Union: A Trust-minimized Bridge for Rootstock

TL;DR

Union is a secure, trust-minimized bridge protocol enabling Bitcoin transfer between Bitcoin and secondary blockchains, employing multi-party cryptography and innovative architecture to enhance security and efficiency.

Contribution

It introduces a novel multi-party bridge protocol with a packet-based architecture, enablers, and flexible light client framework, reducing trust assumptions and improving scalability.

Findings

Secure transfer of BTC between Bitcoin and other blockchains demonstrated.

The protocol operates securely under a 1-of-n honest participant assumption.

Enhanced capital efficiency and scalability through innovative design features.

Abstract

We present Union, a trust-minimized bridge protocol that enables secure transfer of BTC between Bitcoin and a secondary blockchain. The growing ecosystem of blockchain systems built around Bitcoin has created a pressing need for secure and efficient bridges to transfer BTC between networks while preserving Bitcoin's security guarantees. Union employs a multi-party variant of BitVMX, an optimistic proving system on Bitcoin, to create a bridge that operates securely under the assumption that at least one participant remains honest. This 1-of-n honest approach is strikingly different from the conventional honest-majority assumption adopted by practically all federated systems. The protocol introduces several innovations: a packet-based architecture that allows security bonds to be reused for multiple bridge operations, improving capital efficiency; a system of enablers to manage functionaries participation and to enforce penalties; a flexible light client framework adaptable to various blockchain architectures; and an efficient stop watch mechanism to optimize time-lock management. Union is a practical and scalable solution for Bitcoin interoperability that maintains strong security guarantees and minimizes trust assumptions.