SRTPCryptoContext#checkReplay may failed when resume from a paused secure video call

[cmeng-git]

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Description

1. start a secured video call e.g. ZRTP
2. Pause the secured video call
3. Resume the secured video call
4. You may need to repeat steps 2 and 3 to see the problem.

Current behavior

Pending on when the video call is resumed, the checkReplay() may return false because the seqNo is outsider the replay window. This causes video streaming to failed on both sending and receiving ends.
It seems that the video frame is continuously being acquired while video call is paused. However the acquired frames are not relayed to the SRTPCryptoContext for processing. Therefore the packet seqNo continuous to be incremented, without the guessIndex being updated.

Expected Behavior

The secured video call should not failed on resume. It has been verified that if a normal call (no ZRTP) is paused and resumed, it always resume without problem.

Possible Solution

The video streaming acquiring process must also be stopped while call is paused, so the seqNo is not changed until it is resumes.

Steps to reproduce

See Description

Environment details

Secured video call on Jitsi

[GNUDimarik]

I investigated this issue in some fork of jitsi and seems I remember the root cause. When you set call to pause library destroys contexsts and next checkReplay formula works like you started call again and returns wrong values ..
In one day I'll have time for fixing this ...

[ibauersachs]

Are you sure the call context gets destroyed? What I remember is slightly different: Jitsi doesn't send a muted (i.e. just black) video stream if a call is on hold, thus the context never receives data to increment the counters.
This is AFAIK different for audio, where silence is actually transmitted.

[GNUDimarik]

Will check it later. After fixing some issues in jitsi desktop. Honestly don't remember exactly