Censorship Resistance

Censorship resistance is the property of a payment system or network that makes it difficult for any single party, whether a company, government or group of insiders, to stop valid transactions or shut particular users out. In crypto, it is one of the main reasons given for using permissionless blockchains such as Bitcoin and Ethereum: if the rules are followed and the fee is paid, a transaction should eventually be recorded, regardless of who sent it or why.

The idea is the counterpart of censorship, which on blockchains means deliberately excluding or delaying transactions. Censorship resistance depends on decentralization: the more independent parties produce blocks and run infrastructure, and the harder it is to coordinate them, the stronger it is. It is a contested value, since the same feature that protects dissidents and people in unstable countries can also help criminals and sanctioned actors move money.

Picture a town with one post office. If the postmaster decides not to send your letters, you have no other option. Now picture a town where hundreds of independent couriers will carry any sealed envelope for a small fee, and none of them can open it. If one refuses, you simply hand it to the next.

A censorship-resistant network is the second town. No single courier can stop your letter, and to block it completely, almost all of them would have to agree. In practice, some couriers are much bigger than others, and the roads into town can still be watched, so the protection is strong but never total.

Several design choices work together:

• Open participation. Anyone with the right hardware or stake can become a miner or validator, so no gatekeeper decides who produces blocks.
• Rotating block producers. Different parties produce successive blocks, so a transaction excluded by one can be included by another.
• Peer-to-peer relay. Transactions spread through a peer-to-peer network rather than a central server, making them hard to intercept.
• Self-custody. With a non-custodial wallet, users hold their own keys, so no intermediary can freeze their funds at the base layer.
• Economic incentives. Producers who exclude fee-paying transactions give up income, which makes long-term censorship costly.

The Nakamoto coefficient, the smallest number of parties needed to control a network, is often used as a rough measure of how easily it could be censored.

Blockchains are only as censorship-resistant as their weakest layer. After the US Treasury's Office of Foreign Assets Control added the Tornado Cash mixing service to its sanctions list in August 2022, a large share of Ethereum blocks for a time were built by relays and builders that excluded transactions linked to sanctioned addresses, although such transactions were still included by others, with some delay. Other pressure points include:

• Concentration. A small number of mining pools, staking providers or block builders can produce most blocks.
• Infrastructure. Many users reach blockchains through a few hosted node providers, wallets and website front ends, which can block addresses.
• Issuer controls. Fiat-backed stablecoins can be frozen by their issuers, as described in stablecoin freezing.
• On- and off-ramps. Exchanges that convert crypto to cash must follow the law, so users can be cut off where money enters or leaves.

Ethereum researchers have proposed inclusion lists, which would let validators require builders to include certain pending transactions, and other changes to reduce the power of block builders. Bitcoin developers focus on keeping nodes cheap to run and transaction relay open. Some networks use encrypted mempools so that producers cannot see a transaction's contents before including it, which also helps against front-running.

Supporters argue that censorship resistance protects property rights, free expression and access to payments for people shut out by their governments or banks. Regulators and law enforcement point out that it also hinders efforts against money laundering, sanctions evasion and ransomware, and they increasingly apply rules to the companies at the edges of the system, such as exchanges and stablecoin issuers. The tension between these views shapes debates on cryptocurrency regulation and on the design of CeDeFi services, which add central control points by design. A related property is immutability, which concerns whether records can be changed once written.