SC-103: Require EKUs for Cross-Certified Subordinate CAs

[aarongable]

--- Ballot Background ---

The CCADB Policy v2.1 requires that the Extended Key Usage extension exist and have specific values in "cross-certificates [whose] Subject CA’s public key [...] might exist in a publicly-trusted self-signed Root CA certificate whose hierarchy should be considered dedicated to a specific PKI use case".

Simultaneously, several Root Program Policies require that all CA certificates be dedicated as such. For example, the Chrome Policy v1.8 requires that all Subordinate CA Certificates must include the EKU extension and assert only id-kp-serverAuth. Similarly, the Mozilla Policy v3.0 requires that all Subordinate CA Certificates assert either id-kp-serverAuth or both that and id-kp-clientAuth.

This ballot is an attempt to harmonize the Baseline Requirements with those existing root program requirements.

--- Ballot Summary ---

In the Cross-Certified Subordinate CA Certificate Profile, indicate that the Extended Key Usage extension is required in all circumstances. Remove the language distinguishing between "unrestricted" and "restricted" cases. In the table, direct readers to the subsection detailing restricted EKUs.

Remove the contents of the old "unrestricted" subsection, marking it as deprecated.

Rewrite the "restricted" subsection to clearly indicate exactly what combinations of EKU OIDs are acceptable. This is significantly stricter than the current version of the BRs, as the current language allows many combinations of EKU OIDs, as long as the certificate does not also assert id-kp-serverAuth. But it mirrors the restrictions in existing root program policy, and is much simpler to read and understand.

This ballot does not currently have an effective date, as generation of Cross-Certified Subordinate CA Certificates is a comparatively rare event, and this change is only intended to harmonize the BRs with existing root program policy. However, an effective date in the future could be added if others deem it necessary.

This ballot is endorsed by Chris Clements (Google / Chrome) and Ben Wilson (Mozilla).