Cassandra: Distributed Access Control Policies with Tunable Expressiveness

We study the specification of access control policy inlarge-scale distributed systems. Our work on real-worldpolicies has shown that standard policy idioms such as rolehierarchy or role delegation occur in practice in many subtle variants. A policy specification language should therefore be able to express this variety of features smoothly,rather than add them as specific features in an ad hoc way,as is the case in many existing languages.We present Cassandra, a role-based trust managementsystem with an elegant and readable policy specificationlanguage based on Datalog with constraints. The expressiveness (and computational complexity) of the languagecan be adjusted by choosing an appropriate constraint domain. With just five special predicates, we can easily express a wide range of policies including role hierarchy,role delegation, separation of duties, cascading revocation, automatic credential discovery and trust negotiation. Cassandra has a formal semantics for query evaluation andfor the access control enforcement engine. We use a goal-oriented distributed policy evaluation algorithm that is efficient and guarantees termination. Initial performance results for our prototype implementation have been promising.