Revisiting Definitional Foundations of Oblivious RAM for Secure Processor Implementations

Abstract: Oblivious RAM (ORAM) is a renowned technique to hide the access patterns of an application to an untrusted memory. According to the standard ORAM definition presented by Goldreich and Ostrovsky, two ORAM access sequences must be computationally indistinguishable if the lengths of these sequences are identically distributed. An artifact of this definition is that it does not apply to modern ORAM implementations adapted in current secure processors technology because of their arbitrary lengths of memory access sequences depending on programs' behaviors (their termination times). As a result, the ORAM definition does not directly apply; the theoretical foundations of ORAM do not clearly argue about the timing and termination channels. This paper conducts a first rigorous study of the standard Goldreich-Ostrovsky ORAM definition in view of modern practical ORAMs (e.g., Path ORAM) and demonstrates the gap between theoretical foundations and real implementations. A new ORAM formulation which clearly separates out termination channel leakage is proposed. It is shown how this definition implies the standard ORAM definition (for finite length input access sequences) and better fits the modern practical ORAM implementations. The proposed definition relaxes the constraints around the stash size and overflow probability for Path ORAM, and essentially transforms its security argument into a performance consideration problem. Finally, a `strong' ORAM formulation which clearly includes obfuscation of termination leakage is shown to imply our new ORAM formulation and applies to ORAM for outsourced disk storage. In this strong formulation constraints are not relaxed and the security argument for Path ORAM remains complex as one needs to prove that the stash overflows with negligible probability.