Polyinstantation for Cover Stories

In this paper we study the use of polyinstantiation, for the purpose of implementing cover stories in multilevel secure relational database systems. We define a particular semantics for polyinstantiation called PCS (i.e., polyinstantiation for cover stories). PCS allows two alternatives for each attribute (or attribute group) of a multilevel entity: (i) no polyinstantiation, or (ii) polyinstantiation at the explicit request of a user to whom the polyinstantiation is visible. PCS strictly limits the extent of polyinstantiation by requiring that each entity in a multilevel relation has at most one tuple per security class. We demonstrate that PCS provides a natural, intuitive and useful technique for implementing cover stories. A particularly attractive feature of PCS is its run-time flexibility regarding the use of cover stories. A particular attribute may have cover stories for some entities and not for others. Even for the same entity, a particular attribute may be polyinstantiated at some time and not at other times.

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