XPCH: A Cross-Chain Payment Protocol via Connecting the Payment Channel Hubs

Cross-chain asset transfer protocols are vital for blockchain interoperability. While many focus on functionality and scalability, privacy is often overlooked. The Payment Channel Hub (PCH) addresses this by offering a centralized service that enables fast, private off-chain cross-chain transactions by obscuring user transaction relationships. However, current implementations struggle with scalability. Users cannot perform multiple transactions within a single epoch, and the system faces high load concentration, as one hub must lock various currencies to support cross-chain operations. In this research, we put forward a novel privacy-preserving cross-chain payment protocol, XPCH. It supports multiple bidirectional payments within the same epoch, thereby improving scalability and efficiency. It connects multiple PCHs, where each hub only needs to lock two types of coins, thereby reducing the load on individual hubs and mitigating single-point-of-failure risks. To realize our XPCH, we construct the following components. First, we implement an Adaptor Payment Channel to support multiple bidirectional transactions. We then utilize this channel to connect multiple hubs. Since it allows multiple transactions, this forms a multi-hub architecture and enables the system to serve multiple users within the same epoch. In addition, we design the RA2L cryptographic primitives to support privacy-preserving payments across multiple hubs. Moreover, XPCH provides resistance to griefing attacks, atomicity, and unlinkability, and we present a rigorous security proof. We carried out experiments on multiple regtest networks to evaluate the performance of XPCH. Results show that XPCH improves transaction efficiency and scalability and reduces the transaction and volume burdens on each hub.