Re: Comments on draft-stark-expect-ct-00

Eric Rescorla <ekr@rtfm.com> Fri, 25 November 2016 19:21 UTC

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From: Eric Rescorla <ekr@rtfm.com>
Date: Fri, 25 Nov 2016 11:16:08 -0800
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To: Emily Stark <estark@google.com>
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Subject: Re: Comments on draft-stark-expect-ct-00
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On Mon, Nov 21, 2016 at 3:28 PM, Emily Stark <estark@google.com> wrote:

> Summarizing some hallway conversations from IETF:
>
> - Caching in report-only mode: I can be convinced that this is useful,
> in case where you are e.g. rolling out a CT-compliant certificate in
> conjunction with Expect-CT (for example if you have a config that
> turns on CT and also turns on Expect-CT in report-only mode, and the
> config didn't make it out to a few of your servers). Will be
> especially convinced if site owners say that this is how they want it
> to work.
>

I'd in general be interested in hearing from site owners on how they
feel about this header. That would be a good addition to this discussion.

>
> - Policy: One can draw an analogy to HSTS, where a site promises to
> provide a certificate that is valid according to the client's
> definition of valid, including factors that vary across clients
> (variations in trust stores, SHA1 deprecation, etc.). In practice, I
> don't think CT will be more of a foot-gun than HSTS (and certainly
> much less than HPKP) because browsers are in close collaboration to
> work out policies that play nicely with each other.
>

I'm not sure how strong the analogy is here. It's actually a nontrivial
inconvenience
for sites that different browsers have different policies. With that said,
it's not something
I'm willing to make a big deal of if the send of the WG is otherwise.

-Ekr



> On Mon, Nov 14, 2016 at 8:53 PM, Eric Rescorla <ekr@rtfm.com> wrote:
> > On Tue, Nov 15, 2016 at 10:50 AM, Emily Stark <estark@google.com> wrote:
> >>
> >>
> >> >>
> >> >> (https://groups.google.com/d/msg/mozilla.dev.security.
> policy/VJYX1Wnnhiw/ZaJBaKfKBQAJ).
> >> >> That is, eventually, when browsers require CT for all certificates,
> >> >> site owners will have to face this same problem of making sure that
> >> >> all their certificate chains are compliant with the CT policies of
> all
> >> >> the UAs that they care about. So I guess I see the interop problem as
> >> >> somewhat separate, perhaps something that should be addressed on its
> >> >> own when the CT ecosystem and implementations have matured enough
> that
> >> >> UAs are able to standardize on one policy...?
> >> >>
> >> >> To put it another way, I see Expect-CT as a way that individual sites
> >> >> can opt in to the future early ("the future" being when browsers
> >> >> require CT for all certificates), and the future is quite possibly
> >> >> different policies in different browsers, at least for some amount of
> >> >> time.
> >> >
> >> >
> >> > The problem is that as written the future is likely to involve a lot
> of
> >> > bustage.
> >>
> >> I feel like maybe I'm not understanding what you'd like to see
> >> instead. Are you arguing that the Expect-CT draft should contain a
> >> policy like "all EE certs must come with 2 SCTs from different logs",
> >> even if that policy differs from what different browsers plan to
> >> actually enforce for new certificates? Or that browsers shouldn't
> >> require CT for all certificates until they standardize on such a
> >> policy?
> >
> >
> > I'm arguing that we shouldn't define a header that says "you must enforce
> > CT"
> > without defining what "enforce CT" means.
> >
> > -Ekr
> >
> >>
> >> >
> >> >
> >> >> > S 2.1.3.
> >> >> > What's the rationale for not caching the directive in report-only
> >> >> > mode.
> >> >> > If the purpose of the report-only mode is to tell you when you have
> >> >> > nonconforming servers, then don't you want to be able to turn it on
> >> >> > on server A and detect hwen server B is broken? That seems like it
> >> >> > doesn't work if you don't cache.
> >> >>
> >> >> I'm tempted to say "because that's how HPKP does it", but that's
> >> >> probably not the answer you're looking for. :) I'd expect that sites
> >> >> would generally serve the report-only header on all responses
> >> >> unconditionally. I can't really think of a common misconfiguration
> >> >> scenario that would cause a CT violation and would *also* cause the
> >> >> header to not be served, but maybe that's a failure of imagination on
> >> >> my part.
> >> >
> >> >
> >> > Two different independent servers with the same name behind
> >> > a load balancer? Or a server farm where policies are rolled out
> slowly.
> >> >
> >> > -Ekr
> >> >
> >> >>
> >> >>
> >> >> >
> >> >> > -Ekr
> >> >>
> >> >
> >
> >
>