A consensus mechanism is a computer algorithm that makes a blockchain viable. It does this by solving what’s known as the ‘double spend’ problem.

A $10 note, once spent, no longer belongs to you, so you can’t spend it again. A BTC is a string of computer code, and could be copied infinitely. In theory, this means you could make yourself as rich as you liked by simply making copies of your BTC and spending it over and over again.

However, when you send someone a BTC, the transaction is recorded on the blockchain, and the BTC is transferred from your account to the recipient’s account. The record shows that you no longer own the BTC, preventing you from spending it again.

This is all recorded on a distributed ledger for the world to see. Since everyone can see on their copies of the ledger that you’ve spent your BTC, any attempt to spend the same BTC again would be invalidated by the network. The consensus mechanism ensures that all participants agree on the validity of transactions.

Doctoring one transaction is hard enough, but you’d actually also have to change every subsequent transaction since each one references its forerunners.

This would take an incredible amount of computing power and effort. Additionally, to succeed in such an attack, you’d need to control at least 51% of the network’s computing power to alter the blockchain and rewrite the transaction history in your favour.

Bitcoin and Ethereum use different consensus mechanisms. Bitcoin uses a consensus mechanism called proof of work, which requires miners to solve complex mathematical problems to validate transactions and secure the network. Ethereum has transitioned to a consensus mechanism called proof of stake, where users ‘stake’ a certain amount of ether to become a validator of new transactions.