Paper 2024/1391 Original: eprint.iacr.org/2024/1391 ↗

Scalable Equi-Join Queries over Encrypted Database

Abstract

Secure join queries over encrypted databases, the most expressive class of SQL queries, have attracted extensive attention recently. The state-of-the-art JXT (Jutla et al. ASIACRYPT 2022) enables join queries on encrypted relational databases without pre-computing all possible joins. However, JXT can merely support join queries over two tables (in encrypted databases) with some high-entropy join attributes. In this paper, we propose an equi-join query protocol over two tables dubbed JXT+, that allows the join attributes with arbitrary names instead of JXT requiring the identical name for join attributes. JXT+ reduces the query complexity from $O(\ell_1 \cdot \ell_2)$ to $O(\ell_1)$ as compared to JXT, where $\ell_1$ and $\ell_2$ denote the numbers of matching records in two tables respectively. Furthermore, we present JXT++, the \emph{first} equi-join queries across three or more tables over encrypted databases without pre-computation. Specifically, JXT++ supports joins of arbitrary attributes, i.e., all attributes (even low-entropy) can be candidates for join, while JXT requires high-entropy join attributes. In addition, JXT++ can alleviate sub-query leakage on three or more tables, which hides the leakage from the matching records of two-table join. Finally, we implement and compare our proposed schemes with the state-of-the-art JXT. The experimental results demonstrate that both of our schemes are superior to JXT in search and storage costs. In particular, JXT+ (resp., JXT++) brings a saving of 49% (resp., 68%) in server storage cost and achieves a speedup of 51.7$\times$ (resp., 54.3$\times$) in search latency.