Re: [rtcweb] No Interim on SDES at this juncture

Dan Wing <> Thu, 20 June 2013 03:06 UTC

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From: Dan Wing <>
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Date: Wed, 19 Jun 2013 20:05:50 -0700
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Subject: Re: [rtcweb] No Interim on SDES at this juncture
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On Jun 18, 2013, at 12:02 PM, Matthew Kaufman (SKYPE) <> wrote:

> Back on April 25th, I made some claims. I still believe these are true:
> 1. DTLS-SRTP w/EKT is exactly as secure as SDES sent over HTTPS... it is just a different encrypted channel over which the key is set.

It is not exactly as secure.  SDES requires giving the SRTP key to the web server; DTLS-SRTP (with EKT) does not.

In your later emails, you combined the web server's functionality with gatewaying to an SDES-only IP PBX.  This complicates the analysis and makes a fruitful discussion impossible.

> 2. DTLS-SRTP w/EKT requires a more complex media gateway relationship for interworking (as it needs to be in-path for the keying on that side, despite the use of SDES on the other side).

Yes, because DTLS-SRTP is purposefully on the media path, which is what requires the media gateway be on path.

> 3. DTLS-SRTP w/EKT for interworking exposes the key via SDES on the other side of the interworking relationship anyway, so even though there isn't SDES to the browser there's SDES on the other (likely SIP) side.

Yes, I agree that wherever SDES is used, those weaknesses of SDES remain.

> 4. And DTLS-SRTP without EKT fails completely for the cases where the key needs to be set for interworking.

No, it doesn't completely fail.  It requires a decrypt/re-encrypt operation in one direction of the SRTP traffic (from the SDES side to the DTLS-SRTP side).  Perhaps that is what you mean by "fails completely", I don't know.

> 5. And finally, to get the browser-to-browser security guarantees you would need to be A) sure that you're really talking browser to browser and not via something else in path (like a mixer) and B) would really prefer that there be no way that the in-path device be able to force a key (thus you'd want to NOT allow EKT in the browser-to-browser case, even though there's no way for a browser to know for sure what it is talking to... I suppose that *if* you trusted your browser and the browser at the other end (which you need to do anyway, because they could just leak keys or media), you could mandate that browsers set some bit that disallows EKT and hope that the other end respects it)

You're saying identity is not desirable, because of audio mixers?


> Since we can't get interworking at all without either DTLS-SRTP + EKT or SDES, and since the security guarantees of DTLS-SRTP + EKT are the same as those of SDES, and since the interworking gateway is made more complex (because the keying must be in-path), I believe the only possible conclusions are A) We accept that interworking with other SRTP systems is desirable and therefore SDES is the best way to achieve that or B) We prevent any interworking with other RTP or SRTP systems.
> As someone who could make some money sending traffic to/from non-RTCWEB networks, I'm a fan of "A".
> And no, I'm not planning on writing a draft that says to just do what we should be doing anyway.
> Matthew Kaufman
> ________________________________________
> From: [] on behalf of Magnus Westerlund []
> Sent: Tuesday, June 18, 2013 2:56 AM
> To: Hadriel Kaplan
> Cc:
> Subject: Re: [rtcweb] No Interim on SDES at this juncture
> Hadirel and WG,
> Please see my response inline.
> On 2013-06-14 18:58, Hadriel Kaplan wrote:
>> You can't be serious. There was exactly ONE email asking for agenda
>> items, here:
>> It
>> was sent on May 30th.  It gave a generous 6 days to respond.  Luckily
>> no one ever goes on vacation for longer than 5 days. Instead, two
>> people sent a response on June 10th, a tremendous 11 days after the
>> request.  Outrageous!  That's a almost twice as long as they were
>> given!
>> Thank goodness the chairs detected this monstrous breach of
>> procedure, and thwarted the attempts of anarchy!  I mean if people
>> are allowed to respond to emails so tardily, how can we be expected
>> to get things accomplished as quickly as they've so far been in this
>> WG?!?
> Yes, we have only sent a single email this time, being the second round
> in a short time we tried to schedule this meeting.
>> Sure, an interim for this topic has been waiting for many months if
>> not a whole year, and now that people didn't respond in 6 days but
>> took instead 11 days the topic will be delayed indefinitely yet
>> again... but that's no excuse for blatantly flaunting the rules!
> Yes, there has been difficult finding a time could work for this
> meeting. That was why the agenda request time was short.
>> Personally I saw the email on May 30th, and assumed Oscar and Dan
>> would respond to you for agenda time.  I assumed that if no one had
>> submitted agenda items to you, that the WG Chairs would send out an
>> email warning about that, or perhaps even directly email the people
>> who they expected to submit agenda items.
> Yes, you assumed that someone would respond. Rather than you reaching
> out to verify that others would drive the question for you. Hadriel you
> are apparently very interested in this topic. Why don't you ensure that
> an agenda topic is on the agenda for Berlin? The WG chairs are happy to
> receive agenda requests already now.
>>> If you want to discuss this, write a draft describing how how your
>>> additional keying is to be integrated, what the pro and cons of it.
>>> That will enable direct discussion of a proposal. The WG clearly
>>> are opinionated on this matter, but apparently don't have energy to
>>> produce proposals.
>> There *are* drafts.
>> There
>> are even powerpoint slides, sent to the chairs the last time this
>> meeting almost happened but didn't.
> One of those drafts has been expired since February I would like to
> point out.
> The looking at these drafts, they are neither a proposal for what to
> actually do. Dan Wing's draft is argument in general why SDES would be
> bad for security. Oscar Ohlsson's is an argument for why a number of
> potential risks are not a serious issue and what the other gains of
> using security descriptions are.
> From my perspective I really would like to see some progress in at least
> the proposal for actually adding additional keying to ensure that the
> raised issues in drafts or earlier WG meetings and email discussions are
> meet. Additionally I would really like to see some more details for how
> the actual integration of an additional keying mechanism is supposed to
> work.
>> I think the problem must be that those things weren't signed in
>> triplicate, sent in, sent back, queried, lost, found, subjected to
>> public enquiry, lost again, and finally buried in soft peat for three
>> months and recycled as firelighters.
> No, that is not it. This topic has dragged on for various reasons. Yes
> we chairs are clearly to blame for some of them, like being slow to
> attempt to schedule a meeting. However, after that there has been issues
> finding a suitable time where sufficient mass of people could
> participate. There has been time conflicts with other meetings resulting
> in a cancellation, which in hind sight was unnecessary. Then a
> rescheduling, lurk warm response from the WG, limited agenda items and
> another almost collision resulting another cancellation.
> I am sorry that this makes you frustrated. Well, it makes also me
> frustrated. I wished this topic was dealt with and out of the way, but
> it is not. So, if you want it dealt with. Please request agenda time for
> Berlin and ensure to update any drafts or other material to actually
> take into account what actually has happened in the last 15 months.
> Regards
> Magnus Westerlund
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