Opca: Enabling Optimistic Concurrent Access for Multiple Users in Oblivious Data Storage

The challenges of data privacy and security posed by data outsourcing are becoming increasingly prevalent. Oblivious RAM (ORAM)-based oblivious data storage guarantees data confidentiality through data encryption and access pattern obfuscation. However, it suffers from performance degradation and low throughput. To address these issues, the concurrency of ORAM in a multi-user scenario has been explored. We investigate several existing concurrent oblivious data storage solutions and discover that a trusted proxy is used to serve concurrent accesses between users and storage, with processing locks involved in the proxy to ensure correctness and prevent conflicts. The proxy-based system is inherently prone to pessimistic concurrency control, and as the number of users grows, a proxy might become a performance bottleneck, causing significant delays. In this study, we propose Opca, a novel oblivious data storage framework that enables optimistic concurrent access. Opca refines the proxy design by temporally storing multiple versions of modified data with labeled timestamps, committing only the latest version to the storage during a separate processing period. Opca is implemented and evaluated in different real-world storage backends with a scalable number of users, and its performance is compared to alternative schemes. Opca outperforms the state-of-the-art concurrent oblivious storage system TaoStore, which relies on a similar system setting. Our results show that Opca can improve 3.77x throughput and reduce 73.5% response time.