Constants Count: Practical Improvements to Oblivious RAM

Abstract

Oblivious RAM (ORAM) is a cryptographic primitive that hides memory access patterns as seen by untrusted storage. This paper proposes Ring ORAM, the most bandwidth-efficient ORAM scheme for the small client storage setting in both theory and practice. Ring ORAM is the first tree-based ORAM whose bandwidth is independent of the ORAM bucket size, a property that unlocks multiple performance improvements. First, Ring ORAM's overall bandwidth is 2.3× to 4× better than Path ORAM, the prior-art scheme for small client storage. Second, if memory can perform simple untrusted computation, Ring ORAM achieves constant online bandwidth (∼ 60× improvement over Path ORAM for practical parameters). As a case study, we show Ring ORAM speeds up program completion time in a secure processor by 1.5× relative to Path ORAM. On the theory side, Ring ORAM features a tighter and significantly simpler analysis than Path ORAM.