Abstract: We study the problem of privately performing database queries (i.e., keyword searches and conjunctions over them), where a server provides its own database for a client’s query-based access. We propose a cryptographic model for the study of such protocols, by expanding previous well-studied models of keyword search and private information retrieval to incorporate a more practical data model: a time-varying, multi-attribute and multiple-occurrence database table. Our first result is a 2-party private database retrieval protocol. This is the first protocol that preserves query and data privacy on such a practical data model. Like all previous work in private information retrieval and keyword search, this protocol still satisfies server time complexity linear in the database size. Our main result is a private database retrieval protocol in a 3-party model where encrypted data is outsourced to a third party (i.e., a cloud server), satisfying highly desirable privacy and efficiency properties; most notably: (1) no unintended information is leaked to clients or servers, and only minimal ‘access pattern’ information is leaked to the third party; (2) for each query, all parties run in time only logarithmic in the number of database records; (3) the protocol’s runtime is practical for real-life applications, as shown in our implementation where we achieve response time that is only a small constant slower than commercial non-private protocols like MySQL. This is the first protocol that achieves privacy of database and query content with practical performance. Finally, we show a second private database retrieval protocol in the 3-party model for which we can show that no unintended information is leaked to an adversary corrupting both clients and third parties, at an only constant additional performance overhead cost.
Keywords: Privacy for outsourced data, privacy and big data, database as a service
