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

Alan Johnston <alan.b.johnston@gmail.com> Fri, 09 September 2011 19:22 UTC

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Date: Fri, 9 Sep 2011 14:23:31 -0500
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From: Alan Johnston <alan.b.johnston@gmail.com>
To: Eric Rescorla <ekr@rtfm.com>
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Cc: Randell Jesup <randell-ietf@jesup.org>, Jonathan Lennox <jonathan@vidyo.com>, "rtcweb@ietf.org" <rtcweb@ietf.org>
Subject: Re: [rtcweb] AVPF [was: Encryption mandate (and offer/answer)]
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Ekr is correct.  If we allow RTP, which I think is a mistake, then
there is always a downgrade attack.

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.

- Alan -

On Fri, Sep 9, 2011 at 1:35 PM, Eric Rescorla <ekr@rtfm.com>; wrote:
> On Fri, Sep 9, 2011 at 11:11 AM, Matthew Kaufman
> <matthew.kaufman@skype.net>; wrote:
>> On 9/9/11 10:47 AM, Alan Johnston wrote:
>>>
>>>   The default will be SRTP - this can be
>>> expressed in SDP without CapNeg.  Should the RTCWEB clients choose to
>>> instead negotiate RTP, then this could be done with a second SDP
>>> Offer/Answer exchange.
>>
>> I believe you've just designed a downgrade vulnerability.
>
> 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
>