Access control via belnap logic: intuitive, expressive, and analyzable policy composition

Abstract

Access control to IT systems increasingly relies on the ability to compose policies. There is thus bene t in any framework for policy composition that is intuitive, formal (and so \an- alyzable" and \implementable"), expressive, independent of speci c application domains, and yet able to be extended to create domain-speci c instances. Here we develop such a framework based on Belnap logic. An access-control policy is interpreted as a four-valued predicate that maps access requests to either grant, deny, con ict, or unspeci ed { the four values of the Bel- nap bilattice. We de ne an expressive access-control policy language PBel, having composition operators based on the operators of Belnap logic. Natural orderings on policies are obtained by lifting the truth and information orderings of the Belnap bilattice. These orderings lead to a query language in which policy analyses, e.g. con ict freedom, can be speci ed. Policy analysis is supported through a reduction of the validity of policy queries to the validity of propositional formulas on predicates over access requests. We evaluate our approach through rewall policy and RBAC policy examples, and discuss domain-speci c and generic extensions of our policy language.