POPE: Partial Order Preserving Encoding

Recently there has been much interest in performing search queries over encrypted data to enable functionality while protecting sensitive data. One particularly efficient mechanism for executing such queries is order-preserving encryption/encoding (OPE) which results in ciphertexts that preserve the relative order of the underlying plaintexts thus allowing range and comparison queries to be performed directly on ciphertexts. Recently, Popa et al. (SP 2013) gave the first construction of an ideally-secure OPE scheme and Kerschbaum (CCS 2015) showed how to achieve the even stronger notion of frequency-hiding OPE. However, as Naveed et al. (CCS 2015) have recently demonstrated, these constructions remain vulnerable to several attacks. Additionally, all previous ideal OPE schemes (with or without frequency-hiding) either require a large round complexity of O(log n) rounds for each insertion, or a large persistent client storage of size O(n), where n is the number of items in the database. It is thus desirable to achieve a range query scheme addressing both issues gracefully. In this paper, we propose an alternative approach to range queries over encrypted data that is optimized to support insert-heavy workloads as are common in "big data" applications while still maintaining search functionality and achieving stronger security. Specifically, we propose a new primitive called partial order preserving encoding (POPE) that achieves ideal OPE security with frequency hiding and also leaves a sizable fraction of the data pairwise incomparable. Using only O(1) persistent and O(n^e) non-persistent client storage for 0<e<1, our POPE scheme provides extremely fast batch insertion consisting of a single round, and efficient search with O(1) amortized cost for up to O(n^(1-e)) search queries. This improved security and performance makes our scheme better suited for today's insert-heavy databases.