[pkix] Support for delegate certificates

[Phil Lello <[e-mail]>]

Hi all,

Not sure if this has already been discussed (I couldn't find it), or indeed
if this is the most appropriate list, but I have a variation on certificate
chains I'd like considered.

I have a couple of scenarios in mind where it could be useful for the
holder of a certificate, say for *.example.com to act as a CA for issuing
certificate to delegates. Examples include:

  - *.example.com issues a certificate to project1.example.com (to reduce
number of admins trusted with wildcard cert)
  - *.example.com issues a certificate to server.altsvc.com (to support
HTTP's Alt-Svc)
  - *.example.com issues a certificate to [e-mail], who issues a
certificate to www.randompublisher.net

The basic principle is that the certificate (as vetted by a public CA)
identifies the party responsible for the signed content, but allows
generation of certificates for alternate distribution points (or
individuals who can do so).

This would require explicit changes for libraries supporting protocols such
as TLS (assuming the chain verification isn't hopelessly broken) to provide
a notification to the client to identify who's behalf the certificate was
issued on.

I'd like to avoid adding any extra metadata to the delegating certificate,
as that would seem to serve no techincal purpose, and simply be an excuse
for a public CA to charge more.

Does this sound feasible/desirable? The TLS use-case should probably define
extra handshake options to guide the server on selecting an appropriate
cert/logging a useful error.

Best wishes,

Phil Lello