Abstract
Distributed ORAM (DORAM) is a multi-server variant of Oblivious RAM. Originally proposed to lower bandwidth, DORAM has recently been of great interest due to its applicability to secure computation in the RAM model, where circuit complexity and rounds of communication are equally important metrics of efficiency. All prior DORAM constructions either involve linear work per server (e.g., Floram) or logarithmic rounds of communication between servers (e.g., square root ORAM). In this work, we construct the first DORAM schemes in the 2-server, semi-honest setting that simultaneously achieve sublinear server computation and constant rounds of communication. We provide two constant-round constructions, one based on square root ORAM that has \(O(\sqrt{N}\log N)\) local computation and another based on secure computation of a doubly efficient PIR that achieves local computation of \(O(N ^\epsilon )\) for any \(\epsilon > 0\) but that allows the servers to distinguish between reads and writes. As a building block in the latter construction, we provide secure computation protocols for evaluation and interpolation of multivariate polynomials based on the Fast Fourier Transform, which may be of independent interest.
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Notes
- 1.
The exact functionality including the indicator bit is not included in their constructions, but they can be easily be extended with an additional round of a conditional computations.
- 2.
A concurrent work by Boyle et al. [4] relies on an equivalent assumption called Oblivious LDC.
- 3.
For any value of \(t< \log (N +t)\) then the cost of \(\varPi _{store}\) controls, but in our setting we consider a \(t\) greater than that.
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Acknowledgments
We gratefully acknowledge conversations with Daniel Wichs and Jack Doerner for their valuable insights. This material is based upon work supported by the National Science Foundation under Grants 1414119, 1718135, 1750795, and 1931714.
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Hamlin, A., Varia, M. (2021). Two-Server Distributed ORAM with Sublinear Computation and Constant Rounds. In: Garay, J.A. (eds) Public-Key Cryptography – PKC 2021. PKC 2021. Lecture Notes in Computer Science(), vol 12711. Springer, Cham. https://doi.org/10.1007/978-3-030-75248-4_18
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