| Hi Emil,
Thank you for the input on the framework and the double documents from everyone. The points raised by the individuals here are not new and have been discussed before by the WG at several occasions. And for these issues there has be no WG consensus. Specifically on the points regarding double encryption: At IETF 95 double had consensus and got adopted (after 4 design team meetings and 3 IETF meetings). At IETF 96 the WG chairs re-opened the discussions around SSRC mutability and Emil got asked to submit a draft on the security impact of SSRC mutability At IETF 98 SSRC immutability and RTX considerations were discussed but no proposal made on security implications
This is really quite misleading: We did indeed have a discussion on this was on IETF 96 (Berlin) where I was asked to describe how this works in a draft, which we did: https://tools.ietf.org/html/draft-grozev-perc-ssrc-00We then talked about it in Chicago (IETF 98) . No one implied security issues any more. This never was a problem.
I wasn’t a WG chair at the time but quoting from the IETF 96 meeting minutes section about “SSRC Immutability Discussion":
"- Emil asked to submit draft explaining the impact of ssrc mutability on security/ssrc collision/end-point procedures in the perc context.”
And the security considerations section of the above draft is empty. No decision was reached and the topic was left open but it had nothing to do with the security implications of either approach and the lack of consensus was primarily driven by implementation convenience.
While implementation convenience was part of the discussion it was raised a few times that the people in favor of allowing SSRC mutability never provided any written description of why mutating the SSRC is not a problem as pointed out by the design team.
Best Nils The discussion then morphed into RTX/FEC handling for which we've also made submissions.
Eventually, at some point the chairs decided to declare consensus where there wasn't any and the WG just ploughed through. That's where I personally disconnected and I have been waiting for last call to voice these concerns ever since.
The entire WG mode of operation has been an egregious violation of the main principles that the IETF stands for.
Regards, Emil At IETF 99 the double authors and Sergio presented a joint proposal. The WG chairs called for consensus in the room and on the list and concluded that with rough consensus, the proposal got adopted. Best regards Nils & Suhas PERC WG chairs
PERC may be a valid solution for some scenarios, maybe SIP. But in regards of WebRTC, my personal opinion is that it is not well suited and that we should do a fresh start, with an analysis of the requirements and specifics of webrtc, define trust models, role of the js apps, UI/UX, IdP and isolated media streams, key management within browser, compatibility with webrtc 1.0, if we need to support it in SDP or not, QUIC, WASM, etc.. and then check if PERC is able to fulfill them or what parts can be reused, if any. Best regards Sergio
On 02/02/2019 21:42, Bernard Aboba wrote:
Sergio -
In your opinion, what portions of PERC are salvageable, if any? Is this a situation where we need to start over or could some aspect of PERC (e.g. Double if the triple encryption problem were fixed) be suitably modified and then implemented? I think Emil and Bernard have described quite precisely where we are and how we managed to get here.
In my opinion it would be a big mistake to consider PERC as *THE* solution for end to end encryption for multiconferencing, as if there was a one size fits all solution for the problem.
Speaking from a WebRTC perspective, PERC, apart of have taken some controversial technical decisions (OHB as header, the ssrc rewriting issue and reverse the the order of FEC/RTX and SRTP), does not take into consideration the specifics of WebRTC (it could be argued that that was not in the scope of this group), like the role of the js app, the possibility of allowing key management in js, or the interaction with Idp and isolated media streams. Not to speak about the recent discussions about full frame vs per packet encryption or QUIC.
Best regards Sergio
On 02/02/2019 18:42, Emil Ivov wrote:
Emil said:
"The need to do a triple encryption for example is a particularly egregious consequence of the order problem. That’s a problem specific to the “double” documents."
[BA] Can you describe how the need for "triple encryption" arises? The framework document doesn't even mention the issues with ordering of FEC/RTX/RED and encryption, let alone the need for "triple encryption".
One of the goals that some members of the group seemed to have was to allow specific applications to become PERC-compliant without changing the app code itself and by simply replacing the libsrtp library with a PERC-enabled one.
I don’t know that this goal is a direct consequence of the framework’s conceptual approach (contrary to the imposition of key distribution and negotiation). I think it simply carries a promise for some minimal pragmatic value to some implementers.
The issue with this approach is that it leaves hop-by-hop protection mechanisms such FEC and RTC unavailable to the SFU as they are usually performed before SRTP, which would make them e2e encrypted.
The solution to that is simple. One merely needs to perform e2e encryption first, then apply FEC and/or RTX and only then apply the second (hop-by-hop) layer of SRTP.
This approach was referred to as “wedging RTX and FEC” as it places them in between the two encryption operations.
While wedging appeared to have overall support in hallway discussions by all SFU implementors except potentially one, it was mysteriously rejected by a subset of the WG and replaced with the following:
Applications will apply SRTP-double first and, those that need to use FEC and RTX would have to apply them only after.
It was quickly pointed out that this not only destroys the stated “don’t-change-the-app” goal, but also leaves RTX and mostly FEC unprotected and FEC receivers vulnerable to DoS. To this the proponents of this approach simply replied with: “well, those of you who use FEC/RTX will simply do a third round of SRTP”, thus arriving at a total of three encryptions for every packet..
The discussions around this topic were highly contentious.
Hope this makes things clear, Emil
The need to do a triple encryption for example is a particularly egregious consequence of the order problem. That’s a problem specific to the “double” documents.
I would however also say that the decision to bake one specific way of performing key negotiation into the framework rather than leaving it open was both unnecessary and quite problematic.
Emil "on the consensus not reached on this and other topics."
[BA] Out of curiosity, what other topics do you consider to be problematic within the framework? I am aware of at least two others where implementers have chosen different paths than in the PERC framework:
* Order of application of encryption versus FEC/RTX/RED * Whole frame encryption versus payload encryption
With respect to consensus, this is IETF last call, one of whose purposes is to determine whether there is IETF consensus to publish this document as a Proposed Standard. Are you saying that you do not agree that there is an IETF consensus to publish this document as a Proposed Standard? Or that there is no IETF consensus to publish *any* of the PERC WG output as a Proposed Standard?
+1 on ssrc rewriting, and on the consensus not reached on this and other topics. Sent from my iPhone +1, SSRC rewriting is pretty much fundamental to all SFUs out there. Lorenzo Il 2 febbraio 2019 10:19:06 CET, Emil Ivov < emcho@xxxxxxxxx> ha scritto: I want to second that as it is a particularly major problem: not allowing SSRC rewriting makes the entire framework unusable with SFU implementation I represent as well as every other SFU I am familiar with.
I am also not sure that I agree with “SSRC rewriting could not be allowed” is a conclusion that ever had any consensus in PERC, regardless of what WG leadership is trying to make everyone believe. Richard said:
"Again, the answer is clear here, but the document should be clearer. The working group discussed SSRC rewriting several times, and concluded that SSRC rewriting could not be allowed in this system. This decision is reflected, e.g., in the fact that the Double transform does not allow modification of SSRCs."
[BA] Not being able to rewrite SSRCs has some major implications with respect to requirements on PERC endpoints. Typically today's MDD will switch between the simulcast streams provided by an endpoint, forwarding only a single stream to other participants, based on the bandwidth, resolution and framerates. If rewriting of SSRCs is not possible, do PERC endpoints need to be able to receive simulcast? If PERC endpoints do need to be able to receive simulcast, what are the requirements for endpoints? For example, should endpoints expect the MDD to use RID header extensions to identify the incoming simulcast streams?
Receiving of simulcast is tricky because the endpoint is receiving multiple streams with different sequence number spaces which may contain holes because of reordering or loss. This not only complicates the application of RTX, RED and FEC, but also the operation of the endpoint. As a result, as noted in the WEBRTC specification Section 5.4.1, support for reception of simulcast is optional. I am aware of two ORTC implementations that have attempted to support simulcast reception, neither of which is robust in scenarios with considerable loss and/or reordering. And neither implementation supports the RID header extension on received simulcast streams.
On 01/02/2019 17:18, Richard Barnes wrote:
So I would propose we add something like the following to this definition:
"In the context of WebRTC, where control of a session is divided between a _javascript_ application and a browser, the browser acts as the Trusted Endpoint for purposes of this framework (just as it acts as the endpoint for DTLS-SRTP in one-to-one calls).
If we decide to adopt perc (big if) in webrtc, shouldn't this be defined within the https://tools.ietf.org/html/draft-ietf-rtcweb-security-arch-17 doc ?
Optimally, we would not rely on trust in any entities other than the
browser. However, this is unfortunately not possible if we wish to
have a functional system. Other network elements fall into two
categories: those which can be authenticated by the browser and thus
can be granted permissions to access sensitive resources, and those
which cannot be authenticated and thus are untrusted.
WebRTC already IdP as trusted for identity purposes, so it should be up to the RTCWEB group to decide what is a trusted endpoint and what is not in webrtc. As Bernard is stating, we could decide that there are other key management solutions trusted (even in JS or WASM), as for for example is being done in EME: https://github.com/WICG/media-capabilities/blob/master/explainer.md#encryption
Best regards Sergio
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