Union: A Trust-minimized Bridge for Rootstock

TL;DR

Union addresses secure BTC interoperability by replacing honest-majority federations with a 1-of-$N$ honest functionary model implemented via a multi-party BitVMX proving system. It introduces a packet-based security deposit scheme and enablers to share deposits and enforce penalties, a flexible light-client framework for cross-chain validation, and a stop-watch mechanism to optimize time-locks. The approach yields improved capital efficiency, scalable security guarantees, and a generalized bridge architecture adaptable to various blockchain ecosystems while preserving Bitcoin's security. This work provides a concrete, scalable blueprint for trust-minimized Bitcoin interoperability with broad practical impact.

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.

Figures (7)

• Figure 1: BitVMX Dispute Resolution Channel. The prover submits a proof of a computation that a verifier can challenge through a dispute resolution game. Colors blue and red in the diagram represent transactions from the prover and the verifier respectively.
• Figure 2: Transaction graph structure for multi-party BitVMX showing the Locking, Kick-off, dispute, and Unlocking transactions. The dispute resolution component corresponds to the channel scheme depicted in Figure \ref{['fig:dispute']}.
• Figure 3: Transaction graph structure of the dispute resolution game with verifier counter-proof using nested BitVMX.
• Figure 4: Security deposit management in the BitVMX proving system. The system uses shared deposits per packet and enabler outputs to manage functionary participation and enforce penalties.
• Figure 5: Security deposit management in the BitVMX challenge system. The system employs a special transaction known as Kill enablers, which enables functionaries to prevent a malicious party from participating in future prover or challenge operations.