Querying for Queries: Indexes of Queries for Efficient and Expressive IT-PIR

We propose indexes of queries, a novel mechanism for supporting efficient, expressive, and information-theoretically private singleround queries over multi-server PIR databases. Our approach decouples the way that users construct their requests for data from the physical layout of the remote data store, thereby enabling users to fetch data using "contextual" queries that specify which data they seek, as opposed to "positional" queries that specify where those data happen to reside. For example, an open-access eprint repository could employ indexes of queries to let researchers fetch academic articles via PIR queries such as for "this year's 5 most cited papers about PIR" or "the 3 most recently posted papers about PIR". Our basic approach is compatible with any PIR protocol in the ubiquitous "vector-matrix" model for PIR, though the most sophisticated and useful of our constructions rely on some nice algebraic properties of Goldberg's IT-PIR protocol (Oakland 2007). We have implemented our techniques as an extension to Percy++, an open-source implementation of Goldberg's IT-PIR protocol. Our experiments indicate that the new techniques can greatly improve not only utility for private information retrievers but also efficiency for private information retrievers and servers alike.