Secure RTP (SRTP) Media Transport
[Media Transport]

---

Detailed Description

This module implements SRTP as described by RFC 3711, using RFC 4568 as key exchange method. It implements Media Transport to integrate with the rest of PJMEDIA framework.

As we know, media transport is separated from the stream object (which does the encoding/decoding of PCM frames, (de)packetization of RTP/RTCP packets, and de-jitter buffering). The connection between stream and media transport is established when the stream is created (we need to specify media transport during stream creation), and the interconnection can be depicted from the diagram below:

I think the diagram above is self-explanatory.

SRTP functionality is implemented as some kind of "adapter", which is plugged between the stream and the actual media transport that does sending/receiving RTP/RTCP packets. When SRTP is used, the interconnection between stream and transport is like the diagram below:

So to stream, the SRTP transport behaves as if it is a media transport (because it is a media transport), and to the media transport it behaves as if it is a stream. The SRTP object then forwards RTP packets back and forth between stream and the actual transport, encrypting/decrypting the RTP/RTCP packets as necessary.

The neat thing about this design is the SRTP "adapter" then can be used to encrypt any kind of media transports. We currently have UDP and ICE media transports that can benefit SRTP, and we could add SRTP to any media transports that will be added in the future.

Data Structures
struct	pjmedia_srtp_crypto
struct	pjmedia_srtp_setting
struct	pjmedia_srtp_info
Enumerations
enum	pjmedia_srtp_crypto_option {   PJMEDIA_SRTP_NO_ENCRYPTION = 1,   PJMEDIA_SRTP_NO_AUTHENTICATION = 2 }
enum	pjmedia_srtp_use {   PJMEDIA_SRTP_DISABLED,   PJMEDIA_SRTP_OPTIONAL,   PJMEDIA_SRTP_MANDATORY }
Functions
void	pjmedia_srtp_setting_default (pjmedia_srtp_setting *opt)
pj_status_t	pjmedia_transport_srtp_create (pjmedia_endpt *endpt, pjmedia_transport *tp, const pjmedia_srtp_setting *opt, pjmedia_transport **p_tp)
pj_status_t	pjmedia_transport_srtp_start (pjmedia_transport *srtp, const pjmedia_srtp_crypto *tx, const pjmedia_srtp_crypto *rx)
pj_status_t	pjmedia_transport_srtp_stop (pjmedia_transport *srtp)
pj_status_t	pjmedia_transport_srtp_decrypt_pkt (pjmedia_transport *tp, pj_bool_t is_rtp, void *pkt, int *pkt_len)
pjmedia_transport *	pjmedia_transport_srtp_get_member (pjmedia_transport *srtp)

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Enumeration Type Documentation

enum pjmedia_srtp_crypto_option

Crypto option.

Enumerator:

PJMEDIA_SRTP_NO_ENCRYPTION	When this flag is specified, encryption will be disabled.
PJMEDIA_SRTP_NO_AUTHENTICATION	When this flag is specified, authentication will be disabled.

enum pjmedia_srtp_use

This enumeration specifies the behavior of the SRTP transport regarding media security offer and answer.

Enumerator:

PJMEDIA_SRTP_DISABLED	When this flag is specified, SRTP will be disabled, and the transport will reject RTP/SAVP offer.
PJMEDIA_SRTP_OPTIONAL	When this flag is specified, SRTP will be advertised as optional and incoming SRTP offer will be accepted.
PJMEDIA_SRTP_MANDATORY	When this flag is specified, the transport will require that RTP/SAVP media shall be used.

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Function Documentation

void pjmedia_srtp_setting_default

(

pjmedia_srtp_setting *

opt

)

Initialize SRTP setting with its default values.

Parameters:

opt

SRTP setting to be initialized.

pj_status_t pjmedia_transport_srtp_create	(	pjmedia_endpt *	endpt,
		pjmedia_transport *	tp,
		const pjmedia_srtp_setting *	opt,
		pjmedia_transport **	p_tp
	)

Create an SRTP media transport.

Parameters:

	endpt	The media endpoint instance.
	tp	The actual media transport to send and receive RTP/RTCP packets. This media transport will be kept as member transport of this SRTP instance.
	opt	Optional settings. If NULL is given, default settings will be used.
	p_tp	Pointer to receive the transport SRTP instance.

Returns:

PJ_SUCCESS on success.

pj_status_t pjmedia_transport_srtp_start	(	pjmedia_transport *	srtp,
		const pjmedia_srtp_crypto *	tx,
		const pjmedia_srtp_crypto *	rx
	)

Manually start SRTP session with the given parameters. Application only needs to call this function when the SRTP transport is used without SDP offer/answer. When SDP offer/answer framework is used, the SRTP transport will be started/stopped by pjmedia_transport_media_start() and pjmedia_transport_media_stop() respectively.

Please note that even if an RTP stream is only one direction, application will still need to provide both crypto suites, because it is needed by RTCP.

If application specifies the crypto keys, the keys for transmit and receive direction MUST be different.

Parameters:

	srtp	The SRTP transport.
	tx	Crypto suite setting for transmit direction.
	rx	Crypto suite setting for receive direction.

Returns:

PJ_SUCCESS on success.

pj_status_t pjmedia_transport_srtp_stop

(

pjmedia_transport *

srtp

)

Stop SRTP session.

Parameters:

srtp

The SRTP media transport.

Returns:

PJ_SUCCESS on success.

See also:

pjmedia_transport_srtp_start()

pj_status_t pjmedia_transport_srtp_decrypt_pkt	(	pjmedia_transport *	tp,
		pj_bool_t	is_rtp,
		void *	pkt,
		int *	pkt_len
	)

This is a utility function to decrypt SRTP packet using SRTP transport. This function is not part of SRTP transport's API, but it can be used to decrypt SRTP packets from non-network (for example, from a saved file) without having to use the transport framework. See pcaputil.c in the samples collection on how to use this function.

Parameters:

	tp	The SRTP transport.
	is_rtp	Set to non-zero if the packet is SRTP, otherwise set to zero if the packet is SRTCP.
	pkt	On input, it contains SRTP or SRTCP packet. On output, it contains the decrypted RTP/RTCP packet.
	pkt_len	On input, specify the length of the buffer. On output, it will be filled with the actual length of decrypted packet.

Returns:

PJ_SUCCESS on success.

pjmedia_transport* pjmedia_transport_srtp_get_member

(

pjmedia_transport *

srtp

)

Query member transport of SRTP.

Parameters:

srtp

The SRTP media transport.

Returns:

member media transport.