Biblio

This paper analyzes the formalizations of information-theoretic security for the fundamental primitives in cryptography: symmetric-key encryption and key agreement. Revisiting the previous results, we can formalize information-theoretic security using different methods, by extending Shannon's perfect secrecy, by information-theoretic analogues of indistinguishability and semantic security, and by the frameworks for composability of protocols. We show the relationships among the security formalizations and obtain the following results. First, in the case of encryption, there are significant gaps among the formalizations, and a certain type of relaxed perfect secrecy or a variant of information-theoretic indistinguishability is the strongest notion. Second, in the case of key agreement, there are significant gaps among the formalizations, and a certain type of relaxed perfect secrecy is the strongest notion. In particular, in both encryption and key agreement, the formalization of composable security is not stronger than any other formalizations. Furthermore, as an application of the relationships in encryption and key agreement, we simultaneously derive a family of lower bounds on the size of secret keys and security quantities required under the above formalizations, which also implies the importance and usefulness of the relationships.

In this paper, we present an open cloud DRM service provider to protect the digital content's copyright. The proposed architecture enables the service providers to use an on-the fly DRM technique with digital signature and symmetric-key encryption. Unlike other similar works, our system does not keep the encrypted digital content but lets the content creators do so in their own cloud storage. Moreover, the key used for symmetric encryption are managed in an extremely secure way by means of the key fission engine and the key fusion engine. The ideas behind the two engines are taken from the works in secure network coding and secret sharing. Although the use of secret sharing and secure network coding for the storage of digital content is proposed in some other works, this paper is the first one employing those ideas only for key management while letting the content be stored in the owner's cloud storage. In addition, we implement an Android SDK for e-Book readers to be compatible with our proposed open cloud DRM service provider. The experimental results demonstrate that our proposal is feasible for the real e-Book market, especially for individual businesses.