This is storage for Chatelain, a password manager. It holds encrypted blobs and hands them back. There is nothing to log in to here.
It cannot read what it stores. Everything arrives encrypted by the client, and this server holds no key — which is the point, and also why it is unremarkable that you can see this page.
The application is served from somewhere else, on purpose: storage and code are kept apart so that a compromise of one is not a compromise of the other.
Machine-readable at
/v0/<root>/. If you are looking for the
protocol, it is in the project repository.