[论文解读] Hijacking Routes in Payment Channel Networks: A Predictability Tradeoff

Saar Tochner, Stefan Schmid+1 more|arXiv (Cornell University)|Sep 15, 2019

Blockchain Technology Applications and Security参考文献 22被引用 35

一句话总结

本文识别了链下支付网络（Lightning）中的路由劫持 DoS 漏洞，实证分析路由实现，并揭示一个根本权衡：降低可预测性提高安全性，但也增加成本。

ABSTRACT

Off-chain transaction networks can mitigate the scalability issues of today's trustless electronic cash systems such as Bitcoin. However, these peer-to-peer networks also introduce a new attack surface which is not well-understood today. This paper identifies and analyzes, a novel Denial-of-Service attack which is based on route hijacking, i.e., which exploits the way transactions are routed and executed along the created channels of the network. This attack is conceptually interesting as even a limited attacker that manipulates the topology through the creation of new channels can navigate tradeoffs related to the way it attacks the network. Furthermore, the attack also highlights a fundamental design tradeoff for the defender (who determines its own routes): to become less predictable and hence secure, a rational node has to pay higher fees to nodes that forward its payments. We find that the three most common implementations for payment channels in Bitcoin (lnd, C-lightning, Eclair) approach routing differently. We begin by surveying the current state of the Lightning network and explore the routes chosen by these implementations. We find that in the current network nearly 60\% of all routes pass through only five nodes, while 80\% go through only 10 nodes. Thus, a relatively small number of colluding nodes can deny service to a large fraction of the network. We then turn to study an external attacker who creates links to the network and draws more routes through its nodes by asking for lower fees. We find that just five new links are enough to draw the majority (65\% - 75\%) of the traffic regardless of the implementation being used. The cost of creating these links is very low. We discuss the differences between implementations and eventually derive our own suggested routing policy, which is based on a novel combination of existing approaches.

研究动机与目标

• 激发链下支付网络的可扩展性与路由安全性关注。
• 表征路由决策与通道费用如何影响攻击面。
• 对当前 Lightning 实现和网络拓扑进行经验评估。
• 分析勾结的中心节点和外部攻击者的攻击可行性。
• 讨论缓解劫持风险的设计与政策权衡。

提出的方法

• 将支付通道网络建模为一张图，其中边表示通道。
• 调查 lnd、C-lightning 与 Eclair 使用的路由算法及其权重计算。
• 对实时 Lightning Network 数据（2019 年 7 月）进行实证分析，以评估中心性、路径使用和费用。
• 模拟勾结中心节点的 DoS 路由劫持攻击，以及外部攻击者创建新通道的情形。
• 通过集中化中心性与子模优化属性评估攻击者策略。
• 提出受现有方法启发的路由策略混合并讨论防御策略。

实验结果

研究问题

• RQ1 在当前的 Lightning Network 实现中，路由劫持 DoS 攻击的可行性有多高？
• RQ2 需要多少个新通道或哪些中心节点才能劫持大多数路由？
• RQ3 不同的路由实现（lnd、C-lightning、Eclair）在脆弱性和路由行为上有何差异？
• RQ4 在选择路由策略时，防守方的安全性与经济权衡（可预测性的代价）是什么？
• RQ5 有哪些实际的缓解措施或路由策略变更可以在不削弱网络性能的前提下降低劫持风险？

主要发现

• 五个新的低费链接即可劫持大约 65%–75% 的流量（跨实现）。
• 大约 60% 的路径经过前五个节点，80% 经过前十个节点，从而实现定向勾结式干扰。
• 仅需要五个外部链接就能吸引大部分流量（65%–75%），30个链接可使这一比例提高到约 80%。
• 节点大多使用默认费用；缺乏自私路由的证据，表明易受低费劫持策略的影响。
• 路由实现（lnd、C-lightning、Eclair）在路径选择方面存在差异，但都表现出基于中心性的脆弱性；Eclair 的前 3 位随机化提供的保护有限。
• 存在一个根本性设计权衡：通过选择替代路由来降低可预测性可以提升安全性，但会提高防御方的路由费用。