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

Eric Rescorla <> Mon, 14 November 2016 21:09 UTC

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From: Eric Rescorla <>
Date: Tue, 15 Nov 2016 06:04:43 +0900
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To: Emily Stark <>
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Subject: Re: Comments on draft-stark-expect-ct-00
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On Sun, Nov 13, 2016 at 9:47 PM, Emily Stark <> wrote:

> Hi, thanks for the comments. Replies inline.
> > Overall:
> > Is there a reason to not have this be attached to the HSTS header (or,
> > I guess more weakly to HPKP)?  The syntax seems like it allows it. It
> > seems like we go to all the trouble of making these headers extensible
> > (indeed, the syntax in S 2.1) seems almost identical to HSTS but then
> > we define a new header each time.
> If we try to attach it to HSTS or HPKP, the syntax gets kind of
> clunky, which according to my understanding is one of the main reasons
> that HPKP ended up as a separate header too. For example, one might
> want separate max-ages for HSTS and Expect-CT, and one might want to
> configure a report-uri but it should be clear that the report-uri
> applies to Expect-CT but not to HSTS, and then you end up with
> something like this:
> Strict-Transport-Security: max-age=31536000; expect-ct-enforce;
> expect-ct-max-age=86400; expect-ct-report-uri=https://...
> which I don't think would be the end of the world, but it does feel
> kind of clunky and error-prone to me.

In general, my intuition would be that all these parameters other than
the policy should be the same. Is there some compelling use case fo
why they should not?

> At least, it would be nice to merge the report-uri function/description.
> > I am probably missing something, but I don't see the text that defines
> > what the actual semantics of enforcing CT are. The document says stuff
> > like "The UA evaluates each connection to an Expect-CT host for
> > compliance with the UA's Certificate Transparency (CT) policy" which
> > leads me to believe that different UAs might have different policies.
> > Assuming I am correct, this seems like a recipe for interop problems,
> > even with the compliance checking in S 2.3.1. Consider the case where
> > a UA's policy is that you must have CT for every cert in the chain
> > and the operator has two certs, one from a CA which has CT for every
> > cert in the chain and another from a CA which has CT just for the EE
> > cert. If the client gets Expect-CT from a host with the first cert,
> > then it will fail trying to connect to a host with the second cert.
> It is indeed the intention that different UAs can have different
> policies. I think this reflects the reality that browser are not
> necessarily planning to have identical CT policies
> (
> 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

> 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