The name 51% Attack is used to describe one of the phenomena during which the security of cryptocurrencies is threatened.
This is the name for a rather infrequent type of attack that aims to steal or spend the same cryptocurrency twice, which is also known as double-spending.
In the world of cryptocurrencies, this phenomenon occurs when a person, or a group of individuals, is able to control 50% + 1 unit of the network. In order to simplify the name for this, no one uses the term 50% + 1 unit, but 51% Attack.
This attack, which essentially leads to the reorganization of blocks to double consumption, is an attack on the integrity of the blockchain where the actor, by appropriating a significant part of the validation power, manages to rewrite parts of the blockchain to cancel the payment and thus keep both the goods bought and cryptocurrency.
In that sense, 51% Attack refers to an attack on a blockchain, and it is an attack carried out by a group of miners who control more than 50% of the total computer power of the network.
If a malicious entity or group were able to take control of the blockchain network through a 51% Attack, several unintended consequences could occur.
Once a block is mined, it is impossible to change it, because network users would quickly detect and discard the fake version.
However, a group of attackers who control most of the network’s computing power can interfere with the process of registering new blocks.
Another potential problem is that an attacker could lead to a double-spending dilemma, which happens when proof of a transaction is deleted or fake versions of transactions are published on a blockchain without spending cryptocurrency.
This attack generally refers to chains that use proof of work so that the validation is done thanks to mining, i.e. when the attacker then only temporarily needs to have the majority of the computing power of the network in order to safely achieve double the cost.
Such a consensus model was firstly used on the blockchain for bitcoin transactions. Although transactions using bitcoin are protected by blockchain technology, it is not impossible for their security to be hacked, and hackers usually try to sidestep the bitcoin verification system using computer mechanisms and techniques that include false evidence.
In essence, this is a completely logical rule that requires that the transaction does not consume bitcoins that have already been spent.
In other words, if a group controls a blockchain then it has the ability to process bitcoin transfers to its wallet multiple times by turning the blockchain records so that it acts as if the initial transaction never occurred.
In bitcoin and related systems, the connection between each block is made thanks to what is called proof of work, i.e. a situation in which a miner consumes electricity to find a solution to a problem that depends on the context and, if they succeed, they receive a reward in bitcoins.
This mechanism ensures that it is determined that the entity performing the transactions actually owns bitcoin, thus preventing double spending or other fraud.
Each block, as well as each transaction it contains, is subject to rules called consensus rules, which are determined by the protocol and ensure the proper functioning of the system.
The blockchain of each verified transaction is built over time as more and more transactions are added to it.
In this case, each miner participating in the blockchain network acts as a node and goes through an agreed process to confirm the legitimacy of the transaction. In this sense, 51% Attack is decomposed so that three phases can be identified.
It takes time for bitcoin transactions to be verified, as the process involves intensive number processing and complex algorithms that consume a lot of computing power, making it very difficult to duplicate or falsify a blockchain.
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