Towards Payment Channel Watchtowers With Collateral-Free Security and Robustness

Recently, watchtowers emerge as a critical service within payment channel networks (PCNs). Existing payment channels necessitate that channel owners periodically monitor the blockchain to ensure their fund security, or enlist the watchtower services for this task. Presently, watchtower proposals mandate the implementer to provide collateral as a safeguard against its collusion with potential adversaries. However, this collateral substantially inflates the implementation costs, consequently leading to higher service fees for users. Furthermore, most watchtowers are typically operated by a third-party entity, creating a single point of failure. To ameliorate the status quo, we propose a novel approach where PCN nodes collaboratively implement a watchtower system named “SilenTower.” Our proposal is rooted in the fundamental principles of blockchain systems, emphasizing maintenance by a community with an honest majority. SilenTower’s security no longer relies on collateral but rather on the inherent difficulty of a large proportion of collusion. SilenTower also allows inaccessible participants and thus obtains robustness. Through a rigorous theoretical analysis, we demonstrate that participants’ optimal strategy is to remain accessible and faithfully adhere to the SilenTower protocol. Furthermore, we assess the practical performance of SilenTower through comprehensive benchmarking, and the results reveal that it introduces lightweight overheads.