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

Justin Uberti <juberti@google.com> Fri, 09 September 2011 21:27 UTC

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From: Justin Uberti <juberti@google.com>
Date: Fri, 9 Sep 2011 17:29:22 -0400
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To: Randell Jesup <randell-ietf@jesup.org>
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Subject: Re: [rtcweb] AVPF [was: Encryption mandate (and offer/answer)]
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On Fri, Sep 9, 2011 at 5:15 PM, Randell Jesup <randell-ietf@jesup.org>wrote;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.
> 2) Debug/etc tools work better with RTP.  Not important.
> 3) May simplify/improve some E911 cases.  Might be important; likely not.


In Quebec City, the mic discussion also noted a use case for enterprises
that want to log communications.

Regarding #1, the ability to avoid media gateways for "contemporary devices"
has been a design requirement from the start. I think we need to have a
clear understanding that this requirement creates serious, unavoidable
problems before jettisoning it.




> So, effectively it comes down to "is advantage 1 worth the
> complexity/risk?"  Anyone want to defend that
> case?
>
>  - Alan -
>>
>> On Fri, Sep 9, 2011 at 1:35 PM, Eric Rescorla<ekr@rtfm.com>  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
>>>
>>>
> --
> Randell Jesup
> randell-ietf@jesup.org
>
>
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