Introduction On Bitcoin's Security # Bitcoin's breakthrough was solving the double-spending problem using a Proof⁠-⁠of⁠-⁠Work (PoW) mechanism (see whitepaper). Bitcoin remains secure as long as honest miners control the majority of the network's computing power. The reverse situation is known as a 51% attack, which can manifest in different ways: [1] Double⁠-⁠spending by rewriting recent history. A double-spending attack in Bitcoin happens when someone tries to spend the same coins twice. If th...