But he started instead with the simplest of questions: What if an Ethereum owner stored their digital money with a private key—the unguessable, 78-digit string of numbers that protects the currency stashed at a certain address—that had a value of 1?

But the cash had already been taken out of the Ethereum wallet that used it—almost certainly by a thief who had thought to guess a private key of 1 long before Bednarek had.

So he and his colleagues at the security consultant Internet Security Evaluators wrote some code, fired up some cloud servers, and tried a few dozen billion more.

A single Ethereum account seems to have siphoned off a fortune of 45,000 ether—worth at one point more than $50 million—using those same key-guessing tricks.

"Whoever this guy or these guys are, they're spending a lot of computing time sniffing for new wallets, watching every transaction, and seeing if they have the key to them."

To explain how that blockchain banditry works, it helps to understand that the the odds of guessing a randomly generated Ethereum private key is 1 in 115 quattuorvigintillion.

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