Typing illegal information flows as program effects

Specification of information flow policies is classically based on a security labeling and a lattice of security levels that establishes how information can flow between security levels. We present a type and effect system for determining the least permissive relaxation of a given confidentiality policy that allows to type a program, given a fixed security labeling. To this end, sets of illegal information flows are represented as downward closure operators (here referred to as flow kernels) on a given lattice of security levels. Illegal information flows can then be seen as program effects, and their representation as flow kernels subsumes in granularity previous lattice-oriented representations of information flow policies. Effect soundness, optimality and preservation results are presented for the proposed type and effect system, for programs written in a concurrent higher-order imperative lambda-calculus with reference creation. Our type and effect system provides a mechanism for deriving the flow kernel that characterizes the illegal flows that occur within a program, and which can be used to support runtime decisions of compliance to other policies. This point is illustrated by means of an application to a setting where local programs run under the control of a dynamic allowed flow policy.