Towards a Separation of Semantic and CCA Security for Public Key Encryption
We address the question of whether or not semantically secure public-key encryption primitives imply the existence of chosen ciphertext attack (CCA) secure primitives. We show a black-box separation, using the methodology introduced by Impagliazzo and Rudich, for a large non-trivial class of constructions. In particular, we show that if the proposed CCA construction's decryption algorithm does not query the semantically secure primitive's encryption algorithm, then the proposed construction cannot be CCA secure
A Random Server Model for Private Information Retrieval (or How to Achieve Information Theoretic PIR Avoiding Data Replication)
Private information retrieval (PIR) schemes enable users to obtain information from databases while keeping their queries secret from the database managers. We propose a new model for PIR, utilizing auxiliary random servers to provide privacy services for database access. In this model, prior to any on-line communication where users request queries, the database engages in an initial preprocessing setup stage with the random servers. Using this model we achieve the first PIR information theoretic solution in which the database does not need to give away its data to be replicated, and with minimal on-line computation cost for the database. This solves privacy and efficiency problems inherent to all previous solutions. In particular, all previous information theoretic PIR schemes required multiple replications of the database into separate entities which are not allowed to communicate with each other; and in all previous schemes (including ones which do not achieve information theoretic security), the amount of computation performed by the database on-line for every query is at least linear in the size of the database. In contrast, in our solutions the database does not give away its contents to any other entity; and after the initial setup stage, which costs at most O(n log n) in computation, the database needs to perform only O(1) amount of computation to answer questions of users on-line. All the extra on-line computation is done by the auxiliary random servers.