In the proposed mandatory access control model, arbitrary, label changing policies can be expressed. The relatively simple model can capture a wide variety of security policies, including high-water marks, downgrading, separation of duties, and Chinese Walls. The model forms the basis for a tiered approach to the formal development of secure systems, whereby security verification can be spread across what makes up the reference monitor and the security requirement specification. The advantage of this approach is that once a trusted computing base (TCB) is in place, reconfiguring it for different security requirements requires verification of just the new requirements. We illustrate the approach with a number of examples, including one policy that permits high-level subjects to make relabelling requests on low-level objects; the policy is multilevel secure.
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