To share files across wide-area networks, a number of protocols have been deployed, the most commonly used ones being FTP and HTTP. Anonymous FTP, where there is no need for authentication, offers high flexibility because any user can download or upload files to FTP servers. Similarly, in the Web architecture, access is either anonymous or subject to some sort of ad hoc authentication mechanism. This configuration is useful only in the case where file content is not sensitive. In the case where authentication is required, flexibility is greatly reduced. The only users allowed to access the server, in that case, are users who are already known to the system. As with existing network file systems, this restricts the possibility for users outside the same administrative domain to collaborate.