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The content access control problem appears in any context with a set of users and resources. The difference in access rights of the users defines classes where members of a given class have exactly the same access rights. A hierarchy can be defined on the classes. Linear hierarchies constitute a particulary interesting type of hierarchies. They appear in a wide range of applications such as secure multi-layered data streaming and communications within security corps. Many proposals have dealt with key management issues for tree hierarchies but they result in unjustified overhead when applied to linear hierarchies. In this paper, we discuss the general problem of content access control in a hierarchy (CACH). Thereafter, we present the main requirements in key management to ensure confidentiality in linear hierarchies. In particular, we define a model to make a uniform and coherent description of the existing key management schemes. Thereafter, we propose an efficient key management scheme for linear hierarchies that not only provides mechanisms to manage membership changes but also hierarchy shape changes, and we describe it using our model. We conduct intensive simulations which show that our solution scales very well in terms of storage, bandwidth, and computation. Finally, we determine the complexity of some well-known key management schemes and compare them to the complexity of our scheme. This comparison shows that our scheme offers efficient compromises in complexity and overall overheads.

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