Re: [AVTCORE] RTP Header Extension Encryption

Paul Kyzivat <pkyzivat@alum.mit.edu> Fri, 11 September 2020 18:31 UTC

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From: Paul Kyzivat <pkyzivat@alum.mit.edu>
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Subject: Re: [AVTCORE] RTP Header Extension Encryption
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Bernard,

On 9/10/20 5:42 PM, Bernard Aboba wrote:

> There was also some discussion of whether encryption could be negotiated 
> per m-line or just a blanket on/off for all m-lines.  IMHO, negotiating 
> encryption per m-line is more complex, particularly if we also choose to 
> extend the scope of encryption, so as to cover the ID field (e.g. 
> encrypt the entire RTP header extension block).  Extending the scope of 
> encryption means that the entire MID header extension (including the ID 
> field) could be encrypted.
> 
> Having encryption on for some m-lines and off for other m-lines seems 
> like it would open up a number of corner cases.  If some MIDs have RTP 
> header extensions encrypted and others do not, how does an RTP receiver 
> know whether a particular RTP packet it receives has RTP header 
> extensions encrypted or not?
> 
> To determine this, the receiver needs to determine the MID value, but 
> for some packets the MID header extension is encrypted, and for other 
> RTP packets it isn't. The implementer might have to do some error-prone 
> and potentially non-interoperable gymnastics, like using heuristics to 
> guess whether the RTP header extension block is unencrypted or 
> encrypted, or attempting to decrypt the RTP header extension block on 
> all received RTP packets, then checking for a MID header extension to 
> confirm that yes, the RTP header extension block should have been 
> encrypted.
> 
> This complexity can be avoided if RTP header extension encryption is 
> either on or off for all MIDs. It is hard to come up with a use case in 
> which you'd only want some m-lines to have RTP header extension 
> encryption on and you'd want other m-lines to have RTP header extensions 
> sent in the clear. So the added complexity doesn't seem to have a 
> corresponding benefit.

Can you please clarify the scope for which you want the encryption to be 
consistent? Above you variously mention all MIDs and all m-lines. I'm 
concerned with what "all" applies to.

I think I can agree if you are talking about "all within a bundle 
group". Anything broader has major problems.

	Thanks,
	Paul