Secure and Flexible Global File Sharing

Sharing of files is a major application of computer networks, with examples ranging from LAN-based network file systems to wide-area applications such as use of version control systems in distributed software development. Identification, authentication and access control are much more challenging in this complex large-scale distributed environment. In this paper, we introduce the Distributed Credential Filesystem (DisCFS). Under DisCFS, credentials are used to identify both the files stored in the file system and the users that are permitted to access them, as well as the circumstances under which such access is allowed. As with traditional capabilities, users can delegate access rights (and thus share information) simply by issuing new credentials. Credentials allow files to be accessed by remote users that are not known a priori to the server. Our design achieves an elegant separation of policy and mechanism which is mirrored in the implementation. Our prototype implementation of DisCFS runs under OpenBSD 2.8, using a modified user-level NFS server. Our measurements suggest that flexible and secure file sharing can be made scalable at a surprisingly low performance cost.