Paper 2019/480

On the Efficiency of Privacy-Preserving Smart Contract Systems

Karim Baghery

Abstract

Along with blockchain technology, smart contracts have found intense interest in lots of practical applications. A smart contract is a mechanism involving digital assets and some parties, where the parties deposit assets into the contract and the contract redistributes the assets among the parties based on provisions of the smart contract and inputs of the parties. Recently, several smart contract systems are constructed that use zk-SNARKs to provide privacy-preserving payments and interconnections in the contracts (e.g. Hawk [IEEE S&P, 2016] and Gyges [ACM CCS, 2016]). Efficiency of such systems severely are dominated by efficiency of the underlying UC-secure zk-SNARK that is achieved using COCO framework [Kosba et al., 2015] applied on a non-UC-secure zk-SNARK. In this paper, we show that recent progresses on zk-SNARKs, allow one to simplify the structure and also improve the efficiency of both systems with a UC-secure zk-SNARK that has simpler construction and better efficiency in comparison with the currently used ones. To this end, we first show that given a NIZK argument which guarantees non-black-box simulation (knowledge) soundness, one can construct a UC-secure NIZK that has simpler construction and better efficiency than the ones that currently are used in Hawk and Gyges. We believe, new technique can be of independent interest.

Note: This is the full version of paper (published in proceeding of AFRICACRYPT 2019).