[Acme] Onion address validation extension

[Roland Shoemaker <[e-mail]>]

With the recent passage of SC27 at the CA/Browser Forum there are now
acceptable mechanisms for validating v3 Onion addresses for inclusion in DV
certificates. As such it would be good to extend ACME to be able to make
use of these methods. I've written a short draft that covers what is likely
to be the most interesting validation mechanism to CAs (i.e. the one that
doesn't require actually using Tor directly) and would be interested in
thoughts from the WG.

The defined ACME challenge is basically just an adapted version of the
method defined in Appendix C 2.a of version 1.6.9 of the CABF BRs. I think
in general the usage of a CSR as the transport mechanism for the nonces and
key signature are a bit burdensome, and likely to cause some confusion for
implementers (since it introduces yet another place a CSR is
transferred between the client and server, with another non-standard use).
That said I think the first priority in this document is to get out
something that works with current CABF rules. There could be value though
in defining our own validation mechanism that is a bit more
straightforward alongside the existing CABF method and if/when this
document is standardized we could lobby for it's inclusion as a blessed
CABF validation method.

Thoughts?

https://www.ietf.org/id/draft-shoemaker-acme-onion-02.html