encouraged to initialize policy elements
+ * with this function call.  Doing so may allow your code to be
+ * forward compatible with later versions of libSRTP that include more
+ * elements in the crypto_policy_t datatype.
+ * 
+ * @return void.
+ * 
+ */
+
+void
+crypto_policy_set_rtcp_default(crypto_policy_t *p);
+
+/**
+ * @brief crypto_policy_set_aes_cm_128_hmac_sha1_80() sets a crypto
+ * policy structure to the SRTP default policy for RTP protection.
+ *
+ * @param p is a pointer to the policy structure to be set 
+ * 
+ * The function crypto_policy_set_aes_cm_128_hmac_sha1_80() is a
+ * synonym for crypto_policy_set_rtp_default().  It conforms to the
+ * naming convention used in RFC 4568 (SDP Security Descriptions for
+ * Media Streams).
+ * 
+ * @return void.
+ * 
+ */
+
+#define crypto_policy_set_aes_cm_128_hmac_sha1_80(p) crypto_policy_set_rtp_default(p)
+
+
+/**
+ * @brief crypto_policy_set_aes_cm_128_hmac_sha1_32() sets a crypto
+ * policy structure to a short-authentication tag policy
+ *
+ * @param p is a pointer to the policy structure to be set 
+ * 
+ * The function call crypto_policy_set_aes_cm_128_hmac_sha1_32(&p)
+ * sets the crypto_policy_t at location p to use policy
+ * AES_CM_128_HMAC_SHA1_32 as defined in RFC 4568.
+ * This policy uses AES-128
+ * Counter Mode encryption and HMAC-SHA1 authentication, with an
+ * authentication tag that is only 32 bits long.  This length is
+ * considered adequate only for protecting audio and video media that
+ * use a stateless playback function.  See Section 7.5 of RFC 3711
+ * (http://www.ietf.org/rfc/rfc3711.txt).
+ * 
+ * This function is a convenience that helps to avoid dealing directly
+ * with the policy data structure.  You are encouraged to initialize
+ * policy elements with this function call.  Doing so may allow your
+ * code to be forward compatible with later versions of libSRTP that
+ * include more elements in the crypto_policy_t datatype.
+ *
+ * @warning This crypto policy is intended for use in SRTP, but not in
+ * SRTCP.  It is recommended that a policy that uses longer
+ * authentication tags be used for SRTCP.  See Section 7.5 of RFC 3711
+ * (http://www.ietf.org/rfc/rfc3711.txt).
+ *
+ * @return void.
+ * 
+ */
+
+void
+crypto_policy_set_aes_cm_128_hmac_sha1_32(crypto_policy_t *p);
+
+
+
+/**
+ * @brief crypto_policy_set_aes_cm_128_null_auth() sets a crypto
+ * policy structure to an encryption-only policy
+ *
+ * @param p is a pointer to the policy structure to be set 
+ * 
+ * The function call crypto_policy_set_aes_cm_128_null_auth(&p) sets
+ * the crypto_policy_t at location p to use the SRTP default cipher
+ * (AES-128 Counter Mode), but to use no authentication method.  This
+ * policy is NOT RECOMMENDED unless it is unavoidable; see Section 7.5
+ * of RFC 3711 (http://www.ietf.org/rfc/rfc3711.txt).
+ * 
+ * This function is a convenience that helps to avoid dealing directly
+ * with the policy data structure.  You are encouraged to initialize
+ * policy elements with this function call.  Doing so may allow your
+ * code to be forward compatible with later versions of libSRTP that
+ * include more elements in the crypto_policy_t datatype.
+ *
+ * @warning This policy is NOT RECOMMENDED for SRTP unless it is
+ * unavoidable, and it is NOT RECOMMENDED at all for SRTCP; see
+ * Section 7.5 of RFC 3711 (http://www.ietf.org/rfc/rfc3711.txt).
+ *
+ * @return void.
+ * 
+ */
+
+void
+crypto_policy_set_aes_cm_128_null_auth(crypto_policy_t *p);
+
+
+/**
+ * @brief crypto_policy_set_null_cipher_hmac_sha1_80() sets a crypto
+ * policy structure to an authentication-only policy
+ *
+ * @param p is a pointer to the policy structure to be set 
+ * 
+ * The function call crypto_policy_set_null_cipher_hmac_sha1_80(&p)
+ * sets the crypto_policy_t at location p to use HMAC-SHA1 with an 80
+ * bit authentication tag to provide message authentication, but to
+ * use no encryption.  This policy is NOT RECOMMENDED for SRTP unless
+ * there is a requirement to forego encryption.  
+ * 
+ * This function is a convenience that helps to avoid dealing directly
+ * with the policy data structure.  You are encouraged to initialize
+ * policy elements with this function call.  Doing so may allow your
+ * code to be forward compatible with later versions of libSRTP that
+ * include more elements in the crypto_policy_t datatype.
+ *
+ * @warning This policy is NOT RECOMMENDED for SRTP unless there is a
+ * requirement to forego encryption.  
+ *
+ * @return void.
+ * 
+ */
+
+void
+crypto_policy_set_null_cipher_hmac_sha1_80(crypto_policy_t *p);
+
+
+/**
+ * @brief crypto_policy_set_aes_cm_256_hmac_sha1_80() sets a crypto
+ * policy structure to a encryption and authentication policy using AES-256 
+ * for RTP protection.
+ *
+ * @param p is a pointer to the policy structure to be set 
+ * 
+ * The function call crypto_policy_set_aes_cm_256_hmac_sha1_80(&p)
+ * sets the crypto_policy_t at location p to use policy
+ * AES_CM_256_HMAC_SHA1_80 as defined in
+ * draft-ietf-avt-srtp-big-aes-03.txt.  This policy uses AES-256
+ * Counter Mode encryption and HMAC-SHA1 authentication, with an 80 bit
+ * authentication tag.
+ * 
+ * This function is a convenience that helps to avoid dealing directly
+ * with the policy data structure.  You are encouraged to initialize
+ * policy elements with this function call.  Doing so may allow your
+ * code to be forward compatible with later versions of libSRTP that
+ * include more elements in the crypto_policy_t datatype.
+ *
+ * @return void.
+ * 
+ */
+
+void crypto_policy_set_aes_cm_256_hmac_sha1_80(crypto_policy_t *p);
+
+
+/**
+ * @brief crypto_policy_set_aes_cm_256_hmac_sha1_32() sets a crypto
+ * policy structure to a short-authentication tag policy using AES-256
+ * encryption.
+ *
+ * @param p is a pointer to the policy structure to be set 
+ * 
+ * The function call crypto_policy_set_aes_cm_256_hmac_sha1_32(&p)
+ * sets the crypto_policy_t at location p to use policy
+ * AES_CM_256_HMAC_SHA1_32 as defined in
+ * draft-ietf-avt-srtp-big-aes-03.txt.  This policy uses AES-256
+ * Counter Mode encryption and HMAC-SHA1 authentication, with an
+ * authentication tag that is only 32 bits long.  This length is
+ * considered adequate only for protecting audio and video media that
+ * use a stateless playback function.  See Section 7.5 of RFC 3711
+ * (http://www.ietf.org/rfc/rfc3711.txt).
+ * 
+ * This function is a convenience that helps to avoid dealing directly
+ * with the policy data structure.  You are encouraged to initialize
+ * policy elements with this function call.  Doing so may allow your
+ * code to be forward compatible with later versions of libSRTP that
+ * include more elements in the crypto_policy_t datatype.
+ *
+ * @warning This crypto policy is intended for use in SRTP, but not in
+ * SRTCP.  It is recommended that a policy that uses longer
+ * authentication tags be used for SRTCP.  See Section 7.5 of RFC 3711
+ * (http://www.ietf.org/rfc/rfc3711.txt).
+ *
+ * @return void.
+ * 
+ */
+
+void
+crypto_policy_set_aes_cm_256_hmac_sha1_32(crypto_policy_t *p);
+
+
+/**
+ * @brief srtp_dealloc() deallocates storage for an SRTP session
+ * context.
+ * 
+ * The function call srtp_dealloc(s) deallocates storage for the
+ * SRTP session context s.  This function should be called no more
+ * than one time for each of the contexts allocated by the function
+ * srtp_create().
+ *
+ * @param s is the srtp_t for the session to be deallocated.
+ *
+ * @return
+ *    - err_status_ok             if there no problems.
+ *    - err_status_dealloc_fail   a memory deallocation failure occured.
+ */
+
+err_status_t
+srtp_dealloc(srtp_t s);
+
+
+/*
+ * @brief identifies a particular SRTP profile 
+ *
+ * An srtp_profile_t enumeration is used to identify a particular SRTP
+ * profile (that is, a set of algorithms and parameters).  These
+ * profiles are defined in the DTLS-SRTP draft.
+ */
+
+typedef enum {
+  srtp_profile_reserved           = 0,
+  srtp_profile_aes128_cm_sha1_80  = 1,
+  srtp_profile_aes128_cm_sha1_32  = 2,
+  srtp_profile_aes256_cm_sha1_80  = 3,
+  srtp_profile_aes256_cm_sha1_32  = 4,
+  srtp_profile_null_sha1_80       = 5,
+  srtp_profile_null_sha1_32       = 6,
+} srtp_profile_t;
+
+
+/**
+ * @brief crypto_policy_set_from_profile_for_rtp() sets a crypto policy
+ * structure to the appropriate value for RTP based on an srtp_profile_t
+ *
+ * @param p is a pointer to the policy structure to be set 
+ * 
+ * The function call crypto_policy_set_rtp_default(&policy, profile)
+ * sets the crypto_policy_t at location policy to the policy for RTP
+ * protection, as defined by the srtp_profile_t profile.
+ * 
+ * This function is a convenience that helps to avoid dealing directly
+ * with the policy data structure.  You are encouraged to initialize
+ * policy elements with this function call.  Doing so may allow your
+ * code to be forward compatible with later versions of libSRTP that
+ * include more elements in the crypto_policy_t datatype.
+ * 
+ * @return values
+ *     - err_status_ok         no problems were encountered
+ *     - err_status_bad_param  the profile is not supported 
+ * 
+ */
+err_status_t
+crypto_policy_set_from_profile_for_rtp(crypto_policy_t *policy, 
+                                       srtp_profile_t profile);
+
+
+
+
+/**
+ * @brief crypto_policy_set_from_profile_for_rtcp() sets a crypto policy
+ * structure to the appropriate value for RTCP based on an srtp_profile_t
+ *
+ * @param p is a pointer to the policy structure to be set 
+ * 
+ * The function call crypto_policy_set_rtcp_default(&policy, profile)
+ * sets the crypto_policy_t at location policy to the policy for RTCP
+ * protection, as defined by the srtp_profile_t profile.
+ * 
+ * This function is a convenience that helps to avoid dealing directly
+ * with the policy data structure.  You are encouraged to initialize
+ * policy elements with this function call.  Doing so may allow your
+ * code to be forward compatible with later versions of libSRTP that
+ * include more elements in the crypto_policy_t datatype.
+ * 
+ * @return values
+ *     - err_status_ok         no problems were encountered
+ *     - err_status_bad_param  the profile is not supported 
+ * 
+ */
+err_status_t
+crypto_policy_set_from_profile_for_rtcp(crypto_policy_t *policy, 
+                                       srtp_profile_t profile);
+
+/**
+ * @brief returns the master key length for a given SRTP profile
+ */
+unsigned int
+srtp_profile_get_master_key_length(srtp_profile_t profile);
+
+
+/**
+ * @brief returns the master salt length for a given SRTP profile
+ */
+unsigned int
+srtp_profile_get_master_salt_length(srtp_profile_t profile);
+
+/**
+ * @brief appends the salt to the key
+ *
+ * The function call append_salt_to_key(k, klen, s, slen) 
+ * copies the string s to the location at klen bytes following
+ * the location k.  
+ *
+ * @warning There must be at least bytes_in_salt + bytes_in_key bytes
+ *          available at the location pointed to by key.
+ * 
+ */
+
+void
+append_salt_to_key(unsigned char *key, unsigned int bytes_in_key,
+                   unsigned char *salt, unsigned int bytes_in_salt);
+
+
+
+/**
+ * @}
+ */
+
+
+
+/**
+ * @defgroup SRTCP Secure RTCP
+ * @ingroup  SRTP 
+ *
+ * @brief Secure RTCP functions are used to protect RTCP traffic.
+ *
+ * RTCP is the control protocol for RTP.  libSRTP protects RTCP
+ * traffic in much the same way as it does RTP traffic.  The function
+ * srtp_protect_rtcp() applies cryptographic protections to outbound
+ * RTCP packets, and srtp_unprotect_rtcp() verifies the protections on
+ * inbound RTCP packets.  
+ *
+ * A note on the naming convention: srtp_protect_rtcp() has an srtp_t
+ * as its first argument, and thus has `srtp_' as its prefix.  The
+ * trailing `_rtcp' indicates the protocol on which it acts.  
+ * 
+ * @{
+ */
+
+/**
+ * @brief srtp_protect_rtcp() is the Secure RTCP sender-side packet
+ * processing function.
+ * 
+ * The function call srtp_protect_rtcp(ctx, rtp_hdr, len_ptr) applies
+ * SRTCP protection to the RTCP packet rtcp_hdr (which has length
+ * *len_ptr) using the SRTP session context ctx.  If err_status_ok is
+ * returned, then rtp_hdr points to the resulting SRTCP packet and
+ * *len_ptr is the number of octets in that packet; otherwise, no
+ * assumptions should be made about the value of either data elements.
+ * 
+ * @warning This function assumes that it can write the authentication
+ * tag into the location in memory immediately following the RTCP
+ * packet, and assumes that the RTCP packet is aligned on a 32-bit
+ * boundary.
+ *
+ * @param ctx is the SRTP context to use in processing the packet.
+ *
+ * @param rtcp_hdr is a pointer to the RTCP packet (before the call); after
+ * the function returns, it points to the srtp packet.
+ *
+ * @param pkt_octet_len is a pointer to the length in octets of the
+ * complete RTCP packet (header and body) before the function call,
+ * and of the complete SRTCP packet after the call, if err_status_ok
+ * was returned.  Otherwise, the value of the data to which it points
+ * is undefined.
+ *
+ * @return 
+ *    - err_status_ok            if there were no problems.
+ *    - [other]                  if there was a failure in 
+ *                               the cryptographic mechanisms.
+ */
+             
+
+err_status_t 
+srtp_protect_rtcp(srtp_t ctx, void *rtcp_hdr, int *pkt_octet_len);
+
+/**
+ * @brief srtp_unprotect_rtcp() is the Secure RTCP receiver-side packet
+ * processing function.
+ *
+ * The function call srtp_unprotect_rtcp(ctx, srtp_hdr, len_ptr)
+ * verifies the Secure RTCP protection of the SRTCP packet pointed to
+ * by srtcp_hdr (which has length *len_ptr), using the SRTP session
+ * context ctx.  If err_status_ok is returned, then srtcp_hdr points
+ * to the resulting RTCP packet and *len_ptr is the number of octets
+ * in that packet; otherwise, no assumptions should be made about the
+ * value of either data elements.
+ * 
+ * @warning This function assumes that the SRTCP packet is aligned on a
+ * 32-bit boundary.
+ *
+ * @param ctx is a pointer to the srtp_t which applies to the
+ * particular packet.
+ *
+ * @param srtcp_hdr is a pointer to the header of the SRTCP packet
+ * (before the call).  After the function returns, it points to the
+ * rtp packet if err_status_ok was returned; otherwise, the value of
+ * the data to which it points is undefined.
+ *
+ * @param pkt_octet_len is a pointer to the length in octets of the
+ * complete SRTCP packet (header and body) before the function call,
+ * and of the complete rtp packet after the call, if err_status_ok was
+ * returned.  Otherwise, the value of the data to which it points is
+ * undefined.
+ *
+ * @return 
+ *    - err_status_ok          if the RTCP packet is valid.
+ *    - err_status_auth_fail   if the SRTCP packet failed the message 
+ *                             authentication check.
+ *    - err_status_replay_fail if the SRTCP packet is a replay (e.g. has
+ *                             already been processed and accepted).
+ *    - [other]  if there has been an error in the cryptographic mechanisms.
+ *
+ */
+
+err_status_t 
+srtp_unprotect_rtcp(srtp_t ctx, void *srtcp_hdr, int *pkt_octet_len);
+
+/**
+ * @}
+ */
+
+/**
+ * @defgroup SRTPevents SRTP events and callbacks
+ * @ingroup  SRTP
+ *
+ * @brief libSRTP can use a user-provided callback function to 
+ * handle events.
+ *
+ * 
+ * libSRTP allows a user to provide a callback function to handle
+ * events that need to be dealt with outside of the data plane (see
+ * the enum srtp_event_t for a description of these events).  Dealing
+ * with these events is not a strict necessity; they are not
+ * security-critical, but the application may suffer if they are not
+ * handled.  The function srtp_set_event_handler() is used to provide
+ * the callback function.
+ *
+ * A default event handler that merely reports on the events as they
+ * happen is included.  It is also possible to set the event handler
+ * function to NULL, in which case all events will just be silently
+ * ignored.
+ *
+ * @{
+ */
+
+/**
+ * @brief srtp_event_t defines events that need to be handled
+ *
+ * The enum srtp_event_t defines events that need to be handled
+ * outside the `data plane', such as SSRC collisions and 
+ * key expirations.  
+ *
+ * When a key expires or the maximum number of packets has been
+ * reached, an SRTP stream will enter an `expired' state in which no
+ * more packets can be protected or unprotected.  When this happens,
+ * it is likely that you will want to either deallocate the stream
+ * (using srtp_stream_dealloc()), and possibly allocate a new one.
+ *
+ * When an SRTP stream expires, the other streams in the same session
+ * are unaffected, unless key sharing is used by that stream.  In the
+ * latter case, all of the streams in the session will expire.
+ */
+
+typedef enum { 
+  event_ssrc_collision,    /**<
+                            * An SSRC collision occured.             
+                            */
+  event_key_soft_limit,    /**< An SRTP stream reached the soft key
+                            *   usage limit and will expire soon.          
+                            */
+  event_key_hard_limit,    /**< An SRTP stream reached the hard 
+                            *   key usage limit and has expired.
+                            */
+  event_packet_index_limit /**< An SRTP stream reached the hard 
+                            * packet limit (2^48 packets).             
+                            */
+} srtp_event_t;
+
+/**
+ * @brief srtp_event_data_t is the structure passed as a callback to 
+ * the event handler function
+ *
+ * The struct srtp_event_data_t holds the data passed to the event
+ * handler function.  
+ */
+
+typedef struct srtp_event_data_t {
+  srtp_t        session;  /**< The session in which the event happend. */
+  srtp_stream_t stream;   /**< The stream in which the event happend.  */
+  srtp_event_t  event;    /**< An enum indicating the type of event.   */
+} srtp_event_data_t;
+
+/**
+ * @brief srtp_event_handler_func_t is the function prototype for
+ * the event handler.
+ *
+ * The typedef srtp_event_handler_func_t is the prototype for the
+ * event handler function.  It has as its only argument an
+ * srtp_event_data_t which describes the event that needs to be handled.
+ * There can only be a single, global handler for all events in
+ * libSRTP.
+ */
+
+typedef void (srtp_event_handler_func_t)(srtp_event_data_t *data);
+
+/**
+ * @brief sets the event handler to the function supplied by the caller.
+ * 
+ * The function call srtp_install_event_handler(func) sets the event
+ * handler function to the value func.  The value NULL is acceptable
+ * as an argument; in this case, events will be ignored rather than
+ * handled.
+ *
+ * @param func is a pointer to a fuction that takes an srtp_event_data_t
+ *             pointer as an argument and returns void.  This function
+ *             will be used by libSRTP to handle events.
+ */
+
+err_status_t
+srtp_install_event_handler(srtp_event_handler_func_t func);
+
+/**
+ * @}
+ */
+/* in host order, so outside the #if */
+#define SRTCP_E_BIT      0x80000000
+/* for byte-access */
+#define SRTCP_E_BYTE_BIT 0x80
+#define SRTCP_INDEX_MASK 0x7fffffff
+
+#ifdef __cplusplus
+}
+#endif
+
+#endif /* SRTP_H */