Recent databases are implemented as in-memory columnstores. Adjustable encryption offers a solution to encrypted database processing in the cloud.We show that the two technologies play well together by providing an analysis and prototype results that demonstrate the impact of mechanisms at the database side (dictionaries and their compression) and cryptographic mechanisms at the adjustable encryption side (order-preserving, homomorphic, deterministic and probabilistic encryption).

Abstract. In order to perform a join in a deterministically, adjustably encrypted database one has to re-encrypt at least one column. The problem is to select that column that will result in the minimum number of re-encryptions even under an unknown schedule of joins. Naive strategies may perform too many or even infinitely many re-encryptions. We provide two strategies that allow for a much better performance. In particular the asymptotic behavior is O(n 3/2 ) resp. O(n log n) re-encryptions for n columns. We show that there can be no algorithm better than O(n log n). We further extend our result to element-wise re-encryptions and show experimentally that our algorithm results in the optimal cost in 41% of the cases.

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