srtpdec: GStreamer Bad Plugins 1.0 Plugins Reference Manual

## srtpdec srtpdec |

## Properties

guint | replay-window-size | Read / Write
GstStructure * | stats | Read

## Signals

void | clear-keys | Action
GstCaps* | hard-limit | Run Last
GstCaps* | request-key | Run Last
GstCaps* | soft-limit | Run Last
void | remove-key | Action

## Types and Values

struct | GstSrtpDec

## Object Hierarchy

        GObject
        ╰── GInitiallyUnowned
            ╰── GstObject
                ╰── GstElement
                    ╰── GstSrtpDec

## Description

gstrtpdec acts as a decoder that removes security from SRTP and SRTCP packets (encryption and authentication) and out RTP and RTCP. It receives packet of type 'application/x-srtp' or 'application/x-srtcp' on its sink pad, and outs packets of type 'application/x-rtp' or 'application/x-rtcp' on its source pad.

For each packet received, it checks if the internal SSRC is in the list of streams already in use. If this is not the case, it sends a signal to the user to get the needed parameters to create a new stream : master key, encryption and authentication mecanisms for both RTP and RTCP. If the user can't provide those parameters, the buffer is dropped and a warning is emitted.

This element uses libsrtp library. The encryption and authentication mecanisms available are :

Encryption

- AES_ICM 256 bits (maximum security)

- AES_ICM 128 bits (default)

- NULL

Authentication

- HMAC_SHA1 80 bits (default, maximum protection)

- HMAC_SHA1 32 bits

- NULL

Note that for SRTP protection, authentication is mandatory (non-null) if encryption is used (non-null).

Each packet received is first analysed (checked for valid SSRC) then its buffer is unprotected with libsrtp, then pushed on the source pad. If protection failed or the stream could not be created, the buffer is dropped and a warning is emitted.

When the maximum usage of the master key is reached, a soft-limit signal is sent to the user, and new parameters (master key) are needed in return. If the hard limit is reached, a flag is set and every subsequent packet is dropped, until a new key is set and the stream has been updated.

If a stream is to be shared between multiple clients the SRTP rollover counter for a given SSRC must be set in the caps "roc" field when the request-key signal is emitted by the decoder. The rollover counters should have been transmitted by a signaling protocol by some other means. If no rollover counter is provided by the user, 0 is used by default.

#### Example pipelines

    1
    gst-launch-1.0 udpsrc port=5004 caps='application/x-srtp, payload=(int)8, ssrc=(uint)1356955624, srtp-key=(buffer)012345678901234567890123456789012345678901234567890123456789, srtp-cipher=(string)aes-128-icm, srtp-auth=(string)hmac-sha1-80, srtcp-cipher=(string)aes-128-icm, srtcp-auth=(string)hmac-sha1-80, roc=(uint)0' !  srtpdec ! rtppcmadepay ! alawdec ! pulsesink

Receive PCMA SRTP packets through UDP using caps to specify master key and protection.

    1
    gst-launch-1.0 audiotestsrc ! alawenc ! rtppcmapay ! 'application/x-rtp, payload=(int)8, ssrc=(uint)1356955624' ! srtpenc key="012345678901234567890123456789012345678901234567890123456789" ! udpsink port=5004

Send PCMA SRTP packets through UDP, nothing how the SSRC is forced so that the receiver will recognize it.

## Synopsis

### Element Information

plugin | srtp
author | Gabriel Millaire <millaire.gabriel@collabora.com>
class | Filter/Network/SRTP

### Element Pads

name | rtcp_sink
direction | sink
presence | always
details | application/x-srtcp

name | rtp_sink
direction | sink
presence | always
details | application/x-srtp

name | rtcp_src
direction | source
presence | always
details | application/x-rtcp

name | rtp_src
direction | source
presence | always
details | application/x-rtp

## Functions

## Types and Values

### struct GstSrtpDec

    struct GstSrtpDec;

## Property Details

### The `“replay-window-size”` property

      “replay-window-size”       guint

Size of the replay protection window.

Flags: Read / Write

Allowed values: [64,32768]

Default value: 128

### The `“stats”` property

      “stats”                    GstStructure *

Various statistics.

Flags: Read

## Signal Details

### The `“clear-keys”` signal

    void
    user_function (GstSrtpDec *gstsrtpdec,
                   gpointer    user_data)

Clear the internal list of streams

#### Parameters

gstsrtpdec | the element on which the signal is emitted |
user_data | user data set when the signal handler was connected. |

Flags: Action

### The `“hard-limit”` signal

    GstCaps*
    user_function (GstSrtpDec *gstsrtpdec,
                   guint       ssrc,
                   gpointer    user_data)

Signal emited when the stream with `ssrc` has reached the hard limit of utilisation of it's master encryption key. User should provide a new key and new RTP and RTCP encryption ciphers and authentication, and return them wrapped in a GstCaps. If user could not provide those parameters or signal is not answered, the buffers of this stream will be dropped.

#### Parameters

gstsrtpdec | the element on which the signal is emitted |
ssrc | The unique SSRC of the stream |
user_data | user data set when the signal handler was connected. |

Flags: Run Last

### The `“request-key”` signal

    GstCaps*
    user_function (GstSrtpDec *gstsrtpdec,
                   guint       ssrc,
                   gpointer    user_data)

Signal emited to get the parameters relevant to stream with `ssrc` . User should provide the key and the RTP and RTCP encryption ciphers and authentication, and return them wrapped in a GstCaps.

#### Parameters

gstsrtpdec | the element on which the signal is emitted |
ssrc | The unique SSRC of the stream |
user_data | user data set when the signal handler was connected. |

Flags: Run Last

### The `“soft-limit”` signal

    GstCaps*
    user_function (GstSrtpDec *gstsrtpdec,
                   guint       ssrc,
                   gpointer    user_data)

Signal emited when the stream with `ssrc` has reached the soft limit of utilisation of it's master encryption key. User should provide a new key and new RTP and RTCP encryption ciphers and authentication, and return them wrapped in a GstCaps.

#### Parameters

gstsrtpdec | the element on which the signal is emitted |
ssrc | The unique SSRC of the stream |
user_data | user data set when the signal handler was connected. |

Flags: Run Last

### The `“remove-key”` signal

    void
    user_function (GstSrtpDec *gstsrtpdec,
                   guint       ssrc,
                   gpointer    user_data)

Removes keys for a specific SSRC

#### Parameters

gstsrtpdec | the element on which the signal is emitted |
ssrc | The SSRC for which to remove the key. |
user_data | user data set when the signal handler was connected. |

Flags: Action

## See Also

srtpenc