Censorship and Suppression: The Soft Power of 51%

While double-spending is the most spectacular 51% attack, a majority attacker can also use their power to exert Censorship over the network. By controlling which transactions are included in the most-work chain, they can effectively choose who is allowed to participate in the Bitcoin economy.

---

1. Blacklisting Addresses

An attacker can decide that they will never include transactions from a specific set of Bitcoin addresses. 1. Block Building: When building a block, they simply skip any transactions from the blacklisted addresses found in their mempool. 2. Chain Dominance: Because the attacker has 51% of the work, their "clean" chain will eventually outpace any chain that includes the "forbidden" transactions.

---

2. The Feathercoin Attack (Minority Suppression)

What if an honest miner (with 10% hashrate) finds a block that contains a blacklisted transaction?

A 51% attacker can engage in Minority Suppression: 1. Orphaning: The attacker ignores the honest miner's block and builds their own block at the same height. 2. Double-Work: Because the attacker has more hashrate, their branch will likely win. 3. Deterrent: Honest miners realize that if they include blacklisted transactions, their blocks will be orphaned and they will lose their rewards. They are eventually coerced into following the attacker's censorship rules.

---

3. Empty Block Attacks (DOS)

A malicious attacker with 51% hashrate can choose to mine Empty Blocks (blocks containing only the coinbase transaction). * Result: No transactions are processed by the network. * Impact: The Bitcoin network becomes a "frozen" ledger. While coins aren't stolen, they cannot be moved, rendering the network useless for commerce.

---

️ 4. The Nuclear Option: UASF

If a 51% attacker begins censoring the network, the community has a "nuclear option": the User Activated Soft Fork (UASF). * Social Consensus: Users and node operators can agree to update their software to ignore blocks from the attacker's miners. * Economic War: By changing the consensus rules (e.g., changing the PoW algorithm or blacklisting the attacker's specific coinbase outputs), the community can "fire" the miners and migrate to a new, clean chain. * Incentive Alignment: This threat usually prevents miners from attempting overt censorship, as it would destroy the value of their multi-billion dollar ASIC investments.