Bell-La Padula model

Definition
The Bell-La Padula model is

"[a] formal state transition model of computer security policy that describes a set of access control rules for hierarchical (e.g., classified) information structures. In this model the entities in a computer 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 a 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. To determine if a specific access mode is allowed, the clearance of a subject is compared to the classification of the object."