The promise of digital cryptocurrencies like bitcoin is that you don't need to trust the people you send or receive money from because the software makes it technically impossible for anyone to cheat the system. Instead of relying on humans and their flawed judgment, you rely on the laws of mathematics. But a recent attack on the cryptocurrency Ethereum Classic---not to be confused with the original Ethereum project---shows once again how hard it is to remove human frailty from digital systems.

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