[Acme] Remove "Proof of possession" challenge?

[Richard Barnes <[e-mail]>]

Hey ACME folks,

I just updated the editor's draft to change the name of the "proof of
possession of a prior key" challenge to "proof-of-possession-01" (from
"proofOfPossession-01").  But that got me thinking -- do we actually
need this challenge?

If I recall correctly, this was added to the initial version of the
spec because some folks from Let's Encrypt thought that they would use
it as an extra check for high-value domains with known, existing
certificates.  However, they don't seem to have gotten around to
implementing it.

Is anyone aware of CAs out there that would use
"proof-of-possession-01"?  That is, CAs that keep track of existing
certificates and require an applicant for a domain with an existing
cert to prove that they hold the corresponding private key?

If not, maybe we can streamline the spec by removing that challenge
type.  It can always get re-added in a future spec if there turns out
to be a need.

--Richard