The Taos operating system  uses a narrow API to control access to security-aware services, e.g., the file system. It uses credentials for delegation of access rights between principals. DisCFS utilizes these well-known techniques to extend a real world protocol such as NFS. This makes our approach easily portable and more flexible than specialized operating system mechanisms.
The concept of credential-based access control also appears in the Exokernel . In this system, users can create new capabilities at will, but the new capability must be dominated by an existing one. This is similar to our chains of certificates, but is limited by the fact that permissions are hardwired into the system, and the hierarchical capability tree may be only up to 8 levels deep. In our system, certificate chains can be of arbitrary length, and the access policy can consider factors such as time-of-day, so that, for example, leisure-related files may not be available during office hours.