AMR: autonomous coin mixer with privacy preserving reward distribution

Abstract

It is well known that users on open blockchains are tracked by an industry providing services to governments, law enforcement, secret services, and alike. While most blockchains do not protect their users' privacy and allow external observers to link transactions and addresses, a growing research interest attempts to design add-on privacy solutions to help users regain their privacy on non-private blockchains. In this work, we propose to our knowledge the first censorship resilient mixer, which can reward its users in a privacy-preserving manner for participating in the system. Increasing the anonymity set size, and diversity of users, is, as we believe, an important endeavor to raise a mixer's contributed privacy in practice. The paid-out rewards can take the form of governance tokens to decentralize the voting on system parameters, similar to how popular "Decentralized Finance (Defi) farming" protocols operate. Moreover, by leveraging existing Defi lending platforms, AMR is the first mixer design that allows participating clients to earn financial interest on their deposited funds. Our system AMR is autonomous as it does not rely on any external server or a third party. The evaluation of our AMR implementation shows that the system supports today on Ethereum anonymity set sizes beyond thousands of users, and a capacity of over 66,000 deposits per day, at constant system costs. We provide a formal specification of our zk-SNARK-based AMR system, a privacy and security analysis, implementation, and evaluation with both the MiMC and Poseidon hash functions.