[论文解读] Security and Privacy of Lightning Network Payments with Uncertain Channel Balances

Rene Pickhardt, Sergei Tikhomirov+2 more|arXiv (Cornell University)|Mar 15, 2021

Blockchain Technology Applications and Security被引用 7

一句话总结

本文提出了一种概率模型，通过量化通道余额的不确定性，以提升闪电网络支付的可靠性和隐私性。该模型推导出最优的多段支付拆分策略与路径选择启发式方法，将预期支付尝试次数减少20%（在使用BOLT14重平衡的情况下达到48%），同时量化了失败尝试带来的隐私泄露。

ABSTRACT

The Lightning Network (LN) is a prominent payment channel network aimed at addressing Bitcoin's scalability issues. Due to the privacy of channel balances, senders cannot reliably choose sufficiently liquid payment paths and resort to a trial-and-error approach, trying multiple paths until one succeeds. This leaks private information and decreases payment reliability, which harms the user experience. This work focuses on the reliability and privacy of LN payments. We create a probabilistic model of the payment process in the LN, accounting for the uncertainty of the channel balances. This enables us to express payment success probabilities for a given payment amount and a path. Applying negative Bernoulli trials for single- and multi-part payments allows us to compute the expected number of payment attempts for a given amount, sender, and receiver. As a consequence, we analytically derive the optimal number of parts into which one should split a payment to minimize the expected number of attempts. This methodology allows us to define service level objectives and quantify how much private information leaks to the sender as a side effect of payment attempts. We propose an optimized path selection algorithm that does not require a protocol upgrade. Namely, we suggest that nodes prioritize paths that are most likely to succeed while making payment attempts. A simulation based on the real-world LN topology shows that this method reduces the average number of payment attempts by 20% compared to a baseline algorithm similar to the ones used in practice. This improvement will increase to 48% if the LN protocol is upgraded to implement the channel rebalancing proposal described in BOLT14.

研究动机与目标

• 为解决闪电网络中因通道余额未知而导致的支付不可靠问题。
• 对通道余额的不确定性进行建模，并计算给定路径与金额下的支付成功率。
• 推导出最小化预期支付尝试次数的最优多段支付拆分策略。
• 量化重复支付尝试带来的隐私泄露，包括探测侧信道信息的影响。
• 提出一种无需协议升级的路径选择算法，优先选择高成功率路径。

提出的方法

• 作者将通道余额建模为均匀分布的随机变量，并利用负二项分布推导出通过某条路径成功支付的概率。
• 应用负二项分布试验，计算单段与多段支付的预期尝试次数。
• 推导出支付应拆分为多少段才能使预期尝试次数最小化的闭式解析解。
• 利用Kullback-Leibler散度计算每次支付失败尝试所泄露的信息量（即隐私泄露）。
• 提出一种路径选择算法，按先验成功率对路径进行排序，无需协议更改即可提升可靠性。
• 在真实世界LN子图上进行仿真，验证模型并评估不同条件下的性能表现。

实验结果

研究问题

• RQ1 为最小化预期尝试次数，闪电网络支付应拆分为多少段为最佳？
• RQ2 预期支付尝试次数如何随路径长度与通道余额不确定性变化？
• RQ3 重复支付重试在多大程度上会向发送方泄露私人余额信息？
• RQ4 基于先验成功率的路径选择在减少尝试次数方面效果如何？
• RQ5 结合BOLT14通道重平衡方案，所提策略可实现多大性能提升？

主要发现

• 模型表明，对于小额支付，拆分为多段支付并无益处，多段支付所能带来的改进存在上限。
• 所提出的路径选择算法在仿真中相比基线启发式方法，将平均支付尝试次数减少了20%。
• 在使用BOLT14通道重平衡的情况下，改进幅度提升至48%，原因是路径成功率更高。
• 对于给定的服务水平目标σ，预期尝试次数满足闭式表达式 n > log(1−σ)/log(1−s)，其中s为路径成功率。
• 每次支付尝试都会泄露私人余额信息，其泄露量通过Kullback-Leibler散度量化，泄露量取决于跳数与余额分布。
• 研究证实，最小化不确定跳数（而非单纯路径长度）更能有效提升可靠性，因为成功率随跳数呈指数下降。