51% Attack

(possible already with 30%, but 51% is the threshold at which a success is very likely)

If an entity gets control over the majority of the hashing power it cannot:

- steal bitcoins

- spend bitcoins without signatures

- redirect bitcoins

- change past transactions or ownership records

- change the rules of the network

It can:

- only attack future consensus or at best the most recent past (tens of blocks)

- perform a double spend attack (only with their own coins!)

- perform denial-of-service disruption

A seller of high value items can easily protect himself against a double spend attack, by waiting for multiple confirmations until delivering the item (recommendation: at least 6). The more confirmations, the harder it gets to invalidate the transaction with a 51% attack. Waiting 24 h for very high value items corresponds to approximately 144 confirmations, which practically impossible to invalidate.

Consensus attacks would be met with immediate countermeasures by the bitcoin community, making bitcoin hardier, stealthier, and more robust than ever.