Enhancing Oblivious RAM Performance Using Dynamic Prefetching

Oblivious RAM (ORAM) is an established technique to hide the access pattern to an untrusted storage system. With ORAM, a curious adversary cannot tell what data address the user is accessing when observing the bits moving between the user and the storage system. All existing ORAM schemes achieve obliviousness by adding redundancy to the storage system, i.e., each access is turned into multiple random accesses. Such redundancy incurs a large performance overhead. Though traditional data prefetching techniques successfully hide memory latency in DRAM based systems, it turns out that they do not work well for ORAM. In this paper, we exploit ORAM locality by taking advantage of the ORAM internal structures. Though it might seem apparent that obliviousness and locality are two contradictory concepts, we challenge this intuition by exploiting data locality in ORAM without sacrificing provable security. In particular, we propose an ORAM prefetching technique called dynamic super block scheme and comprehensively explore its design space. The dynamic super block scheme detects data locality in the program’s working set at runtime, and exploits the locality in a data-independent way. Our simulation results show that with dynamic super block scheme, ORAM performance without super blocks can be significantly improved. After adding timing protection to ORAM, the average performance gain is 25.5% (up to 49.4%) over the baseline ORAM and 16.6% (up to 30.1%) over the best ORAM prefetching technique proposed previously.