What Is the ‘Longest Chain Rule’ in Blockchain Consensus?

The longest chain rule, also known as the Nakamoto consensus, is a fundamental principle in Proof-of-Work blockchains. It dictates that nodes should always consider the chain with the most cumulative Proof-of-Work (i.e. the most blocks or the highest total difficulty) as the valid, canonical chain.

An attacker in a 51% attack attempts to secretly build a longer, malicious chain to replace the honest one.