Biblio

Data loss is perceived as one of the major threats for cloud storage. Consequently, the security community developed several challenge-response protocols that allow a user to remotely verify whether an outsourced file is still intact. However, two important practical problems have not yet been considered. First, clients commonly outsource multiple files of different sizes, raising the question how to formalize such a scheme and in particular ensuring that all files can be simultaneously audited. Second, in case auditing of the files fails, existing schemes do not provide a client with any method to prove if the original files are still recoverable. We address both problems and describe appropriate solutions. The first problem is tackled by providing a new type of "Proofs of Retrievability" scheme, enabling a client to check all files simultaneously in a compact way. The second problem is solved by defining a novel procedure called "Proofs of Recoverability", enabling a client to obtain an assurance whether a file is recoverable or irreparably damaged. Finally, we present a combination of both schemes allowing the client to check the recoverability of all her original files, thus ensuring cloud storage file recoverability.

Proofs of Data Possession/Retrievability (PoDP/PoR) schemes are essential to cloud storage services, since they can increase clients' confidence on the integrity and availability of their data. The majority of PoDP/PoR schemes are constructed from homomorphic linear authentication (HLA) schemes, which decrease the price of communication between the client and the server. In this paper, a new subclass of authentication codes, named ε-authentication codes, is proposed, and a modular construction of HLA schemes from ε-authentication codes is presented. We prove that the security notions of HLA schemes are closely related to the size of the authenticator/tag space and the successful probability of impersonation attacks (with non-zero source states) of the underlying ε-authentication codes. We show that most of HLA schemes used for the PoDP/PoR schemes are instantiations of our modular construction from some ε-authentication codes. Following this line, an algebraic-curves-based ε-authentication code yields a new HLA scheme.