Permissionless blockchains promise to be resilient against censorship by a single entity. This suggests that deterministic rules, and not third-party actors, are responsible for deciding if a transaction is appended to the blockchain or not. In 2022, the U.S. Office of Foreign Assets Control (OFAC) sanctioned a Bitcoin mixer and an Ethereum application, putting the neutrality of permissionless blockchains to the test. In this paper, we formalize quantify and analyze the security impact of blockchain censorship. We start by defining censorship, followed by a quantitative assessment of current censorship practices. We find that 46% of Ethereum blocks were made by censoring actors that intend to comply with OFAC sanctions, indicating the significant impact of OFAC sanctions on the neutrality of public blockchains. We further uncover that censorship not only impacts neutrality, but also security. We show how after Ethereum's move to Proof-of-Stake (PoS) and adoption of Proposer-Builder Separation (PBS) the inclusion of censored transactions was delayed by an average of 85%. Inclusion delays compromise a transaction's security by, e.g., strengthening a sandwich adversary. Finally we prove a fundamental limitation of PoS and Proof-of-Work (PoW) protocols against censorship resilience.

翻译：无许可区块链承诺能够抵抗单一实体的审查，这意味着决定交易是否被追加到区块链上的应当是确定性规则，而非第三方参与者。2022年，美国海外资产控制办公室（OFAC）对一款比特币混币器和一个以太坊应用实施制裁，对无许可区块链的中立性构成了考验。本文对区块链审查的安全性影响进行了形式化定义、量化评估与分析。我们首先定义了审查的概念，随后对当前审查实践进行了定量评估。研究发现，46%的以太坊区块由意图遵守OFAC制裁的审查参与者生成，这表明OFAC制裁对公共区块链的中立性产生了显著影响。我们进一步揭示，审查不仅影响中立性，还威胁安全性。在以太坊转向权益证明（PoS）并采用提议者-构建者分离（PBS）后，受审查交易的纳入延迟平均增加了85%。纳入延迟通过例如强化三明治攻击者的方式，损害了交易的安全性。最后，我们证明了PoS和工作量证明（PoW）协议在抗审查能力上存在根本性局限。