EP-ORAM: Efficient NVM-Friendly Path Eviction for Ring ORAM in Hybrid Memory

Recent studies showed that only ORAM (oblivious RAM) can securely protect memory access patterns (i.e., data privacy) on modern computer systems. Ring ORAM is a promising ORAM protocol as it demands O(1) memory accesses for servicing each user memory request. However, Ring ORAM exhibits low memory utilization, i.e., its memory requirement is 4.8× of the protected user space. While adopting NVM (non-volatile memory) can alleviate the memory requirement, a simple implementation tends to introduce large performance degradation, preventing its adoption in practice.In this paper, we propose EP-ORAM, an NVM-friendly Ring ORAM implementation on DRAM/NVM hybrid memory. EP-ORAM is developed based on two key observations: (1) for tree-based Ring ORAM memory organization, saving bottom levels in NVM can dramatically reduce the DRAM memory requirement; (2) the tradeoffs among Ring ORAM operations expose design opportunities without security compromise. We, therefore, propose to save the bottom levels of the ORAM tree in NVM and shorten the path of EvictPath operation, which not only mitigates the number of NVM writes but also speeds up the execution. Our experimental results show that, under the design constraints of similar performance as the baseline that saves two bottom levels in NVM, EP-ORAM helps to save three levels in NVM, achieving 50% DRAM space reduction. In addition, EP-ORAM reduces the NVM writes by 15%.