Unstoppable Wallets: Chain-assisted Threshold ECDSA and its Applications

Abstract

The security and usability of cryptocurrencies and other blockchain-based applications depend on the secure management of cryptographic keys. However, current approaches for managing these keys often rely on third parties, trusted to be available at a minimum, and even serve as custodians in some solutions, creating single points of failure and limiting the ability of users to fully control their own assets. In this work we first revisit the problem of threshold ECDSA by considering the commonly admissible 'server-aided' model, namely, the presence of a semi-honest and non-colluding service provider. Then, we leverage that model and consider cases where that 'server' is distributed, introducing the novel concept of unstoppable wallets; hence eliminating any single point of failure. Unstoppable wallets are programmable threshold ECDSA wallets that allow users to co-sign transactions with a confidential smart contract, rather than a singular third-party. We construct highly efficient threshold ECDSA protocols that form the basis of unstoppable wallets and prove their security in the server-aided model, achieving the standard notion of fairness and robustness even in case of a dishonest majority among the signers. Our protocols minimize the write-complexity for threshold ECDSA key-generation and signing, while reducing communication and computation overhead. We provide a proof-of-concept implementation of these protocols, written in a smart contract language, deployed on the Secret Network - a blockchain that plays the role of the server. Using that deployment, we showcase the protocols' applicability for two interesting applications, policy checking and wallet exchange, as well as their efficiency by demonstrating low gas costs and fees.