51% Attacks
A majority block-production attacker can rewrite recent history or censor transactions but cannot forge private-key signatures.
This is the most famous blockchain attack and one of the most misunderstood.
The Intuition
If an attacker can create valid blocks faster than the honest network, they can build an alternative history and reveal it later.
See it concretely
If the official class notes are whichever notebook has the most verified pages, the fastest page writer can replace recent pages — but still cannot forge another student's signature.
Tempting — but wrong
The precise version
In Proof of Work, majority hash power can outpace honest cumulative work. Practical impacts include double-spending attacker-controlled transactions, censorship, reorganization of recent blocks, and confidence loss. In Proof of Stake, analogous attacks depend on validator weight, finality, fork choice, and slashing rules.
attackerPower > honestPower \Rightarrow attackerChainGrowth > honestChainGrowthCheck your understanding
Can a 51% attacker spend coins without the private key?
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Who is most exposed to double-spend risk?
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- List what a majority attacker can and cannot do.
- Explain double-spend reorganization.
- Explain why confirmations matter.
- Compare PoW majority power with PoS finality assumptions.
What can a 51% attacker generally not do?