Absorbing covers and intransitive non-interference

The paper gives necessary and sufficient conditions for a system to satisfy intransitive non-interference. Security is defined in terms of allowable flows of information among action domains as represented by an interferes relation /spl sim/>. We examine properties of special sets called basis elements generated from the relation /spl sim/> and introduce the notion of absorbing covers which is associated with the standard unwinding theorems for non-interference. Our approach separates the equivalence relation arguments from the non-interference properties, and as a by product, we develop a decision procedure for non-interference. An upper bound on the number of iterations needed for termination of the procedure is provided.<>