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

Justin Uberti <juberti@google.com> Fri, 09 September 2011 18:52 UTC

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From: Justin Uberti <juberti@google.com>
Date: Fri, 9 Sep 2011 14:53:50 -0400
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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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On Fri, Sep 9, 2011 at 2:35 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.
>

We can of course expose the encryption state to the application so that it
can decide if it wants to proceed with/warn about the downgrade to plain old
RTP.