Private mappings

[masomel]

Section 4.3.1 of the CONIKS paper and section 5 of the CONIKS 2.0 report describes a mechanism for making name-to-key mappings in the server private. This is mostly to protect the visibility of public keys via encryption, but could potentially be extended to hiding usernames as well. Only a whitelisted group of users could then access the private mappings. Implementing this requires a whitelisting protocol as well as a corresponding verification protocol.