If it is API only, why not just use simple token auth? If you handle registration over API then that is just a simple username/password or whatever you want, just compare it to the database. The hard part of auth is either the UI (which does not exist for API endpoints beyond json or graphql or so) and third-party auth like google or github or so (which ueberauth handles wonderfully). Auth_pipe is looking useful soon as well and would also be fantastic for API endpoints. What more are you wanting beyond those?
