[Bug 703264] srtp: Key is required even if cipher and auth are null

[GStreamer (bugzilla.gnome.org)]

https://bugzilla.gnome.org/show_bug.cgi?id=703264
  GStreamer | gst-plugins-bad | 1.x

Morgan Lindqvist <Morgan.Lindqvist> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
             Status|RESOLVED                    |REOPENED
         Resolution|FIXED                       |

--- Comment #3 from Morgan Lindqvist <Morgan.Lindqvist at ericsson.com> 2013-08-06 14:03:58 UTC ---
This fix did not solve the whole problem.

The request was to be able to disable the SRTP element if the remote side did
not support SRTP (without need for modifying the pipeline).

The above solution only makes it possible to disable encryption and
authentication for RTP and encryption for RTCP without having to provide a key.
The authentication is still there for RTCP. Since the SRTP RTCP receiver
requires the authentication information the current solution drops all incoming
RTCP reports.

The logic in the srtp library that correctly drops the RTCP messages can be
found here.
https://github.com/cisco/libsrtp/blob/master/srtp/srtp.c#L1895

The logic that always set the header can be found here:
https://github.com/cisco/libsrtp/blob/master/srtp/srtp.c#L1707

The reason for this behaviour is that the specification requires that RTCP
always is authenticated when SRTP is used.

One way of solving this would be to enable pass-through if all encryption and
authentication is disabled. This would also avoid copying of buffers.

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