On mutually exclusive roles and separation-of-duty

Separation-of-duty (SoD) is widely considered to be a fundamental principle in computer security. A static SoD (SSoD) policy states that in order to have all permissions necessary to complete a sensitive task, the cooperation of at least a certain number of users is required. Role-based access control (RBAC) is today's dominant access-control model. It is widely believed that one of RBAC's main strengths is that it enables the use of constraints to support policies, such as separation-of-duty. In the literature on RBAC, statically mutually exclusive roles (SMER) constraints are used to enforce SSoD policies. In this paper, we formulate and study fundamental computational problems related to the use of SMER constraints to enforce SSoD policies. We show that directly enforcing SSoD policies is intractable (coNP-complete), while checking whether an RBAC state satisfies a set of SMER constraints is efficient; however, verifying whether a given set of SMER constraints enforces an SSoD policy is also intractable (coNP-complete). We discuss the implications of these results. We show also how to generate SMER constraints that are as accurate as possible for enforcing an SSoD policy.