Re: [expect-ct] Is expect-ct policy intended for long-term use? (plus: no user recourse)

Emily Stark <estark@google.com> Thu, 24 November 2016 01:08 UTC

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From: Emily Stark <estark@google.com>
Date: Wed, 23 Nov 2016 17:03:27 -0800
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To: "=JeffH" <Jeff.Hodges@kingsmountain.com>
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Subject: Re: [expect-ct] Is expect-ct policy intended for long-term use? (plus: no user recourse)
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I anticipate Expect-CT to be useful more than a year and less than 5 years.
Within 1-2 years, I expect/hope several browsers will be requiring CT for
all new certificates. They can still implement Expect-CT to protect sites
against backdating and against certificates that were issued before the
date that they started requiring CT for all new certs.

Once a browser is requiring CT for *all* certificates (e.g. because the
maximum validity period has elapsed beyond the date that the browser began
requiring CT for all new certs), then I don't think Expect-CT is useful for
that browser anymore.

On Wed, Nov 23, 2016 at 4:47 PM, =JeffH <Jeff.Hodges@kingsmountain.com>
wrote:

> WRT "Expect-CT" <https://tools.ietf.org/html/draft-stark-expect-ct> (aka
> "the I-D" in the below)...
>
> Is the expect-ct policy intended to be used long-term by servers?
>
> I.e., is this server-declared expect-ct policy only a stop-gap until all
> browsers natively enforce their vendors' "ct policies"?
>
> At first glance, it seems the answer is "yes, expect-ct has long-term
> usefulness" given the language in
> <https://tools.ietf.org/html/draft-stark-expect-ct-00#section-2.1.2>,
> i.e., a host's declaration of expect-ct policy is stating that the UA
> must terminate any connection to that host (and port?) that does not
> satisfy the UA's ct policy.
>
> However, given this..
>
> On Sunday, November 13, 2016 at 4:47 AM, Emily Stark wrote:
> > That is, eventually, when browsers require CT for all certificates,
> > [...] 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)
>
> ..it sounds like the browsers intend to do that in any case, and if so, on
> what timescale?
>
> I.e., is it worthwhile to go through all the work to formally define
> Expect-CT in an RFC?
>

I'm not sure. This is part of the reason why I uploaded this as an
experimental draft. I'm not 100% sure what's the right process or venue is
for a mechanism that is not meant to stick around forever.


>
> Though, if there is some functionality that a server-declared expect-ct
> policy stipulates that is not intended to be implemented by default in
> near- to intermediate-term, then formally specifying Expect-CT perhaps has
> a reasonable cost-benefit regardless. Or also if explicit server-declared
> "expect-ct" policy would be useful to the long-tail of HTTPS clients other
> than the dominant browsers.
>
> Perhaps one should consider having the expect-ct policy additionally mean
> that there is "no user recourse" to connection termination as a result of
> CT-policy violation. I note the I-D does not presently state that.
>
> See <https://tools.ietf.org/html/rfc6797#section-12.1> for how this is
> discussed in HSTS. You might consider adding "no user recourse" to a "UA
> implementation advice" section.
>

That seems reasonable to include, though I don't think "no user recourse"
is enough benefit to justify keeping Expect-CT around after it has
otherwise exhausted its usefulness.


>
> Though, like any of this (including HSTS), the browsers could in the
> future decide that they will have a "no user recourse" policy by default
> for all secure transport establishment failures. It's a question of how far
> in the future might that occur (in order to justify
> present-to-intermediate-term work).
>
> HTH,
>
> =JeffH
>
>
>
>