On the power of an honest majority in three-party computation without broadcast

Bar Alon, Ran Cohen, Eran Omri, Tom Suad

Research output: Chapter in Book/Report/Conference proceedingConference contributionpeer-review

1 Scopus citations

Abstract

Fully secure multiparty computation (MPC) allows a set of parties to compute some function of their inputs, while guaranteeing correctness, privacy, fairness, and output delivery. Understanding the necessary and sufficient assumptions that allow for fully secure MPC is an important goal. Cleve (STOC’86) showed that full security cannot be obtained in general without an honest majority. Conversely, by Rabin and Ben-Or (FOCS’89), assuming a broadcast channel and an honest majority enables a fully secure computation of any function. Our goal is to characterize the set of functionalities that can be computed with full security, assuming an honest majority, but no broadcast. This question was fully answered by Cohen et al. (TCC’16) – for the restricted class of symmetric functionalities (where all parties receive the same output). Instructively, their results crucially rely on agreement and do not carry over to general asymmetric functionalities. In this work, we focus on the case of three-party asymmetric functionalities, providing a variety of necessary and sufficient conditions to enable fully secure computation. An interesting use-case of our results is server-aided computation, where an untrusted server helps two parties to carry out their computation. We show that without a broadcast assumption, the resource of an external non-colluding server provides no additional power. Namely, a functionality can be computed with the help of the server if and only if it can be computed without it. For fair coin tossing, we further show that the optimal bias for three-party (server-aided) r-round protocol remains Θ(1 / r) (as in the two-party setting).

Original languageEnglish
Title of host publicationTheory of Cryptography - 18th International Conference, TCC 2020, Proceedings
EditorsRafael Pass, Krzysztof Pietrzak
PublisherSpringer Science and Business Media Deutschland GmbH
Pages621-651
Number of pages31
ISBN (Print)9783030643775
DOIs
StatePublished - 2020
Externally publishedYes
Event18th International Conference on Theory of Cryptography, TCCC 2020 - Durham, United States
Duration: 16 Nov 202019 Nov 2020

Publication series

NameLecture Notes in Computer Science (including subseries Lecture Notes in Artificial Intelligence and Lecture Notes in Bioinformatics)
Volume12551 LNCS
ISSN (Print)0302-9743
ISSN (Electronic)1611-3349

Conference

Conference18th International Conference on Theory of Cryptography, TCCC 2020
Country/TerritoryUnited States
CityDurham
Period16/11/2019/11/20

Bibliographical note

Publisher Copyright:
© International Association for Cryptologic Research 2020.

Keywords

  • Broadcast
  • Coin flipping
  • Honest majority
  • Impossibility result
  • Multiparty computation
  • Point-to-point communication

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