Group PJMEDIA_TRANSPORT_SRTP

group PJMEDIA_TRANSPORT_SRTP

Media transport adapter to add SRTP feature to existing transports.

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:

pjproject/pjmedia/docs/xml/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:

pjproject/pjmedia/docs/xml/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.

Enums

enum pjmedia_srtp_crypto_option

Crypto option.

Values:

PJMEDIA_SRTP_NO_ENCRYPTION = 1

When this flag is specified, encryption will be disabled.

PJMEDIA_SRTP_NO_AUTHENTICATION = 2

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.

Values:

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.

enum pjmedia_srtp_keying_method

This enumeration specifies SRTP keying methods.

Values:

PJMEDIA_SRTP_KEYING_SDES

Session Description (SDES).

PJMEDIA_SRTP_KEYING_DTLS_SRTP

DTLS-SRTP.

PJMEDIA_SRTP_KEYINGS_COUNT

Number of keying method.

Functions

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.

Return

PJ_SUCCESS on success.

Parameters
  • endpt: The media endpoint instance.

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_srtp_enum_crypto(unsigned *count, pjmedia_srtp_crypto crypto[])

Enumerate available SRTP crypto name.

Return

PJ_SUCCESS on success.

Parameters
  • count: On input, specifies the maximum length of crypto array. On output, the number of available crypto initialized by this function.

  • crypto: The SRTP crypto array output.

pj_status_t pjmedia_srtp_enum_keying(unsigned *count, pjmedia_srtp_keying_method keying[])

Enumerate available SRTP keying methods.

Return

PJ_SUCCESS on success.

Parameters
  • count: On input, specifies the maximum length of keying method array. On output, the number of available keying method initialized by this function.

  • crypto: The SRTP keying method array output.

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.

Return

PJ_SUCCESS on success.

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.

pj_status_t pjmedia_transport_srtp_dtls_get_fingerprint(pjmedia_transport *srtp, const char *hash, char *buf, pj_size_t *len)

Get fingerprint of local DTLS-SRTP certificate.

Return

PJ_SUCCESS on success.

Parameters
  • srtp: The SRTP transport.

  • hash: Fingerprint hash algorithm, currently valid values are “SHA-256” and “SHA-1”.

  • buf: Buffer for fingerprint output. The output will be formatted as “SHA-256/1 XX:XX:XX…” and null terminated.

  • len: On input, the size of the buffer. On output, the length of the fingerprint.

pj_status_t pjmedia_transport_srtp_dtls_start_nego(pjmedia_transport *srtp, const pjmedia_srtp_dtls_nego_param *param)

Manually start DTLS-SRTP negotiation 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 DTLS-SRTP negotiation will be handled by pjmedia_transport_media_create(), pjmedia_transport_media_start(), pjmedia_transport_media_encode_sdp(), and pjmedia_transport_media_stop().

When the negotiation completes, application will be notified via SRTP callback on_srtp_nego_complete(), if set. If the negotiation is successful, SRTP will be automatically started.

Note that if the SRTP member transport is an ICE transport, application should only call this function after ICE negotiation is completed successfully.

Return

PJ_SUCCESS on success.

Parameters
  • srtp: The SRTP transport.

  • param: DTLS-SRTP nego parameter.

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.

Return

PJ_SUCCESS on success.

Parameters
  • srtp: The SRTP transport.

  • tx: Crypto suite setting for transmit direction.

  • rx: Crypto suite setting for receive direction.

pj_status_t pjmedia_transport_srtp_stop(pjmedia_transport *srtp)

Stop SRTP session.

Return

PJ_SUCCESS on success.

See

pjmedia_transport_srtp_start()

Parameters
  • srtp: The SRTP media transport.

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.

Return

PJ_SUCCESS on success.

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.

pjmedia_transport *pjmedia_transport_srtp_get_member(pjmedia_transport *srtp)

Query member transport of SRTP.

Return

member media transport.

Parameters
  • srtp: The SRTP media transport.

struct pjmedia_srtp_crypto
#include <transport_srtp.h>

This structure describes an individual crypto setting.

struct pjmedia_srtp_cb
#include <transport_srtp.h>

Structure containing callbacks to receive SRTP notifications.

struct pjmedia_srtp_setting
#include <transport_srtp.h>

Settings to be given when creating SRTP transport. Application should call pjmedia_srtp_setting_default() to initialize this structure with its default values.

struct pjmedia_srtp_info
#include <transport_srtp.h>

This structure specifies SRTP transport specific info. This will fit into buffer field of pjmedia_transport_specific_info.

struct pjmedia_srtp_dtls_nego_param
#include <transport_srtp.h>

This structure specifies DTLS-SRTP negotiation parameters.