Paper 2021/727

SoK: Privacy-Preserving Computing in the Blockchain Era

Ghada Almashaqbeh and Ravital Solomon

Abstract

Cryptocurrency and blockchain continue to build on an innovative computation model that has paved the way for a large variety of applications. Privacy is a huge concern for cryptocurrencies and blockchains as most of these systems log everything in the clear. This has resulted in several academic and industrial initiatives to address privacy. Starting with the UTXO model of Bitcoin, initial works brought confidentiality and anonymity to payments. Recent works have expanded to support more generalized forms of private computation. Such solutions tend to be highly involved as they rely on advanced cryptographic primitives and creative techniques to handle issues related to dealing with private records (e.g. concurrency and private coin tracking to prevent double spending). This situation makes it hard to comprehend the current state-of-the-art, much less build on top of it. To address these challenges, we provide a systematization of knowledge for privacy-preserving solutions in blockchain. To the best of our knowledge, our work is the first of its kind. After motivating design challenges, we study the zero-knowledge proof systems used in supporting blockchain privacy, categorizing them based on their key features and limitations. Then, we develop a systematization of knowledge framework--using which we classify the state-of-the-art privacy preserving solutions based on several dimensions such as supported functionality, work model, where the private computation is performed, and cryptographic primitives used. Finally, we touch upon challenges including limited functionality, practicality, and accommodating new developments.