Re: [TLS] Enforcing Protocol Invariants

Jana Iyengar <jri.ietf@gmail.com> Tue, 12 June 2018 23:25 UTC

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From: Jana Iyengar <jri.ietf@gmail.com>
Date: Tue, 12 Jun 2018 16:25:01 -0700
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To: davidben@chromium.org
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Subject: Re: [TLS] Enforcing Protocol Invariants
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I'm emerging from lurking to just say this: I think this is a fantastic
idea, and thank you for doing it.
FWIW and IMO, you're better off summarizing the codepoints you are using
and planning to use on a wiki somewhere instead of in registries, given the
timescales and the lifetime of these codepoints.

- jana

On Tue, Jun 12, 2018 at 9:28 AM David Benjamin <davidben@chromium.org>;
wrote:

> Hi all,
>
> Now that TLS 1.3 is about done, perhaps it is time to reflect on the
> ossification problems.
>
> TLS is an extensible protocol. TLS 1.3 is backwards-compatible and may be
> incrementally rolled out in an existing compliant TLS 1.2 deployment. Yet
> we had problems. Widespread non-compliant servers broke on the TLS 1.3
> ClientHello, so versioning moved to supported_versions. Widespread
> non-compliant middleboxes attempted to parse someone else’s ServerHellos,
> so the protocol was further hacked to weave through their many defects.
>
> I think I can speak for the working group that we do not want to repeat
> this adventure again. In general, I think the response to ossification is
> two-fold:
>
> 1. It’s already happened, so how do we progress today?
> 2. How do we avoid more of this tomorrow?
>
> The workarounds only answer the first question. For the second, TLS 1.3
> has a section which spells out a few protocol invariants
> <https://tlswg.github.io/tls13-spec/draft-ietf-tls-tls13.html#rfc.section.9.3>;.
> It is all corollaries of existing TLS specification text, but hopefully
> documenting it explicitly will help. But experience has shown specification
> text is only necessary, not sufficient.
>
> For extensibility problems in servers, we have GREASE
> <https://tools.ietf.org/html/draft-ietf-tls-grease-01>;. This enforces the
> key rule in ClientHello processing: ignore unrecognized parameters. GREASE
> enforces this by filling the ecosystem with them. TLS 1.3’s middlebox woes
> were different. The key rule is: if you did not produce a ClientHello, you
> cannot assume that you can parse the response. Analogously, we should fill
> the ecosystem with such responses. We have an idea, but it is more involved
> than GREASE, so we are very interested in the TLS community’s feedback.
>
> In short, we plan to regularly mint new TLS versions (and likely other
> sensitive parameters such as extensions), roughly every six weeks matching
> Chrome’s release cycle. Chrome, Google servers, and any other deployment
> that wishes to participate, would support two (or more) versions of TLS
> 1.3: the standard stable 0x0304, and a rolling alternate version. Every six
> weeks, we would randomly pick a new code point. These versions will
> otherwise be identical to TLS 1.3, save maybe minor details to separate
> keys and exercise allowed syntax changes. The goal is to pave the way for
> future versions of TLS by simulating them (“draft negative one”).
>
> Of course, this scheme has some risk. It grabs code points everywhere.
> Code points are plentiful, but we do sometimes have collisions (e.g. 26 and
> 40). The entire point is to serve and maintain TLS’s extensibility, so we
> certainly do not wish to hamper it! Thus we have some safeguards in mind:
>
> * We will document every code point we use and what it refers to. (If the
> volume is fine, we can email them to the list each time.) New allocations
> can always avoid the lost numbers. At a rate of one every 6 weeks, it will
> take over 7,000 years to exhaust everything.
>
> * We will avoid picking numbers that the IETF is likely to allocate, to
> reduce the chance of collision. Rolling versions will not start with 0x03,
> rolling cipher suites or extensions will not be contiguous with existing
> blocks, etc.
>
> * BoringSSL will not enable this by default. We will only enable it where
> we can shut it back off. On our servers, we of course regularly deploy
> changes. Chrome is also regularly updated and, moreover, we will gate it on
> our server-controlled field trials
> <https://textslashplain.com/2017/10/18/chrome-field-trials/> mechanism.
> We hope that, in practice, only the last several code points will be in use
> at a time.
>
> * Our clients would only support the most recent set of rolling
> parameters, and our servers the last handful. As each value will be
> short-lived, the ecosystem is unlikely to rely on them as de facto
> standards. Conversely, like other extensions, implementations without them
> will still interoperate fine. We would never offer a rolling parameter
> without the corresponding stable one.
>
> * If this ultimately does not work, we can stop at any time and only have
> wasted a small portion of code points.
>
> * Finally, if the working group is open to it, these values could be
> summarized in regular documents to reserve them, so that they are
> ultimately reflected in the registries. A new document every six weeks is
> probably impractical, but we can batch them up.
>
> We are interested in the community’s feedback on this proposal—anyone who
> might participate, better safeguards, or thoughts on the mechanism as a
> whole. We hope it will help the working group evolve its protocols more
> smoothly in the future.
>
> David
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