Many high-level security requirements are about the allowed flow of information in programs, but are difficult to make precise because they involve selective downgrading. Quite a few mutually incompatible and ad-hoc approaches have been proposed for specifying and enforcing downgrading policies. Prior surveys of these approaches have not provided a unifying technical framework. Notions from epistemic logic have emerged as a good approach to policy semantics but are considerably removed from well developed static and dynamic enforcement techniques. We develop a unified framework for expressing, giving meaning and enforcing information downgrading policies that builds on commonly known and widely deployed concepts and techniques, especially static and dynamic assertion checking. These concepts should make information flow accessible and enable developers without special training to specify precise policies. The unified framework allows to directly compare different policy specification styles and enforce them by leveraging existing techniques.
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