Preserving confidentiality of security policies in data outsourcing

Recent approaches for protecting information in data outsourcing scenarios exploit the combined use of access control and cryptography. In this context, the number of keys to be distributed and managed by users can be maintained limited by using a public catalog of tokens that allow key derivation along a hierarchy. However, the public token catalog, by expressing the key derivation relationships, may leak information on the security policies (authorizations) enforced by the system, which the data owner may instead wish to maintain confidential. In this paper, we present an approach to protect the privacy of the tokens published in the public catalog. Consistently with the data outsourcing scenario, our solution exploits the use of cryptography, by adding an encryption layer to the catalog. A complicating issue in this respect is that this new encryption layer should follow a derivation path that is "reversed" with respect to the key derivation. Our approach solves this problem by combining cryptography and transitive closure information. The result is an efficient solution allowing token release and traversal of the key derivation structure only to those users authorized to access the underlying resources. We also present experimental results that illustrate the behavior of our technique in large settings.