In the past few days, through the DEFCON event, Michael Stai, Director of the Technical Office at Pyrofex, a blockchain software development company, recounted how he helped recover the private keys of Bitcoin worth more than $300,000.

The keys were locked in a zip file, and Stay was assigned the task of decrypting the file and recovering the Bitcoin keys inside.

$100,000 Bonus to Redeem $300,000 in Bitcoin:

Stay reviewed and researched more in-depth on how to decrypt zip files, after a Russian citizen contacted him on LinkedIn in October last year telling him what had happened to him and how he had bought $10,000 worth of bitcoins in January 2016, and he forgot his word. Passage of the zip file containing the private bitcoin keys.

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This Russian asked “Stay” to decode the file, saying:

If we can find the password, I will thank you.

On the initial analysis of the problem, Tell “Stay” said the problem and its solution would require a sum of $100,000, for the Russian man to accept the offer.

Where the value of the stored bitcoin was about $300,000, meaning that he would still make a profit even if he paid $100,000.

Narrowing down the possibilities for accessing the decoder key:

The Russian man used a zip file based on ZIP 2.0 Legacy encryption.

Although the ciphers were designed by “crypto hobbyists” decades ago, it was impossible to access them in the usual ways.

Fortunately, the Russian man knew the zip program and the version he used to lock his bitcoin keys, as he also had a timestamp related to the day the file was created.

Stay began to limit the cryptographic key findability by continuing to narrow the odds down to the ‘Quintillions’ probability of being found with a probability of 1 behind 18 zeros.

Decryption attempt and failure:

Stay continued working on the decoding and hired Pyrofex CEO Nash Foster to implement cryptanalysis code and run it on general-purpose Nvidia Tesla GPUs.

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According to Stay, this greatly improved the detection of possibilities, and reduced the time required to access the required file.

Foster said:

Stay ended up being able to do a more efficient job with cryptanalysis, so we spent more time developing the attack but then needed to run it for about a week.

This saved the man a lot of money on infrastructure costs.

Ten years ago, there was no way to do this without building special purpose hardware, and the cost would probably have exceeded the value of the bitcoins he would like to mine

Stay and Foster had to work with only media notes about the file to be decrypted because the Russian man did not fully trust them regarding the file’s contents.

They can, after all, steal his bitcoins after cracking the encryption of the zip file.

The directors of Pyrofex started working in February of this year with everything they had.

Ten days later, all their attempts had failed, and the concern grew that it might take so long to set things right.

keep trying:

STAY reviewed the process, hoping to find an anomaly and come up with an idea to calculate the number or code needed as a starting point for the random number generator used in the zip coding scheme.

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The Russian guy combed through the test data and found that the GPU didn’t process the correct password on the first try.

The Pyrofex duo identified the issue and re-launched the attack using an alternative “Stay” method.

Finally, they got in and were able to retrieve the private bitcoin keys.

Infrastructure costs are set at $6000-7000, which is very little compared to the $100,000 requested.

The two sides eventually agreed to pay the Russian man $25,000 to decrypt the file.

Foster commented on the operation and its success by saying:

Projects like this are totally unusual.

If some of the bits and details are a little different, for example if he used a slightly newer version of the zip file, decrypting it would be impossible.

But in this particular case there was something we could do.

While Stay and Foster helped the Russian man recover his bitcoins locked in a zip file, others like Harry Denley helped recover stolen crypto funds from the clutches of hackers, which we pointed out in the article:

Security experts help salvage a portion of the stolen Ethereum coins

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