Optimizing ORAM and Using It Efficiently for Secure Computation

Abstract

Oblivious RAM (ORAM) allows a client to access her data on a remote server while hiding the access pattern (which locations she is accessing) from the server. Beyond its immediate utility in allowing private computation over a client’s outsourced data, ORAM also allows mutually distrustful parties to run secure-computations over their joint data with sublinear on-line complexity. In this work we revisit the tree-based ORAM of Shi et al. [20] and show how to optimize its performance as a stand-alone scheme, as well as its performance within higher level constructions. More specifically, we make several contributions:

• We describe two optimizations to the tree-based ORAM protocol of Shi et al., one reducing the storage overhead of that protocol by an O(k) multiplicative factor, and another reducing its time complexity by an O(logk) multiplicative factor, where k is the security parameter. Our scheme also enjoys a much simpler and tighter analysis than the original protocol.

• We describe a protocol for binary search over this ORAM construction, where the entire binary search operation is done in the same complexity as a single ORAM access (as opposed to logn accesses for the naive protocol). We then describe simple uses of this binary-search protocol for things like range queries and keyword search.

• We show how the ORAM protocol itself and our binary-search protocol can be implemented efficiently as secure computation, using somewhat-homomorphic encryption.

Since memory accesses by address (ORAM access) or by value (binary search) are basic and prevalent operations, we believe that these optimizations can be used to significantly speed-up many higher-level protocols for secure computation.