Practical Frequency-Hiding Order-Preserving Encryption with Improved Update

Order-preserving encryption (OPE) that preserves the numerical ordering of plaintexts is one of the promising solutions of cloud security. In 2013, an ideally secure OPE, which reveals no additional information except for the order of underlying plaintexts, was proposed, along with the notion (mutable encryption) that ciphertexts can be changed. Unfortunately, even the ideally secure OPE can be vulnerable by inferring the underlying frequency of repeated plaintexts. To solve this problem, in 2015, Kerschbaum designed a frequency-hiding OPE (FH-OPE) scheme based on the notion of a randomized order under the strengthened security model. Later, Maffei et al. has shown that Kerschbaum’s model is imprecise, which means no such OPE scheme can exist. Moreover, they provided a new FH-OPE scheme under the corrected security model. However, their scheme requires the order information of all the encrypted plaintexts as an input; therefore, it causes relatively high overhead during encryption. In this work, we propose a more efficient FH-OPE based on Maffei et al.’ s security model and also present an improved update algorithm suitable for duplicate plaintexts.