Efficient Private Information Retrieval Over Unsynchronized Databases

Web search histories can reveal detailed and sensitive information about people. Private information retrieval (PIR) tackles this potential privacy violation by allowing users to retrieve the $w$ th record of a database without revealing $w$ to the server. However, most known PIR schemes are either very inefficient (and therefore unlikely to gain traction in a practical sense) or reliant on some restrictive assumptions. In this paper, we consider an efficient class of schemes called multi-server PIR. Multi-server PIR assumes that the client communicates with multiple, non-colluding servers, each possessing an identical copy of the database. Significant prior work has gone towards relaxing the anti-collusion assumption, but the literature does not address the assumption that servers store perfectly-synchronized databases. This seems implausible, especially if servers are not meant to collude. We propose the first multi-server PIR scheme to return the desired record even when servers\´ databases are not perfectly synchronized. Our scheme asymptotically has the same computational and communication complexity as state-of-the-art PIR schemes for synchronized databases; this comes at the expense of probabilistic success and two rounds of communication (most existing schemes require only one). Additionally, this approach efficiently processes multiple concurrent PIR queries.