The 51% attack can be defined as a situation whereby a miner or a group of miners are trying to gain control of more than 50% of a network mining power, hash rate, or computing power of a blockchain. Without Cryptocurrencies miners, there will be no trust in the Cryptocurrency world because there will be no one to confirm the different transactions in a blockchain.
A miner is a person that is responsible for the adding of transactions in a blockchain by finding a correct answer to a puzzle. He finds the answer to the mathematical puzzle by using high computational power because the higher the computational power, the more likely will find answers to the puzzle that will allow him to add the transaction to the blockchain.
How does a 51% attack happen?
Not all Cryptocurrency miners are truthful. Some are there to scam people of their hard earn money and one of the ways this is done is by initiating a 51% attack. Once a transaction is initiated on a blockchain, it remains among the list of unconfirmed transactions until a miner confirms it. This is the point where corrupt miners try to highjack the confirmation from genuine miners. At the point of confirmation, a genuine miner who has already gotten an answer to the mathematical puzzle will try to share the answer with other miners. On the part of the corrupt miner, it keeps such an answer to himself.
Such an act implies that two versions of blockchain will be formed with the genuine miners being in charge of one of the blockchain while the scammers are in charge of the other blockchain that does not broadcast the answer to the mathematical puzzle to the original network. The corrupt miner continues to work on its blockchain to make it seem original.
Furthermore, blockchain is built in such a way that it follows the longest chain which is seen as the legitimate blockchain. The corrupt miners know that the more he adds blocks to his chain, the more it will be considered legitimate. So, what he does is that he spends a lot of money so that he can have a high computing power which will, in turn, lead to the adding of blocks to a chain faster. Once he achieves the threshold to be considered the longest blockchain, he will end up announcing it to the network as part of the 51% attack initiation process.
The 51% attack leads to the reversal of transactions in the blockchain. For example, if a person decided to buy a Tesla car with 0.5 bitcoin and at the point of confirmation in the blockchain a 51% attack occurred, the corrupt miner will get the Tesla car that has been ordered for by the unknown person and also get the 0.5 bitcoin and so the name “double-spend”.
Susceptibility of a coin to a 51% attack
It is will very difficult for a 51% attack to occur in a blockchain that is responsible for the storing of bitcoin transactions. This is because the corrupt miners will need computing power that will be more than that of millions of miners around the world. And no matter how high a computer is, it cannot compete with millions of other computers.
Although there are some other ways in which a blockchain can sustain 51% attack. If a blockchain has a bug in it, it can be attacked easily especially when it’s a smaller blockchain.
Therefore, the bigger the blockchain the more difficult it is to be attacked. This is part of the reason why bitcoin has never suffered a 51% attack.