Data outsourcing is emerging today as a successful paradigm allowing users and organizations to exploit external services for the distribution of resources. A crucial problem to be addressed in this context concerns the enforcement of selective authorization policies and the support of policy updates in dynamic scenarios. In this paper, we present a novel solution to the enforcement of access control and the management of its evolution. Our proposal is based on the application of selective encryption as a means to enforce authorizations. Two layers of encryption are imposed on data: the inner layer is imposed by the owner for providing initial protection, the outer layer is imposed by the server to reflect policy modifications. The combination of the two layers provides an efficient and robust solution. The paper presents a model, an algorithm for the management of the two layers, and an analysis to identify and therefore counteract possible information exposure risks.