There are currently two existing open-source server implementations of the Bitcache REST API: one, bitcached, is written in Ruby and is included in the Bitcache reference implementation; the other is written in PHP and available as an add-on module for the Drupal content management framework.
Note that the question of authentication is entirely orthogonal to this API; authentication would typically be implemented using standard HTTP methods, but a cookie-based session, or indeed a publicly-available repository, is equally compatible with the REST API.
(More documentation coming soon.)
To be written.
To be written.