Practical and Privacy-Preserving Policy Compliance for Outsourced Data.

We consider a scenario for data outsourcing that supports performing database queries in the following three-party model: a client interested in making database queries, a data owner providing its database for client access, and a server (e.g., a cloud server) holding the (encrypted) outsourced data and helping both other parties. In this scenario, a natural problem is that of designing efficient and privacy-preserving protocols for checking compliance of a client's queries to the data owner's query compliance policy. We propose a cryptographic model for the study of such protocols, defined so that they can compose with an underlying database retrieval protocol (with no query compliance policy) in the same participant model. Our main result is a set of new protocols that satisfy a combination of natural correctness, privacy, and efficiency requirements. Technical contributions of independent interest include the use of equality-preserving encryption to produce highly practical symmetric-cryptography protocols (i.e., two orders of magnitude faster than "Yao-like" protocols), and the use of a query rewriting technique that maintains privacy of the compliance result.