Fair and Robust Multi-party Computation Using a Global Transaction Ledger

Abstract

Classical results on secure multi-party computation (MPC) imply that fully secure computation, including fairness (either all parties get output or none) and robustness (output delivery is guaranteed), is impossible unless a majority of the parties is honest. Recently, cryptocurrencies like Bitcoin where utilized to leverage the fairness loss in MPC against a dishonest majority. The idea is that when the protocol aborts in an unfair manner (i.e., after the adversary receives output) then honest parties get compensated by the adversarially controlled parties.Our contribution is three-fold. First, we put forth a new formal model of secure MPC with compensation and we show how the introduction of suitable ledger and synchronization functionalities makes it possible to express completely such protocols using standard interactive Turing machines (ITM) circumventing the need for the use of extra features that are outside the standard model as in previous works. Second, our model, is expressed in the universal composition setting with global setup and is equipped with a composition theorem that enables the design of protocols that compose safely with each other and within larger environments where other protocols with compensation take place; a composition theorem for MPC protocols with compensation was not known before. Third, we introduce the first robust MPC protocol with compensation, i.e., an MPC protocol where not only fairness is guaranteed (via compensation) but additionally the protocol is guaranteed to deliver output to the parties that get engaged and therefore the adversary, after an initial round of deposits, is not even able to mount a denial of service attack without having to su↵er a monetary penalty. Importantly, our robust MPC protocol requires only a constant number of (coin-transfer and communication) rounds.

Instantaneous Decentralized Poker

“…Please see [6,31,32,30] for additional details on the model including the necessary modifications to UC. Please also see [28] which extensively treats these modifications, proposes alternative models, and uses protocol compilers different from GMW.…”

Progress in Cryptology – LATINCRYPT 2017

“…After the initial suggestion many cryptocurrencies using variants of the GHOST rule were proposed and implemented. The most popular among them, Ethereum [6] has received substantial research attention [16,15,4,25,12,21]. Ethereum is essentially a Bitcoin-like system where transaction processing is Turing-complete and thus it can be used to implement any public functionality in a distributed way.…”

Zero-Collateral Lotteries in Bitcoin and Ethereum

“…As is typical for modelling cryptocurrencies [18,34,35,30,24,23], we assume that the underlying synchronicity of the communication network is such that transactions submitted to the blockchain system are accepted within a bounded time and that timestamp information available to the smart contract is approximately synchronized.…”