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Relationship-based access control: its expression and enforcement through hybrid logic

Access control policy is typically defined in terms of attributes, but in many applications it is more natural to define permissions in terms of relationships that resources, systems, and contexts may enjoy. The paradigm of relationship-based access control has been proposed to address this issue, and modal logic has been used as a technical foundation.
 We argue here that hybrid logic -- a natural and well-established extension of modal logic -- addresses limitations in the ability of modal logic to express certain relationships.
 We identify a fragment of hybrid logic to be used for expressing relationship-based access-control policies, show that this fragment supports important policy idioms, and demonstrate that it removes an exponential penalty in existing attempts of specifying complex relationships such as "at least three friends". We also capture the previously studied notion of relational policies in a static type system.