Paper 2019/583

Atomic Multi-Channel Updates with Constant Collateral in Bitcoin-Compatible Payment-Channel Networks

Christoph Egger and Pedro Moreno-Sanchez and Matteo Maffei

Abstract

Current cryptocurrencies provide a heavily limited transaction throughput that is clearly insufficient to cater to their growing adoption. Payment-channel networks (PCNs) have emerged as an interesting solution to the scalability issue and are currently deployed by popular cryptocurrencies such as Bitcoin and Ethereum. While PCNs do increase the transaction throughput by processing payments off-chain and using the blockchain only as a dispute arbitrator, they unfortunately require high collateral (i.e., they lock coins for a non-constant time along the payment path) and are restricted to payments in a path from sender to receiver. These issues have severe consequences in practice. The high collateral enables griefing attacks that hamper the throughput and utility of the PCN. Moreover, the limited functionality hinders the applicability of current PCNs in many important application scenarios. Unfortunately, current proposals do not solve either of these issues, or they require Turing-complete language support, which severely limits their applicability. In this work, we present AMCU, the first protocol for atomic multi-channel updates and reduced collateral that is compatible with Bitcoin (and other cryptocurrencies with reduced scripting capabilities). We provide a formal model in the Universal Composability framework and show that AMCU realizes it, thus demonstrating that AMCU achieves atomicity and state privacy. Moreover, the reduced collateral mitigates the consequences of griefing attacks in PCNs while the (multi-payment) atomicity achieved by AMCU opens the door to new applications such as credit rebalancing and crowdfunding that are not possible otherwise. Moreover, our evaluation results demonstrate that AMCU has a performance in line with that of the Lightning Network (the most widely deployed PCN) and thus is ready to be deployed in practice.