Shaduf++: Non-Cycle and Privacy-Preserving Payment Channel Rebalancing

Payment channels serve as a promising solution to improving the scalability of permissionless blockchains, each of which allows two users to perform off-chain payments with almost unlimited frequency. Nevertheless, the payment channel risks encountering fund depletion, which threatens the availability of off-chain payments. The most recent method needs a cycle-based channel rebalancing procedure, which requires a fair leader and users with rebalancing demands forming directed cycles in the network. Therefore, its large-scale applications are restricted. Moreover, the privacy issue is not considered, leading to the leakage of rebalancing information. In this work, we introduce Shaduf++, a novel non-cycle off-chain rebalancing protocol that offers a new solution for users to shift coins among any channels to which they belong directly without relying on the cycle setting, and therefore could be applied to more general rebalancing scenarios. Furthermore, utilizing the cryptography primitives, including the commitment and the range proof schemes, Shaduf++ preserves the privacy for the rebalancing operations. We provide the details of Shaduf++ and formally prove its security under the Universal Composability framework. Our prototype demonstrates its feasibility and the experimental evaluation shows that Shaduf++ enhances the Lighting Network performance in payment success ratio and volume.