In this paper, we present a family of distributed ORAM constructions that follow the hierarchical approach of Goldreich and Ostrovsky [GO96]. We enhance known techniques, and develop new ones, to take better advantage of the existence of multiple servers. By plugging efficient known hashing schemes in our constructions, we get the following results:
1. For any number $m\geq 2$ of servers, we show an $m$-server ORAM scheme with $O(\log N/\log\log N)$ overhead, and block size $\Omega(\log^2 N)$. This scheme is private even against an $(m-1)$-server collusion. 2. A three-server ORAM construction with $O(\omega(1)\cdot\log N/\log\log N)$ overhead and a block size almost logarithmic, i.e. $\Omega(\log^{1+\epsilon}N)$.
We also investigate a model where the servers are allowed to perform a linear amount of light local computations, and show that constant overhead is achievable in this model, through a simple four-server ORAM protocol. From theoretical viewpoint, this is the first ORAM scheme with asymptotic constant overhead, and polylogarithmic block size, that does not use homomorphic encryption. Practically speaking, although we do not provide an implementation of the suggested construction, evidence from related work (e.g. [DS17]) confirms that despite the linear computational overhead, our construction is practical, in particular when applied to secure computation.
Category / Keywords: Oblivious RAM, Multi-Server Setting, Secure Computation, Private Storage Original Publication (in the same form): IACR-PKC-2019 Date: received 17 Jan 2019 Contact author: eyalk at cs technion ac il, tamer mour at gmail com Available format(s): PDF | BibTeX Citation Version: 20190125:203209 (All versions of this report) Short URL: ia.cr/2019/048