Bitcoin 的安全性是利用「當整個社群夠大時，不會有單一團體擁有超過 50% 的計算能力」的假設上發展演算法及防護機制：(參考「Weaknesses - Bitcoin」)
An attacker that controls more than 50% of the network's computing power can, for the time that he is in control, exclude and modify the ordering of transactions. This allows him to:
- Reverse transactions that he sends while he's in control. This has the potential to double-spend transactions that previously had already been seen in the block chain.
- Prevent some or all transactions from gaining any confirmations
- Prevent some or all other miners from mining any valid blocks
不過讓人噴飯的消息出來了，即使在 Bitcoin 這麼火熱的情況下，仍然有單一團體計算能力已經超過 50%：「Largest Bitcoin Mining Pool Pledges Not To Execute '51% Attack'」。