Private information retrieval

Publicly accessible databases are an indispensable resource for retrieving up-to-date information. But they also pose a significant risk to the privac y of the ur;er, since a curious database operator can follow the user's que ries and infer what the user is after. Indeed, in cases where the users' in tentions are to be kept secret, users are often cautious about accessing th e database. It can be shown that when accessing a single database, to compl etely guarantee the privacy of the user, the whole database should be down- loaded; namely it bits should be communicated (where n is the number of bit s in the database). In this work, we investigate whether by replicating the database; more effi cient solutions to the private retrieval problem can be obtained. We descri be schemes that enable a user to access k replicated copies of a database ( k greater than or equal to 2) and privately retrieve information stored in the database. This means that each individual server (holding a replicated copy of the database) gets no information on the identity of the item retri eved by the user. Our schemes use the replication to gain substantial savin g. In particular, we present a two-server scheme with communication complex ity O(n(1/3)).