Before diving into 51% attack we will first overview of how the bitcoin blockchain works. The word blockchain divided into 2 words block and chain. a block is the transaction record and chain is the series of these transactions connected together.

blockchain in bitcoin
blockchain in bitcoin network

so who performing these transactions in bitcoin network, these are miners doing these transactions and making blockchain secure, wait to make blockchain secure what does it means? actually, every miner is a node/computer in the big blockchain network. Every miner has the complete validated blockchain records and when a new transaction occurs it will be broadcasted to the network and each miner store this transaction into their blockchain to keep the blockchain updated in its storage, In return, these miners got some bitcoins. Any computer can join the bitcoin network to start mining.

Think of these miners are the security guard of the bitcoin network, This system is used to protect against double spending and modification of previous transactions.

blockchain nodes
blockchain nodes

because when a transaction is initiated it will be added only into blockchain when it is validated by other nodes. If more than one miner has a different length of blockchain, then according to bitcoin protocol longest chain will be selected and broadcasted to the network, then each miner will store this longest chain.

Now we have litter understanding of blockchain Lets dive into 51% attack.

What is 51% Attack in bitcoin

When a transaction is initiated it is added into blockchain making a long series of block transactions. For example, if someone established the huge no of miners into the bitcoin network to achieve 51% hash power of the entire network. now 51% hash power is under control of him.

it means he can mine faster than the other miners into the network as

” majority is an authority “

51% attack is sometimes also called as majority attack. With 51% he can built blockchain transactions faster making longest chain than the other miners.

51% attacker's chain
51% attacker’s chain

The attacker can spend its bitcoin as many times as he wants, the network transactions are in his control. He can send bitcoins and return this transaction back by deleting it and does not allow the validation of transactions because he is now the boss of the network. he can add or delete the transactions but can’t alter it.

As the attacker is mining fast and according to bitcoin protocol (Proof of work), Longest chain will be selected, the remaining miners will consider this corrupted blockchain records as valid and they will update their blockchain transactions also with this longest chain.

51% attack is actually achievable?

Actually not really, because there are huge miners working, to achieve 51% hash power is impractical. Because according to cryptoSlate it requires almost 1.4$ billion cost for electricity which as much as the Morroco entire country consuming, excluding the advanced machinery cost and place where to put these miners. No one is going to put this huge money on it as in return he will be in lose.

Are other blockchain vulnerable ?

The small altcoins which are working on bitcoin’s protocol (POW) are vulnerable, due to less no of miners. Especially newly launched altcoins with POW protocol are vulnerable because in the beginning they may have less no of miners and anyone can get 51% hash power to steal their coins.

For example Bitcoin gold is built on bitcoin’s protocol. It news of 51% was floating.

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