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

Eric Rescorla <ekr@rtfm.com> Fri, 09 September 2011 18:33 UTC

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From: Eric Rescorla <ekr@rtfm.com>
Date: Fri, 9 Sep 2011 11:35:16 -0700
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To: Matthew Kaufman <matthew.kaufman@skype.net>
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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 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