Re: [rtcweb] AVPF [was: Encryption mandate (and offer/answer)]

Dzonatas Sol <> Fri, 09 September 2011 21:58 UTC

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Date: Fri, 09 Sep 2011 15:02:29 -0700
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Subject: Re: [rtcweb] AVPF [was: Encryption mandate (and offer/answer)]
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On 09/09/2011 02:15 PM, Randell Jesup wrote:
> On 9/9/2011 3:23 PM, Alan Johnston wrote:
>> Ekr is correct.  If we allow RTP, which I think is a mistake, then
>> there is always a downgrade attack.
> Yes, that's true.  The same issue was involved in the best-effort-srtp 
> draft, which unfortunately
> was dropped because CapNeg would "solve" it.  (For historical note, 
> it's still not "solved"
> because CapNeg support is >>>> more complex than best-effort-srtp and 
> not generally deployed,
> and I doubt ever will be ala SDPng (though I'm not close to status on 
> CapNeg.)
> Hmmm.  A real downgrade attack requires that the signalling be 
> compromised.  I wonder if there
> are characteristics of a webrtc transaction that could help avoid this 
> sort of attack (for example,
> a secondary way out-of-scope here for the app to know ahead of time if 
> the target will need to
> be downgraded).  Or some way for the service to vouch for the 
> downgrade (i.e. wasn't a MITM).
> You have to trust the service, but in this case you're doing so to 
> this degree anyways.
>> My point was that if we must support insecure media, we could avoid
>> the complexity of CapNeg by not requiring a single pass non-secure
>> media negotiation.
> There is another option.  I talked about services that wanted to 
> support PSTN  could decide if they
> were willing to support a downgrade.  The application could know it's 
> calling a PSTN gateway and
> if it does know that, avoid a media gateway by not offering encrypted 
> media.
> I see a significant use-case for some services will be calling PSTN 
> numbers and services, much
> as it is now for VoIP.
> Yes, a bunch of new non-legacy services wouldn't use/want it.  But the 
> app for a PSTN-using service
> could specifically allow it.
> So the question comes down to what's the advantage to using 
> unencrypted RTP?
> 1) No media gateway needed.  This is the big one.  Saves on $$$, saves 
> on delay (sometimes a lot),
>     may save on complexity in a PBX type of situation.
>     But is there an issue due to ICE requirements?  If those can't be 
> turned off safely too, that kills this
>     whole discussion I think.

The ICE toggle already exists. Perhaps you meant non-media in the 
insecure audio-only state? Also known as high-fidelity.

> 2) Debug/etc tools work better with RTP.  Not important.
> 3) May simplify/improve some E911 cases.  Might be important; likely not.
> So, effectively it comes down to "is advantage 1 worth the 
> complexity/risk?"  Anyone want to defend that
> case?

The virtual "stateless" driver supposedly doesn't exist unless you want 
to pass DAE only for CapNeg and rely on object recognition and painful 
convex optimizations.

Again, shape is reliable, not a risk. Do you let the federated-servers 
know these traffic-shapes?

>> - Alan -
>> On Fri, Sep 9, 2011 at 1:35 PM, Eric Rescorla<>  wrote:
>>> Unless I'm missing something, if you (a) support an insecure mode 
>>> and (b) allow
>>> negotiation of insecure vs. secure, there's not really any way to
>>> avoid a downgrade
>>> issue; the attacker can always pretend not to support security and 
>>> how do you
>>> know better? Obviously, it helps if you can negotiate the use or 
>>> non-use of
>>> media security over a secure-ish signaling channel, but that doesn't 
>>> reduce
>>> the threat from the signaling service.
>>> Best,
>>> -Ekr

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