Re: [TLS] Roman Danyliw's No Objection on draft-ietf-tls-sni-encryption-05: (with COMMENT)

Roman Danyliw <rdd@cert.org> Thu, 26 September 2019 21:38 UTC

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From: Roman Danyliw <rdd@cert.org>
To: Christian Huitema <huitema@huitema.net>, The IESG <iesg@ietf.org>
CC: "draft-ietf-tls-sni-encryption@ietf.org" <draft-ietf-tls-sni-encryption@ietf.org>, "tls-chairs@ietf.org" <tls-chairs@ietf.org>, "tls@ietf.org" <tls@ietf.org>
Thread-Topic: [TLS] Roman Danyliw's No Objection on draft-ietf-tls-sni-encryption-05: (with COMMENT)
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Date: Thu, 26 Sep 2019 21:38:20 +0000
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Subject: Re: [TLS] Roman Danyliw's No Objection on draft-ietf-tls-sni-encryption-05: (with COMMENT)
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Hi Christian!

Thanks for all of the updates.  I have a remaining items are described inline.

To bring up a new item, there was new text introduced in -06 of Section 5 to which I strongly object.  Specifically:

"Replacing clear text SNI transmission by an encrypted variant will	
also thwart MITM interferences that are sometimes described as	
legitimate.  As explained in Section 2.3, alternative solutions will	
have to be developed.”

I read this paragraph as addressing the operational practices outlined in Section 2.1.  I think it is inappropriate to refer to some of these operational practices as being "sometimes described as legitimate".

> -----Original Message-----
> From: Christian Huitema [mailto:huitema@huitema.net]
> Sent: Wednesday, September 25, 2019 3:47 PM
> To: Roman Danyliw <rdd@cert.org>rg>; The IESG <iesg@ietf.org>
> Cc: draft-ietf-tls-sni-encryption@ietf.org; tls-chairs@ietf.org; tls@ietf.org
> Subject: Re: [TLS] Roman Danyliw's No Objection on draft-ietf-tls-sni-
> encryption-05: (with COMMENT)
> 
> Hello Roman,
> 
> A lot of the fixes that you suggested are incorporated in the draft-07 that was
> just released. I think the last version addresses your concerns, but you may
> of course want to verify.
> 
> On 9/25/2019 7:27 AM, Roman Danyliw wrote:
> > Hi Christian!
> >
> > Thanks for the detailed responses and the helpful background.  Below are a
> number of proposed text block replacements to clarify my intent (instead of
> more questions).
> >
> > Roman
> >
> >> -----Original Message-----
> >> From: iesg [mailto:iesg-bounces@ietf.org] On Behalf Of Christian
> >> Huitema
> >> Sent: Wednesday, September 18, 2019 10:14 PM
> >> To: Roman Danyliw <rdd@cert.org>rg>; The IESG <iesg@ietf.org>
> >> Cc: draft-ietf-tls-sni-encryption@ietf.org; tls-chairs@ietf.org;
> >> tls@ietf.org
> >> Subject: Re: [TLS] Roman Danyliw's No Objection on
> >> draft-ietf-tls-sni-
> >> encryption-05: (with COMMENT)
> >>
> >> Thanks for the feedback, Roman. Comments in line.
> >>
> >> On 9/18/2019 4:40 AM, Roman Danyliw via Datatracker wrote:
> >>> ** Section 1.  Per “More and more services are colocated on
> >>> multiplexed servers, loosening the relation between IP address and
> >>> web service”, completely agree.  IMO, unpacking “multiplexed
> >>> servers” is worthwhile to explain the subsequent text because it
> >>> motivates the loss of visibility due to encryption with network only
> monitoring.
> >> “Multiplex’ happens at two levels:
> >>> -- co-tenants (e.g., virtual hosting) – multiple services on the
> >>> same server (i.e., an IP/port doesn’t uniquely identify the service)
> >>>
> >>> -- cloud/cdn  – a given platform hosts the services/servers of a lot
> >>> of organization (i.e., looking up to what netblock an IP belongs
> >>> reveals little)
> >>
> >> OK, will try to incorporate your text.
> > Thanks.
> 
> Changes incorporated in first paragraph of section 1.

The text -07 works for me.  Thanks for adding this extra bit.

> >
> >>> ** Section 2.1.  Per “The SNI was defined to facilitate management
> >>> of servers, though the developers of middleboxes soon found out that
> >>> they could take advantage of the information.  Many examples of such
> >>> usage are reviewed in [RFC8404].”,
> >>>
> >>> -- Can’t middleboxes also help facilitate the management of servers?
> >>> This text seems to take a particular view on middleboxes which
> >>> doesn't
> >> seem appropriate.
> >>
> >> It is pretty clear that the load balancer in front of a server farm
> >> will need access to the service ID, and must be able to retrieve the
> decrypted SNI.
> >> There may be other examples, such as DoS mitigation boxes. The
> >> "unanticipated usage" comes typically from middle-boxes that are not
> >> in the same management domain as either the client or the server. Is
> >> there an established way to designate those?
> > I'm not sure I understand the original of the requirement that the client
> and server being in the same management domain.
> >
> > RFC3546's definition of SNI opens with:
> >    [TLS] does not provide a mechanism for a client to tell a server the
> >    name of the server it is contacting.  It may be desirable for clients
> >    to provide this information to facilitate secure connections to
> >    servers that host multiple 'virtual' servers at a single underlying
> >    network address.
> >
> > It seems to me that if we are trying to channel original intent, then only the
> virtual server use case applies.  I'd propose:
> >
> > OLD
> > The SNI was defined to facilitate management of servers, though the
> developers of middleboxes soon found out that they could take advantage
> of the information.  Many examples of such usage are reviewed in
> [RFC8404].
> >
> > NEW
> > The SNI was defined to facilitate secure connections to servers that host
> multiple 'virtual' servers at a single underlying network address [RFC3546].
> However, addition management and security practices emerged making use
> of this information.  Examples of such usage are reviewed in [RFC8404].
> >
> > This language would let you distinguish all of the middle box behaviors
> done by operators and enterprises from a possible [RFC7258] attacker.

As I noted in my reply to Ben, I don't follow the current language for anticipated/unanticipated.

https://mailarchive.ietf.org/arch/msg/tls/54cEpjUcIZqNW_y7Eb344XLQp0Y

> >>> -- RFC8404 describes a number of middlebox practices, but only
> >>> Section
> >>> 6.2 explicitly discusses SNI, and of the examples list here, only
> >>> one comes from RFC8404.
> >> A few of the examples also come in the "deep packet inspection"
> >> sections of 8404. But rather than going in a long discussion, I would
> >> rather rewrite the sentence as: Many examples of such usage are
> >> reviewed in [@?RFC8404], other examples came out during discussions of
> this draft.

The -07 text works for me.  Thanks.

> >>> ** Section 2.1. The “monitoring and identification of specific sites”
> >>> isn’t symmetric to the other examples – it is rather generic.  The
> >>> other examples, identify a what/who (e.g., ISP, firewall) + action
> >>> (e.g.,
> >> block, filter).
> >>> Also, to implement most of the other example, “monitoring and
> >>> identification of specific sites” needs to be done.
> > I still think this needs to be cleaned up in some way.  IMO, I'd drop it.
> 
> Was rewritten in new section 2.1 as "Filtering or censorship of specific
> services for a variety of reasons."

I'm still struggling with the specificity here and the overlap.  Dropping the censorship and it reads "filtering of specific services for a variety of reasons".  That's exactly what bullet 2 (content filtering by operators) and bullet 3 (enterprise firewalls for NSFW)

What's unique about this bullet -- is it who is doing this filter/censorship? where it is happening? What tech is being used?

> >>> ** Section 2.1.  Why is parental controls in quotes?  In RFC8404, it is not.
> >>> The quotes could be read as a judgement on the practice.
> >> See answer to Alissa. Removing the quotes.
> > Thanks.
> >
> >>> ** Section 2.1.  Per “The SNI is probably also included in the
> >>> general collection of metadata by pervasive surveillance actors”, I
> >>> recommend against speculation and instead simply stating that SNI
> >>> would be interesting meta-data for a RFC7258 attacker.
> >> Yes, Mirja made a similar comment. Proposed replacement:
> >>
> >> The SNI is probably also included in the general collection of
> >> metadata by pervasive surveillance actors, for example to identify
> >> services used by surveillance targets.
> > IMO, explicitly linking it to the draft would help.
> >
> > OLD:
> > The SNI is probably also included in the general collection of
> > metadata by pervasive surveillance actors, for example to identify
> > services used by surveillance targets.
> >
> > NEW:
> > The SNI could be included in the general collection of metadata by
> > pervasive monitoring attacker [RFC7258], for example to identify
> > services used by surveillance targets.
> 
> I missed the reference to 7258. Will add it in the next version.

Recommend adding the RFC7258 reference.  As I noted above, the semantics of "probably" vs. "could".  "probably" suggests a confidence to me.

> >
> >>> ** Section 2.2.  Per “One reason may be that, when these RFCs were
> >>> written, the SNI information was available through a variety of
> >>> other means”, what would those “other means” be?
> >> The list includes at a minimum:
> >>
> >> Clear text exchanges amenable to deep packet inspection (DPI), server
> >> certificates send in clear text during TLS/SSL exchanges, DNS names
> >> of servers in clear text DNS queries, and server specific IP
> >> addresses in packet headers.
> >>
> >> I guess I could write that all, but it makes the text a bit
> >> redundant, since the following paragraphs do discuss server
> >> certificates, DNS names and IP addresses.
> > I understand.  I didn't read it that way.  My recommendation isn't to
> describe the "other means" (as it is described below), but to be clear on the
> obvious, what is the SNI information.
> >
> > OLD:
> > One reason may be that, when these RFCs were written, the SNI
> > information was available through a variety of other means.
> >
> > NEW:
> > One reason may be that when the RFCs were written, the name of the
> server the being contacted by the client (i.e., the SNI) was evident through
> other means.

The new text in -07 reads more clearly to me.  Thanks for this change.

> >>> ** Section 2.3.  Per “Deploying SNI encryption will help thwarting
> >>> most of
> >> the
> >>> ‘unanticipated’ SNI usages described in Section 2.1, including
> >>> censorship
> >> and
> >>> pervasive surveillance.”:
> >>>
> >>> -- Why the quotes around "unanticipated" SNI usage?
> >> Removing the quotes. Otherwise, you will be convinced that the
> >> authors believe that all middle-boxes are the spawn of the devil...
> > Thanks.
> >
> >>> -- One person’s censorship is another person’s threat mitigation,
> >>> policy enforcement for a network they own, or parental controls (per
> >>> the list in Section 2.1) – recommend being more precise on the order
> >>> of “Deploying
> >> SNI
> >>> encryption will {break | reduce the efficacy of } the operational
> >>> practices
> >> and
> >>> techniques used in middleboxes described in Section 2.1”.
> >> OK. I will try to make the text just stick to the facts:
> >>
> >> Deploying SNI encryption thwarts most of the unanticipated SNI usages
> >> described in (#snileak). It reduces the efficacy of the operational
> >> practices and techniques implement in middle-boxes. Most of these
> >> functions can, however, be realized by other means.

Thanks for this.

> > Works for me.  However, I'd drop "Most of these functions can, however,
> be realized by other means" because this opens the debate on how exactly,
> etc.

I still recommend dropping the "other means" text because that opens debate.

> >>> ** Section 2.3.  Per “It will also thwart functions that are
> >>> sometimes described as legitimate”, what functions are those?  I’d
> >>> recommend
> >> eliminating
> >>> this sentence as it reads like a value judgement on existing
> >>> practices (which doesn’t seem germane for discussing requirements).

Now that censorship got added to the list in Section 2.1, you likely want to:

s/including censorship and pervasive surveillance/including pervasive surveillance/

Otherwise, LGTM.  Thanks.

> >>> ** Section 3.  Per “Over the past years, there have been multiple
> >>> proposals
> >> to
> >>> add an SNI encryption option in TLS.”, can these past proposals be
> >>> cited so future readers can learn from them.
> >> We are describing here a series of design proposed in the TLS working
> >> group over the years. The whole point of the draft is to provide the
> >> results of the analyses, as an easy to read kind of "threat model",
> >> without requiring readers to wade through years of archives. If you
> >> really are interested, you can indeed do just that but I would not
> >> encourage the approach...
> > I understand.  It doesn't seem practical to quote mailing list threads.
> There is actually one such quote in the "Acknowledgement" section.

No problem.

> >>> ** Section 3.4. The existence of designs were alluded to but not
> >>> cited.  Be specific with citation.
> >>>
> >>> ** Section 3.7.1. The rational for including this discussion about
> >>> ALPN isn’t clear as it doesn’t suggest new requirements for SNI
> encryption.
> >> Got comments about that already, and updated the text.

The new intro text is Section 1 explains it better.  Thank you for that preamble.

> >>> ** Section 4.  I got hung-up on the description of Section 4
> >>> describing a “solution”.  Is Section 4 (and the related subsections)
> >>> describing an operational practice or a notional reference
> >>> architecture?  The text reads
> >> one
> >>> part “people are doing” and another part “people could do”.
> >> Yes, I get that. Our point is to describe this solution as part of
> >> explaining why we really want a TLS level solution, not just HTTP.
> >> Not sure that I can change much here.
> > See below.  I think I may have an approach for clarity.
> >
> >>> ** Section 4.  Per “In the absence of TLS-level SNI encryption, many
> >>> sites
> >> rely
> >>> on an "HTTP Co-Tenancy" solution”, this seems like a strong of a
> >>> statement about utilization of this architecture explicitly to hide
> >>> the hidden.example.com SNI.  Can you provide a citation for a sense
> >> penetration.
> >>
> >> That's really hard, because it is all cloak and dagger stuff. The one
> >> well known example is the encrypted messaging application "Signal",
> >> that was censored in Egypt during the "Arab Spring" events. They were
> >> hosted by Google, and apparently programmed their app to just connect
> >> to "https://google.com/", and then use "host: signal.org" in the HTTP
> >> headers, evading censorship. It is not clear at all to what amount
> >> they synchronized with Google when doing that. And I don't think that
> >> anybody ever spoke openly about this.
> > Thanks for the details.  Let me step back and try to restate my concern.  My
> feedback on the assertion that "many sites rely on an HTTP Co-Tenancy" and
> the question above about the "browser plugin" all come from my
> misunderstanding of the purpose of Section 4.   Is it describing a commonly
> accepted practice already done or a notional reference architecture.  IMO,
> given the framing of the rest of the document, it should be the latter.
> >
> > The first paragraph states that "many sites" use this approach which
> suggested to me an existing best common practice.  However, as you
> clarified, there is little evidence that can be provided beyond signal.  To me
> the statement of "many sites" can't be supported.  My thinking is that this
> could be easy cleared up simply avoiding the discussion about adoption by
> saying:
> >
> > OLD:
> > In the absence of TLS-level SNI encryption, many sites rely on an
> > "HTTP Co-Tenancy" solution.  The TLS connection is established with
> > the fronting server, and HTTP requests are then sent over that
> > connection to the hidden service.
> >
> > NEW:
> > In the absence of TLS-level SNI encryption, a site could adopt an "HTTP Co-
> Tenancy" architecture to protect the SNI information.  In such an
> architecture, the client establishes a TLS connection with a fronting server,
> and the HTTP requests are then sent over that connection to the hidden
> service.
> >
> > Related to my confusion is also the new text added to Section 1 of -06,
> "This document does not present the design of a solution, but provides
> guidelines for evaluating proposed solutions."  However, the current text in
> Section 4 is explicitly states it is providing a solution.  The sub-section of
> Section 4.x assume the solution in Section 4.0 and describe the follow-on
> work.  Section 2 - 3 do lay out the means for evaluation nicely.  Perhaps,
> something on the order of:
> >
> > OLD:
> > This document does not present the design of a solution, but provides
> guidelines for evaluating proposed solutions.
> >
> > NEW:
> > This provides guidelines on evaluating solution and proposes an
> architecture to mitigate the threats created by an unencrypted SNI using
> existing approaches.
> 
> 
> I added the reference to a paper by Fified et al. that describes the "domain
> fronting" solution and some of its deployments.
> 
> >
> >>> ** Section 4.  Per the bullet “since this is an HTTP-level
> >>> solution”, I recommend citing that it fails on the requirement
> >>> identified in Section 3.7 (instead of enumerating a list of
> >>> protocols)
> >> Yes. Already fixed.
> > Thanks.
> >
> >>> ** Section 4.  The opening of this section noted that “many sites”
> >>> rely on
> >> the
> >>> architecture described in this section. Later, it is noted that “a
> >>> browser extension that support[s] HTTP Fronting” is a necessary
> >>> architecture
> >> component.
> >>>   Can a few citations be made to the popular extensions.
> >> The "Signal" deployment used a service specific app. The trick of
> >> using https://fronting + Host: hidden is really easy to pull in an
> >> app. To do that in a browser does indeed require an extension, that's
> >> pretty much a statement of fact.
> > Makes sense.  Back to my earlier comment about "many sites", if this text is
> describing a specific solution/best practice vs. a reference architecture.  If it is
> the former, then what's actually done needs to be described (i.e., an app-
> based approach).  If it is the latter, the text is fine.
> 
> Draft 07 added text describing actual deployments.

The use of the [domfront] citation works for me and addresses my concerns.  Two nits:

** I'd recommend using the URL to the paper from conference site itself: https://petsymposium.org/2015/papers/03_Fifield.pdf

** I'd also recommend adding a sentence to the last paragraph of Section 1, "This document does not present the design ...", to foreshadow that you'll discuss an alternative approach even if encrypted SNI isn't realized yet.

Thanks,
Roman