[cabfpub] (Eventually) requiring id-kpServerAuth for all certs in the chain?

Ryan Sleevi sleevi at google.com
Wed Nov 5 17:37:35 UTC 2014

Under the current audit requirements, a CA can issue an intermediate that
has full power and capability of issuing trusted TLS certs, and then
declare it out of scope of audit, because they don't "intend" for it to
issue TLS certs.

This hand-wave does not require disclosure to auditors, nor to root
programs, and yet still provides the same risk profile as incidents such as
DigiNotar or India CCA.

Gerv is proposing a technical means to express this intent, so that it is
clear to all parties the risk profile involved. Gerv's proposal is "opt-in"
- that is, by a certain timeframe, all CAs must reissue to conform to the
scheme, and anyone not conforming to the scheme is out of scope, and UAs
will be updated to require certs conform.

My proposal is "opt out" - by declaring every certificate transitively
issued by a root to be in scope for the BRs, the burden is placed upon the
CA to use technical means to exclude their "non-TLS" intermediates from
being in scope. This is weaker in some ways (it lacks the hard enforcement
on the client side that Gerv proposes), but as both schemes rely on
auditors, I believe they represent the same effective risk profiles.
On Nov 5, 2014 9:29 AM, "kirk_hall at trendmicro.com" <kirk_hall at trendmicro.com>

> Gerv, I'm slow on this argument, but I'm trying to figure out why markers
> in intermediates are important.
> Under the current scheme, it seems the trusted status of the root is
> what's important, not the status of intermediates.  CAs must get WebTrust/
> ETSI audits covering their operations from those roots to be trusted and
> included in the browser root store.  The audits are supposed to cover all
> (SSL) operations from that root.  Ultimately, it's a binary yes/no decision
> on whether to keep the root in the root store based on audit results, plus
> compliance with other root program requirements.  I suppose rogue
> intermediates from the roots can also be explicitly untrusted by browsers
> if needed.
> What are the objectives of the proposals to put markers (EKUs or OIDs) in
> intermediates?  Is it not possible to meet those objectives using the
> current system of trusted root / audit of all (SSL) operations from that
> root?
> Just curious.
> -----Original Message-----
> From: public-bounces at cabforum.org [mailto:public-bounces at cabforum.org] On
> Behalf Of Gervase Markham
> Sent: Monday, November 03, 2014 11:46 AM
> To: CABFPub
> Subject: [cabfpub] (Eventually) requiring id-kpServerAuth for all certs in
> the chain?
> Hi everyone,
> I wonder if the BRs should say that all non-root certs in a chain issued
> for SSL server use, which were issued after <date>, should have EKU
> id-kpServerAuth in them. Date would be, say, six months from now.
> This is primarily aimed at intermediates; EE certs all currently have this
> anyway. It would mean that, over time (years) as intermediates got
> replaced, we could eventually move to a position where it was entirely
> clear what certs were intended for Web PKI SSL use and what certs were not.
> Currently, any intermediate in the world issued by a publicly-trusted root
> can issue for SSL, even those intermediates which are not intended for such
> use. This leads to numerous problems, including the question of whether
> such intermediates need to be covered by a BR audit. Once this change had
> filtered through, it would be clear - they would not be.
> AIUI, EKU "chaining" (i.e. requiring an EKU to be present all the way up
> the chain) is not standard, but is implemented in NSS and elsewhere.
> I know this is a thing which only pays off in the long term, but I still
> think it's worth it. Does this make any sense, or have I missed something
> obvious? :-)
> Gerv
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