Paper 2020/456

General Congestion Attack on HTLC-Based Payment Channel Networks

Zhichun Lu, Runchao Han, and Jiangshan Yu

Abstract

Payment Channel Networks (PCNs) have been a promising approach to scale blockchains. However, PCNs have limited liquidity: large-amount or multi-hop payments may fail. The major threat of PCNs liquidity is payment griefing, where the adversary who acts as the payee keeps withholding the payment, so that coins involved in the payment cannot be used for routing other payments before the payment expires. Payment griefing gives adversaries a chance to launch the congestion attack, where the adversary griefs a large number of payments and paralyses the entire PCN. Understanding congestion attacks, including their strategies and impact, is crucial for designing PCNs with better liquidity guarantees. However, existing research has only focused on the specific attacking strategies and specific aspects of their impact on PCNs. We fill this gap by studying the general congestion attack. Compared to existing attack strategies, in our framework each step serves an orthogonal purpose and is customisable, allowing the adversary to focus on different aspects of the liquidity. To evaluate the attack's impact, we propose a generic method of quantifying PCNs' liquidity and effectiveness of the congestion attacks. We evaluate our general congestion attacks on Bitcoin's Lightning Network, and show that with direct channels to 1.5% richest nodes, and $\sim$ 0.0096 BTC of cost, the adversary can launch a congestion attack that locks 47% ($\sim$280 BTC) coins in the network; reduces success rate of payments by 16.0%$\sim$60.0%; increases fee of payments by 4.5%$\sim$16.0%; increases average attempts of payments by 42.0%$\sim$115.3%; and increase the number of bankruptcy nodes (i.e., nodes with insufficient balance for making normal-size payments) by 26.6%$\sim$109.4%, where the amounts of payments range from 0.001 to 0.019 BTC.