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Low-Cost Hiding of the Query Pattern

Several attacks have shown that the leakage from the access pattern in searchable encryption is dangerous. Attacks exploiting leakage from the query pattern are as dangerous, but less explored. While there are known, lightweight countermeasures to hide information in the access pattern by padding the ciphertexts, the same is not true for the query pattern. Oblivious RAM hides the query patterns, but requires a logarithmic overhead in the size of the database and hence will become even slower as data grows. In this paper we present a query smoothing algorithm to hide the frequency information in the query pattern of searchable encryption schemes by introducing fake queries. Our method only introduces a constant overhead of 7 to 13 fake queries per real query in our experiments. Furthermore, we show that our query smoothing algorithm can also be applied to range-searchable encryption schemes and then prevents all recent plaintext recovery attacks.