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An Optimal Distributed Discrete Log Protocol with Applications to Homomorphic Secret Sharing

  • Itai Dinur
  • Nathan Keller
  • Ohad Klein
Conference paper
Part of the Lecture Notes in Computer Science book series (LNCS, volume 10993)

Abstract

The distributed discrete logarithm (DDL) problem was introduced by Boyle et al. at CRYPTO 2016. A protocol solving this problem was the main tool used in the share conversion procedure of their homomorphic secret sharing (HSS) scheme which allows non-interactive evaluation of branching programs among two parties over shares of secret inputs.

Let g be a generator of a multiplicative group \(\mathbb {G}\). Given a random group element \(g^{x}\) and an unknown integer \(b \in [-M,M]\) for a small M, two parties A and B (that cannot communicate) successfully solve DDL if \(A(g^{x}) - B(g^{x+b}) = b\). Otherwise, the parties err. In the DDL protocol of Boyle et al., A and B run in time T and have error probability that is roughly linear in M/T. Since it has a significant impact on the HSS scheme’s performance, a major open problem raised by Boyle et al. was to reduce the error probability as a function of T.

In this paper we devise a new DDL protocol that substantially reduces the error probability to \(O(M \cdot T^{-2})\). Our new protocol improves the asymptotic evaluation time complexity of the HSS scheme by Boyle et al. on branching programs of size S from \(O(S^2)\) to \(O(S^{3/2})\). We further show that our protocol is optimal up to a constant factor for all relevant cryptographic group families, unless one can solve the discrete logarithm problem in a short interval of length R in time \(o(\sqrt{R})\).

Our DDL protocol is based on a new type of random walk that is composed of several iterations in which the expected step length gradually increases. We believe that this random walk is of independent interest and will find additional applications.

Keywords

Homomorphic secret sharing Share conversion Fully homomorphic encryption Discrete logarithm Discrete logarithm in a short interval Random walk 

Notes

Acknowledgements

The authors would like to thanks Elette Boyle, Niv Gilboa, Yuval Ishai and Yehuda Lindell for discussions and helpful suggestions regarding this work.

This research was supported by the European Research Council under the ERC starting grant agreement no. 757731 (LightCrypt) and by the BIU Center for Research in Applied Cryptography and Cyber Security in conjunction with the Israel National Cyber Bureau in the Prime Minister’s Office.

The first author was additionally supported by the Israeli Science Foundation through grant No. 573/16.

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Copyright information

© International Association for Cryptologic Research 2018

Authors and Affiliations

  1. 1.Department of Computer ScienceBen-Gurion UniversityBeershebaIsrael
  2. 2.Department of MathematicsBar-Ilan UniversityRamat GanIsrael

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