[论文解读] Censorship Resistance in On-Chain Auctions

Elijah Fox, Mallesh M. Pai+1 more|arXiv (Cornell University)|Jan 1, 2023

Blockchain Technology Applications and Security被引用 3

一句话总结

本文提出一种在多提案者区块链中采用条件性小费机制，以增强链上拍卖的审查抵抗能力。通过为多个提案者提供不对称奖励，激励其包含高小费交易，使审查成本变得极高——审查成本为 kT，而预期小费仅为 kt——从而确保交易及时包含，并在敌对共谋下仍能保持拍卖的完整性。

ABSTRACT

Modern blockchains guarantee that submitted transactions will be included eventually; a property formally known as liveness. But financial activity requires transactions to be included in a timely manner. Classical liveness does not guarantee this, particularly in the presence of a motivated adversary who benefits from censoring transactions. We define censorship resistance as the amount it would cost the adversary to censor a transaction for a fixed interval of time as a function of the associated tip. This definition has two advantages, first it captures the fact that transactions with a higher miner tip can be more costly to censor, and therefore are more likely to swiftly make their way onto the chain. Second, it applies to a finite time window, so it can be used to assess whether a blockchain is capable of hosting financial activity that relies on timely inclusion. We apply this definition in the context of auctions. Auctions are a building block for many financial applications, and censoring competing bids offers an easy-to-model motivation for our adversary. Traditional proof-of-stake blockchains have poor enough censorship resistance that it is difficult to retain the integrity of an auction when bids can only be submitted in a single block. As the number of bidders n in a single block auction increases, the probability that the winner is not the adversary, and the economic efficiency of the auction, both decrease faster than 1/n. Running the auction over multiple blocks, each with a different proposer, alleviates the problem only if the number of blocks grows faster than the number of bidders. We argue that blockchains with more than one concurrent proposer can have strong censorship resistance. We achieve this by setting up a prisoner’s dilemma among the proposers using conditional tips.

研究动机与目标

• 将审查抵抗能力形式化为交易小费和时间窗口的函数，以捕捉现实世界中的金融时间需求。
• 分析现有权益证明区块链在敌对审查下是否能够支持对时间敏感的链上拍卖。
• 设计一种机制，利用多个并发提案者和条件性小费，增强拍卖中的审查抵抗能力。
• 证明条件性小费在提案者之间制造了囚徒困境，确保高包含概率和低审查成本。

提出的方法

• 将审查抵抗能力定义为在固定时间窗口内审查一笔小费为 t 的交易所需付出的成本。
• 建立一个链上第二价格拍卖模型，竞标者向提案者提交小费以争取交易被包含。
• 引入条件性小费：若仅一名提案者包含交易，则获得 T；若多个提案者都包含，则每人仅获得 t ≪ T，从而创造协调激励。
• 分析均衡行为，即所有提案者因个体高回报而均选择包含该交易。
• 证明审查所有 k 名提案者需支付 kT 的成本，尽管预期小费仅为 kt，因此审查成本变得不可行。
• 将该机制应用于提升 MEV、预言机数据流和乐观汇总方案的安全性，通过减少提案者租金提取。

实验结果

研究问题

• RQ1 传统的单提案者权益证明区块链在敌对审查下能否支持在经济上可行的链上拍卖？
• RQ2 当单一共谋竞标者可贿赂提案者时，竞标者数量如何影响拍卖效率和胜者完整性？
• RQ3 何种机制设计可使在多提案者区块链中审查高小费交易的成本变得极高？
• RQ4 条件性小费能否创造一种合谋概率为零的聚合均衡，同时保持出价隐私？
• RQ5 与现有解决方案（如私有内存池或数据可用性层）相比，条件性小费机制在缓解审查方面有何优势？

主要发现

• 审查抵抗能力被定义为在固定时间窗口内审查一笔小费为 t 的交易所需付出的成本，同时捕捉了小费敏感性和时间约束。
• 在单提案者拍卖中，随着诚实竞标者数量的增加，共谋竞标者获胜的概率持续上升，并攫取越来越多的剩余价值。
• 在多个区块中由不同提案者负责运行拍卖，仅当区块数量的增长速度快于竞标者数量时，才能真正提升审查抵抗能力。
• 在多个并发提案者（k > 1）的情况下，条件性小费制造了囚徒困境：每个提案者均有激励包含交易以获得 T，导致审查所有 k 名提案者的成本为 kT ≫ kt。
• 该机制实现了合谋概率为零的聚合均衡，保护了出价隐私，且预期小费降低至 kt。
• 该方法显著降低了提案者租金——例如，自合并以来以太坊上 MEV-boost 的支付金额高达 2.45 亿美元，该模型可使其大幅减少。