51% attack

A 51% attack is what could happen if a single party gains control of more than half of a blockchain's power to add blocks — more than half the mining computation on a proof-of-work chain, or more than half the staked funds on a proof-of-stake chain. With a majority, the attacker can consistently outpace everyone else and bend the chain's history to their advantage. The name marks the tipping point: just past half, the honest majority is no longer the majority.

It is important to understand what such an attacker can and cannot do. They can refuse to include certain transactions, and they can rewrite recent history to reverse their own payments — the classic double-spend, where they spend coins, then secretly build a longer chain in which that spend never happened. What they cannot do is steal coins from other people's wallets or forge signatures, because those are protected by cryptography the attacker's majority does not touch.

The reason large chains are safe is economics, not magic. On a major proof-of-work network, amassing the majority of all mining hardware and electricity would cost an enormous fortune, and succeeding would likely crash the value of the very coin the attacker is spending to attack — so it rarely pays off. On proof-of-stake chains, a majority attacker risks having their entire stake destroyed by slashing. The defense, in both cases, is that honest behavior is simply far cheaper than cheating.