The paper focuses on the protection and authentication mechanisms used to implement security in a computer system. E.g., a system where users are isolated from each other but some pieces of information are shared by everyone. A user can implement in her program ways of restricting access not provided by the system.
Protection mechanisms provides ways of permitting access.
Knowledge of how the protection mechanism works should not compromise it. A system with completely isolated users See Fig. Memory is divided into variable-size segments. Each segment contains a user program. One segment contains the supervisor program which performs CPU scheduling.