Sequentially Consistent Concurrent Encrypted Multimaps

Encrypted data structures are essential for designing efficient encrypted search algorithms and secure databases. However, a critical aspect that has not been adequately addressed is the concurrent nature of modern databases, which allow multiple operations to be executed simultaneously. Agarwal, Kamara, and Moataz (Asiacrypt 2024) recently initiated the study of concurrent encrypted data structures and introduced formalisms for their design and analysis.Building on their foundational work, we adapt their security definitions to support sequential consistency instead of linearizability. While linearizability offers a strong correctness guarantee by ensuring operations appear to occur instantaneously, sequential consistency allows operations to be executed in a consistent order without immediate synchronization across clients, making it more efficient for concurrent environments. We present a new concurrent encrypted multimap (EMM), denoted as SCM, which achieves sequential consistency and provides significant improvements in both asymptotic and empirical efficiency compared to their linearizable EMM scheme, TST.Additionally, we develop a benchmarking suite designed to assess the performance of concurrent EMMs, extending the widely used YCSB benchmark to accommodate multimaps that allow multiple values to be associated with a single key. Our results demonstrate that SCM outperforms TST across various workloads and datasets, especially as the number of concurrent operations increases. In our experiments with 16 concurrent clients, SCM has up to 357× faster P95 read latency (with the best read performance as the overall percentage of reads decreases) and up to 69× faster P95 write latency (with the best write performance as the percentage of writes approaches 50%) than TST, demonstrating SCM effectively balances efficiency and correctness.