What, indeed, is intransitive noninterference?

Abstract

This paper argues that Haigh and Young's definition of noninterference for intransitive security policies admits information flows that are not in accordance with the intuitions it seeks to formalise. Several alternative definitions are discussed, which are shown to be equivalent to the classical definition of noninterference with respect to transitive policies. Rushby's unwinding conditions for intransitive noninterference are shown to be sound and complete for one of these definitions, TA-security. Access control systems compatible with a policy are shown to be TA-secure, and it is also shown that TA-security implies that the system can be interpreted as an access control system. © Springer-Verlag Berlin Heidelberg 2007.