Other attributes
51 percent attack happens when an individual miner or group of miners takes control of more than 50% of the network's computing power. Controlling over half of the network gives the attackers the power to disrupt the network. That is usually achieved by renting mining hash power from a third party.
How a 51% Attack Works
Bitcoin and other cryptocurrencies are based on blockchains, a form of a distributed ledger. These digital files record every transaction made on a cryptocurrency's network and are available to all users—and the general public—for review. As a result, no one can spend a coin twice. (So-called "private blockchains" introduce permissions to prevent certain users in the general public from seeing all the data on a blockchain.)
As its name implies, a blockchain is a chain of blocks, which are bundles of data that record all completed transactions during a given period. For bitcoin, a new block is generated approximately every 10 minutes. Once a block is finalized or mined, it cannot be altered since a fraudulent version of the public ledger would quickly be spotted and rejected by the network's users.
However, by controlling the majority of the computing power on the network, an attacker or group of attackers can interfere with the process of recording new blocks. They can prevent other miners from completing blocks, theoretically allowing them to monopolize the mining of new blocks and earn all of the rewards.
KEY TAKEAWAYS
Blockchains are distributed ledgers that record every transaction made on a cryptocurrency's network.
A 51% attack is an attack on a blockchain by a group of miners who control more than 50% of the network's mining hash rate.
Attackers with majority control of the network can interrupt the recording of new blocks by preventing other miners from completing blocks.
Changing historical blocks is difficult due to the hard-coding of past transactions into Bitcoin software.
51% Attack Real-World Examples
Krypton and Shift, two blockchains based on Ethereum, suffered 51% attacks in August 2016.
In May of 2018, Bitcoin Gold, at the time the 26th-largest cryptocurrency, suffered a 51% attack. The malicious actor or actors controlled a vast amount of Bitcoin Gold's hash power, such that even with Bitcoin Gold repeatedly attempting to raise the exchange thresholds, the attackers were able to double-spend for several days, eventually stealing more than $18 million worth of Bitcoin Gold. Bitcoin Gold was hit again in 2020.
Recently, the Bitcoin SV (BSV) network suffered an attack in August 2021.
51% Attack vs. 34% Attack
The tangle, a distributed ledger that is fundamentally distinct from a blockchain but designed to accomplish similar goals, could theoretically succumb to an attacker deploying over a third of the network's hash rate, referred to as a 34% attack.
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