It all happened overnight and in a matter of minutes. Ben Zhou, CEO of the cryptocurrency exchange Bybit, made a series of routine transfers from his home computer. A short while later, his company called to inform him that his reserves of Ethereum, the second most-used cryptocurrency after Bitcoin, worth $1.5 billion, had vanished. By then, the ethers had already been transferred to thousands of other people’s digital wallets. Bybit had just suffered the largest theft in history. Five days later, the FBI confirmed what some analysts suspected from the outset: the attack was the work of Lazarus, a hacking...