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

[Alan Johnston <[e-mail]>]

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 <[e-mail]> wrote:
> On Fri, Sep 9, 2011 at 11:11 AM, Matthew Kaufman
> <[e-mail]> 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
>