[cabfpub] Continuing the discussion on CAA

Tue Oct 25 18:54:49 UTC 2016

As mentioned during the meeting, this doesn't sound fundamentally
problematic, just exceptionally technically complex - and we know that the
greater the technical complexity, the harder it is for CAs to implement.

It's unclear whether your proposal is, in effect, suggesting that CAs
should "re-implement" DNS within their validation databases. Isn't an
alternative solution for the CA to run a recursive resolver within their
infrastructure, have their CAA lookups utilize this internal recursive
resolver (either immediately or within some tightly-bounded time, as
appropriate for operational concerns), and then allow that recursive
resolver to properly implement the DNS RFCs (which includes caching based
on TTL)

I think this would split the difference between the concerns you raised,
and Jacob raised, in as much as you still benefit from the 'don't
communicate with outside services' goal, by utilizing a CA-maintained
recursive resolver that strictly implements the DNS specification, while
also allowing some flexibility with the integration of the systems.

Certainly, my 'ideal' world would be no 'implicit' caching (that is, always
"check" CAA at issuance time), but for which CAs can utilize a recursive
resolver as part of their CA infrastructure, and that recursive resolver
implements any DNS-defined caching semantics. This at least gives relying
parties - and perhaps more importantly, auditors - objective criteria for
which they can threat model and evaluate these systems.

Does that make sense?

On Tue, Oct 25, 2016 at 11:43 AM, Rick Andrews <Rick_Andrews at symantec.com>
wrote:

> Ryan, one week may be appropriate for reuse of cached CAA records, but
> during the face-to-face meeting in Redmond, I realized that the
> time-to-live (TTL) of the CAA record could make that completely ineffective.
>
>
>
> RFC 6844 doesn’t provide guidance on what TTL should be used on a CAA, but
> if a domain owner sets a TTL longer than one week, repeated checks by the
> CA would be served from cache and wouldn’t be served from the authoritative
> name server.
>
>
>
> What do you think of allowing the CA to cache the CAA record for the TTL
> specified in the record? We could augment that with instructions to domain
> owners to pay careful attention to the TTL they specify in their CAA
> records.
>
>
>
> -Rick
>
>
>
> *From:* Public [mailto:public-bounces at cabforum.org] *On Behalf Of *Ryan
> Sleevi via Public
> *Sent:* Tuesday, October 25, 2016 8:22 AM
> *To:* Gervase Markham <gerv at mozilla.org>
> *Cc:* public at cabforum.org
> *Subject:* Re: [cabfpub] Continuing the discussion on CAA
>
>
>
>
>
>
>
> On Tue, Oct 25, 2016 at 1:57 AM, Gervase Markham via Public <
> public at cabforum.org> wrote:
>
> I think this is definitely worth exploring, and I am confident we can
> work out some reasonable parameters. However, I wonder if, if we are not
> checking CAA at every issuance, it would be wise for CAs to be required
> to implement a "no more certs, please" procedure where the customer can
> tell the CA to throw away all cached validation information, including
> the CAA check results. This could be automated in circumstances where
> the customer has a login.
>
>
>
> I think it may also be useful to do this for non-customers, but domain
> holders.
>
>
>
> Consider the following certificate: https://crt.sh/?id=35360532
>
>
>
> It was issued 2016-09-26 to a customer on Google service's
>
> Since roughly 2016-03-31, googleusercontent.com has had a CAA record of 0
> issue symantec.com
>
>
>
> So why was this certificate issued? Well, as Jacob mentioned in
> https://cabforum.org/pipermail/public/2016-October/008576.html , Let's
> Encrypt checks CAA at validation time, not issuance time. Despite our CAA
> record helpfully preventing any new validations, this particular user had a
> pre-existing cached validation, according to Let's Encrypt, and so the new
> certificate was issued using the pre-existing validation.
>
>
>
> Ironically/unfortunately, it was this pre-existing validation (
> https://crt.sh/?id=14639682 ) that lead us to place CAA on the domain to
> begin with.
>
>
>
> Now, I'm not suggesting Let's Encrypt has misissued this certificate -
> it's why I've been continuing to call it 'unauthorized' issuance - and with
> respect to the BRs, everything LE did was correct. In particular, the
> re-use of cached information is fully permitted in the BRs (as many CAs
> know). However, from a our perspective, this was certainly 'a surprise' and
> not intended.
>
>
>
> So if we're going that route, I do believe we need to set a much tighter
> upper-bound than the currently permissible 39 months. Unlike WHOIS
> information, for which we know some registrars don't provide or some
> registries make it considerably more difficult (c.f. the CAPTCHA/OCR issues
> of Comodo recently), CAA is something that any domain holder can express or
> maintain themselves, and any CA can query.
>
>
>
> So it seems like one week or so should be the upper-bound to how long this
> information can be re-used.
>
>
>
> As for how we're dealing with this unauthorized issuance, at least with an
> LE model, we need to submit every one of these 'unauthorized' certificates
> as problem reports, and hope that Let's Encrypt agrees with us on the basis
> of domain holder, and then hope that the associated cached data is thrown
> away. Further, for what it's worth, the BRs do not require that such cached
> info be thrown away on a problem report - that's simply Let's Encrypt going
> above and beyond the BRs to do what is common sense and necessary for
> security, and which other CAs may not be at that same level of maturity.
>
>
>
> > 2) If a customer has a single base domain and needs to issue 6 million
> certs
> > an hour for the various sub domains, then there isn't a way for the CA to
> > simply accept the base domain's CAA record.
>
> I'm not sure how to address this without changing the way CAA works.
> AIUI it's specced to work from the requested domain down to the root. So
> I'm not sure I'd say this problem is "easily solved". Does PHB have a
> comment?
>
>
>
> I'm not PHB, but you're absolutely correct that it's "not how CAA works".
>
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