51% Attack: Principles, Impact, and Mitigation
51%An attack refers to a situation in which a single participant or organization controls more than half of a blockchain network’s computing power, potentially enabling it to reorganize recent blocks, disrupt transaction confirmations, or carry out double-spending. Its risk is related to the network’s size, resource distribution, and consensus mechanism.
51%What is an attack?
In blockchains that rely on computing power to maintain consensus, an attacker that controls more than half of the network's hash power may outweigh honest participants, alter the order in which transactions are processed, prevent some transactions from being confirmed, or create a mining monopoly. However, possessing majority hash power does not mean the attacker can arbitrarily control every rule of the blockchain.
How an Attack Can Cause Double-Spending
An attacker may first pay tokens on the public chain and, after the counterparty delivers the asset, build a branch chain from a block preceding the payment that excludes that payment. If the attacker's hash power enables this chain to become the subsequent record accepted by the network, the original payment may be excluded, allowing the attacker to potentially spend the same tokens again.
Limits of Capability and Number of Confirmations
Majority hash power can disrupt transaction inclusion and reorganize recent history, but it cannot forge other users' valid signatures or cause blocks that violate issuance rules to be accepted by normal validating nodes. Reorganizing history requires redoing the relevant proof-of-work; as transactions are covered by more subsequent blocks, the cost of rewriting them is typically higher, although the number of confirmations is not an absolute security guarantee.
Risk Differences and Mitigation Approaches
The risk in hash-power-based networks depends on the scale and distribution of honest hash power, and controlling substantial hash power typically requires significant resource costs. Key mitigations include reducing the concentration of mining resources and maintaining independent validation; merely increasing the number of ordinary nodes that do not contribute hash power cannot directly prevent majority-hash-power attacks. Different consensus mechanisms should be assessed according to their respective resources and rules to determine attack thresholds.
Original Tutorial
https://www.binance.com/zh-CN/academy/glossary/51-percent-attack
Bitcoin Proof-of-Work and Validation Rules
https://developer.bitcoin.org/devguide/block_chain.html