Cover Stories for Key Attributes—Expanded Database Access Control

Abstract

In this chapter, we extend the multi-level secure (MLS) data model to include nonkey related cover stories so that key attributes can have different values at different security levels. MLS data models require the classification of data and users into multiple security levels. In MLS systems, cover stories allow information provided to users at lower security levels to differ from information provided to users at higher security levels. Previous versions of the MLS model did not permit cover stories for key attributes because the key is used to relate the various cover stories for a particular entity. We present the necessary model changes and modifications to the relational algebra, which are required to implement cover stories for keys. We demonstrate the improvements made by these changes, illustrate the increased expressiveness of the model, and determine the soundness of a database, based on the described concepts.