A formal state transition model of computer security policy that describes a set of access control rules. In this formal model, the entities in a computer system are divided into abstract sets of subjects and objects. The notion of a secure state is defined, and it is proven that each state transition preserves security by moving from secure state to secure state, thereby inductively proving that the system is secure. A system state is defined to be “secure” if the only permitted access modes of subjects to objects are in accordance with a specific security policy. In order to determine whether or not a specific access mode is allowed, the clearance of a subject is compared to the classification of the object, and a determination is made as to whether the subject is authorized for the specific access mode. See star property (*-property) and simple security property.