[cabfpub] Definition of an SSL certificate

Rick Andrews Rick_Andrews at symantec.com
Thu Dec 19 19:32:14 UTC 2013

As mentioned on the call this morning, we learned several months ago about the differences in browser checks for SSL certs, so Ben created a page on the wiki and I populated it with info that others had provided: https://www.cabforum.org/wiki/Browser%20Behavior. If there's a burning need to make this public, we can consider moving it to the main web site.


From: public-bounces at cabforum.org [mailto:public-bounces at cabforum.org] On Behalf Of Jeremy Rowley
Sent: Thursday, December 19, 2013 8:06 AM
Subject: [cabfpub] Definition of an SSL certificate

We are looking to clarify the scope of the BRs.  Right now the BR scope is very loose and subjective: "This version of the Requirements only addresses Certificates intended to be used for authenticating servers  accessible through the Internet."

This loose definition excludes internal names (which are not typically accessible through the Internet), a type of certificate the BRs are clearly designed to regulate (see 11.1.4).  In addition, a CA could easily issue a certificate outside of the BRs  that could later be used in a TLS/SSL attack simply because the certificate wasn't intended to be used for SSL.

Issuance of certificates outside the BRs may not be intentional, especially by CAs who aren't regularly issuing SSL certificates.  These CAs may not be aware that the BRs apply to their certificates and may not realize their client certificates could be used for SSL since "authenticating servers" is not a well-defined term.

Clarifying the scope will ensure that every CA is aware of the minimum standards and what they cover.  Originally, the idea was to tie the scope to the values in the EKU.  Unfortunately, there are several obstacles to this approach:

1)      Grid Certificates require serverAuth in the EKU.  It's unclear whether these certs should be covered.

2)      Per 5280, browsers treat the absence of an EKU and the anyEKU as serverAuth, meaning they are enabled for TLS Server Authentication.

3)      The FBCA requires anyEKU in several certificates.  These are clearly client certificates and are outside the BR scope.

4)      Qualified certificates in the EU have either the anyEKU present or omit the EKU.  They are client certs and clearly not covered by the BRs.  However, they can be used  for server authentication.

For qualified certificates, Moudrick provided the following guidance:

"Certificates using applications MAY require that the extended key usage extension be present and that a particular purpose be indicated in order for the certificate to be acceptable to that application.

If a CA includes extended key usages to satisfy such applications, but does not wish to restrict usages of the key, the CA can include the special KeyPurposeId anyExtendedKeyUsage ***in addition to the particular key purposes required by the applications***.

So a QC pretending to be RFC 5280/BR and ETSI (web server QCs) compliant would have to at least have:

QC + [anyEKU] + id-kp-serverAuth + {DV/OV/EV}

I'm quite confident that the absolute majority of QCs issued so far (that have anyEKU, see Mark Janssen's 08/08/2013 - thank you Stephen) do not contain any DV/OV/EV policy IDs, so why not accept them as BR compliant but not sufficient for TSL/SSL establishment?

In order for a QC to have a TSL/SSL capability, BR may require:

QC + {{id-kp-serverAuth and/or id-kp-clientAuth} + {DV/OV/EV}} (optionally no anyKEY allowed).

A practical interpretation: a WEB server that also does some web site related document/content signing."

There appears to be a significant conflict between the CAB Forum's work and the standards set by other bodies.  In particular, their use of the anyEKU or omission of an EKU is permitting the realm of client certs to overlap SSL certs.  Approaching each government body to stop this practice is not feasible and will take a very long time to complete

Hopefully this summary helps inspire ideas on where we can go from here.  I'm looking forward to ideas.



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