Hierarchical and Shared Access Control

Access control ensures that only the authorized users of a system are allowed to access certain resources or tasks. Usually, according to their roles and responsibilities, users are organized in hierarchies formed by a certain number of disjoint classes. Such hierarchies are implemented by assigning a key to each class, so that the keys for descendant classes can be efficiently derived from classes higher in the hierarchy. However, pure hierarchical access may represent a limitation in many real-world cases. In fact, sometimes it is necessary to ensure access to a resource or task by considering both its directly responsible user and a group of users possessing certain credentials. In this paper, we first propose a novel model that generalizes the conventional hierarchical access control paradigm, by extending it to certain additional sets of qualified users. Afterward, we propose two constructions for hierarchical key assignment schemes in this new model, which are provably secure with respect to key indistinguishability. In particular, the former construction relies on both symmetric encryption and perfect secret sharing, whereas, the latter is based on public-key threshold broadcast encryption.