CHOO-PIR: Hint-Based Private Information Retrieval with Commodity Servers

抄録

In private information retrieval (PIR), a client retrieves an entry from a database without letting the database server know which entry is retrieved. Recently, hint-based PIR schemes have been proposed, where the client receives some hints about the database in the setup phase before the actual query begins. As an advantage, these schemes achieve sublinear database server online computation. On the other hand, the client needs to preprocess the hints with the database server, which can be costly. Moreover, the client needs to store some amount of hints, even if the client plans to query only once or a few times.

In this paper, we propose the first hint-based PIR with commodity servers, which we call CHOO-PIR. The purpose of the commodity servers is to manage the hints, so that the client does not need to be involved in the preprocessing, while the scheme can maintain the sublinear online computation. In order to preserve the privacy of the client against the commodity servers, we apply fully homomorphic encryption (FHE) or secret sharing (SS) schemes. Our FHE-based scheme achieves both client computational cost of e Õ(1) and database server online computational cost of e Õ(\sqrt{n}), where n is the database size. While the FHE-based scheme requires publickey cryptography, we can avoid such operations in the SS-based scheme, with a trade-off of client computational cost and online communication cost. The actual performance of the SS-based scheme is demonstrated through implementation results. The results show that CHOO-PIR has an advantage when the database size is large and the number of queries issued by resource-constrained clients is small.