#27: Proof-of-work algorithm in blockchain

Let’s try to cheat the blockchain. If my wallet has exactly one bitcoin, I can’t spend it twice. Once it’s written into an immutable blockchain, everyone knows my wallet is empty. However, what if I purposefully create and announce two blocks at the same time. With the same parent block. For example, in one of the blocks there’s a 1 bitcoin from my wallet spent on drugs. In the other block I spent that same bitcoin on unlicensed firearms. You know, things you do with cryptocurrencies. There are two competing blocks, each having a different version of the history. There is no central database so the blockchain network has no way of figuring out which block is valid and which is not. Well, they both technically are.

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