Computer security mechanisms involve the design of computing hardware, software as well as operational procedures. Some of these ideas are illustrated in the following sections using the Unix operating system. Access to a Unix computer system is controlled through an account password. The passwords must be stored in the system so that the system may authenticate individual users. The password database is encrypted as briefly described in the following Unix manual page. This password encryption scheme [Gram 84,Morr 79] was developed by Robert H. Morris.