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Secure RTP (SRTP) Media Transport

Media transport adapter to add SRTP feature to existing transports. More...

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

pj_status_t pjmedia_srtp_init_lib (pjmedia_endpt *endpt)
 
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_transportpjmedia_transport_srtp_get_member (pjmedia_transport *srtp)
 

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:

media-transport.PNG

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:

media-srtp-transport.PNG

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.

Enumeration Type Documentation

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.

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.

Function Documentation

pj_status_t pjmedia_srtp_init_lib ( pjmedia_endpt endpt)

Initialize SRTP library. This function should be called before any SRTP functions, however calling pjmedia_transport_srtp_create() will also invoke this function. This function will also register SRTP library deinitialization to pj_atexit(), so the deinitialization of SRTP library will be performed automatically by PJLIB destructor.

Parameters
endptThe media endpoint instance.
Returns
PJ_SUCCESS on success.
void pjmedia_srtp_setting_default ( pjmedia_srtp_setting opt)

Initialize SRTP setting with its default values.

Parameters
optSRTP 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
endptThe media endpoint instance.
tpThe actual media transport to send and receive RTP/RTCP packets. This media transport will be kept as member transport of this SRTP instance.
optOptional settings. If NULL is given, default settings will be used.
p_tpPointer 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
srtpThe SRTP transport.
txCrypto suite setting for transmit direction.
rxCrypto suite setting for receive direction.
Returns
PJ_SUCCESS on success.
pj_status_t pjmedia_transport_srtp_stop ( pjmedia_transport srtp)

Stop SRTP session.

Parameters
srtpThe 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
tpThe SRTP transport.
is_rtpSet to non-zero if the packet is SRTP, otherwise set to zero if the packet is SRTCP.
pktOn input, it contains SRTP or SRTCP packet. On output, it contains the decrypted RTP/RTCP packet.
pkt_lenOn 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
srtpThe SRTP media transport.
Returns
member media transport.

 


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