There are two main types of security in database systems: discretionary security and mandatory security. Discretionary security restricts access to data items at the discretion of the owner. Most commercial database management systems (DBMS) employ some form of discretionary security by controlling access privileges and modes of data users (Griffiths & Wade, 1976). Discretionary security is not adequate in a multilevel secure environment, however, because it does not prevent Trojan horse attacks and provides a low level of assurance. Mandatory security, on the other hand, restricts access of data items to cleared database users. It is widely exploited in military applications and provides a high assurance.