The Quest To Liberate $300,000 of Bitcoin From an Old ZIP File

A few quintillion possible decryption keys stand between a man and his cryptocurrency. From a report: In October, Michael Stay got a weird message on LinkedIn. A total stranger had lost access to his bitcoin private keys — and wanted Stay’s help getting his $300,000 back. It wasn’t a total surprise that The Guy, as Stay calls him, had found the former Google security engineer. Nineteen years ago, Stay published a paper detailing a technique for breaking into encrypted zip files. The Guy had bought around $10,000 worth of bitcoin in January 2016, well before the boom. He had encrypted the private keys in a zip file and had forgotten the password. He was hoping Stay could help him break in. In a talk at the Defcon security conference this week, Stay details the epic attempt that ensued.

[…] “If we find the password successfully, I will thank you,” The Guy wrote with a smiley face. After an initial analysis, Stay estimated that he would need to charge $100,000 to break into the file. The Guy took the deal. After all, he’d still be turning quite the profit.
“It’s the most fun I’ve had in ages. Every morning I was excited to get to work and wrestle with the problem,” says Stay, who today is the chief technology officer of the blockchain software development firm Pyrofex. “The zip cipher was designed decades ago by an amateur cryptographer — the fact that it has held up so well is remarkable.” But while some zip files can be cracked easily with off-the-shelf tools, The Guy wasn’t so lucky. That’s partly why the work was priced so high. Newer generations of zip programs use the established and robust cryptographic standard AES, but outdated versions — like the one used in The Guy’s case — use Zip 2.0 Legacy encryption that can often be cracked. The degree of difficulty depends on how it’s implemented, though. “It’s one thing to say something is broken, but actually breaking it is a whole different ball of wax,” says Johns Hopkins University cryptographer Matthew Green.

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