Cryptographic obfuscation for smart contracts: Trustless bitcoin bridge and more

Abstract

Privacy protection for smart contracts is currently inadequate. Existing solutions for privacy-preserving smart contracts either support only a limited class of smart contracts or rely on noncryptographic assumptions.

We propose a cryptographic obfuscation scheme for smart contracts based on existing blockchain mechanisms, standard cryptographic assumptions, and witness encryption. In the proposed scheme, an obfuscated smart contract does not reveal its algorithm and hardcoded secrets and preserves encrypted states. Any user can provide it with encrypted inputs and allow an untrusted third party to execute it. Although multiparty computation (MPC) among dynamically changing users is necessary, its privacy is protected if at least one user is honest. If the MPC does not finish within a period of time, anyone can cancel and restart it. The proposed scheme also supports decentralized obfuscation where even the participants of the obfuscation process cannot learn secrets in the obfuscated smart contract unless all of them are malicious. As its applications, we present a new trustless bitcoin bridge mechanism that exposes no secret key and privacy-preserving anti-money laundering built into smart contracts.