Private Key Brute Forced With Only 8 Words

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Twitter user Alistair Milne launched a contest where he would unveil one word of a 12-word seed phrase periodically over a month. The wallet contained a whole bitcoin. John Cantrell was able to brute force the seed phrase with only 8 of the words, extracting the funds prematurely before the last 4 words were unveiled. Ultimately, John was declared the winner of the contest and kept the full amount.

This is a global/international case not involving a specific country.

About None

"Alistair Milne, CIO of the Altana Digital Currency Fund, orchestrated a challenge on Twitter where the winner would get an entire Bitcoin. Starting in May, he periodically published hints to a 12-word seed phrase for a wallet address that contained one Bitcoin. Whoever picked up all the clues could use the phrase to unlock the Bitcoin wallet and take the Bitcoin inside."

"Alistair Milne tweeted that he planned to giveaway 1 Bitcoin in a wallet generated using a 12-word mnemonic." "The private keys to [a] 1BTC wallet were generated from a 12-word mnemonic seed. Over the next ~30 days I will be releasing the words (or a clue to a word) on my various social media pages."

"Milne planned to post the last three or four words in one go. This was an attempt to prevent someone from brute-forcing the address open (by continuously guessing words until a combination worked). But his plan failed. With just eight words, Cantrell was able to guess the remaining words, find the right combination and unlock the wallet."

"With 8 known words there are 2^40 (~1.1 trillion) possible mnemonics." "To test a single mnemonic we have to generate a seed from the mnemonic, master private key from the seed, and an address from the master private key."

"I ported all necessary code for generating and checking a mnemonic (SHA-256, SHA-512, RIPEMD-160, EC Addition, EC Multiplication) to OpenCL C which is a programming language to run code on a GPU." "I wrote a server application that would orchestrate the distribution of work into batches of ~16 million mnemonics to a pool of GPU workers. Each GPU worker would ask the server for the next batch of work to do, perform the work, and log the result back to the server."

"I spent ~$350 renting GPUs from vast.ai (plus ~$75 for free from Azure)." "I was worried about other people doing the same and is why I included a .01 BTC miner fee. I didn’t think even this would be enough and thought there could be a ‘race to zero’ where people continually increased the fee trying to get the miners to include their transaction in the next block."

"Creating a contest that isn’t won by software is difficult. I’d like to pay-it-forward and try crafting one myself."

This is a global/international case not involving a specific country.

The background of the exchange platform, service, or individuals involved, as it would have been seen or understood at the time of the events.

Include:

• Known history of when and how the service was started.
• What problems does the company or service claim to solve?
• What marketing materials were used by the firm or business?
• Audits performed, and excerpts that may have been included.
• Business registration documents shown (fake or legitimate).
• How were people recruited to participate?
• Public warnings and announcements prior to the event.

Don't Include:

• Any wording which directly states or implies that the business is/was illegitimate, or that a vulnerability existed.
• Anything that wasn't reasonably knowable at the time of the event.

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The Reality

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• When the service was actually started (if different than the "official story").
• Who actually ran a service and their own personal history.
• How the service was structured behind the scenes. (For example, there was no "trading bot".)
• Details of what audits reported and how vulnerabilities were missed during auditing.

What Happened

The specific events of the loss and how it came about. What actually happened to cause the loss and some of the events leading up to it.

Key Event Timeline - Private Key Brute Forced With Only 8 Words

Date	Event	Description
June 16th, 2020 7:49:00 PM	Main Event	Expand this into a brief description of what happened and the impact. If multiple lines are necessary, add them here.

Total Amount Lost

The total amount lost has been estimated at $9,000 USD.

How much was lost and how was it calculated? If there are conflicting reports, which are accurate and where does the discrepancy lie?

Immediate Reactions

How did the various parties involved (firm, platform, management, and/or affected individual(s)) deal with the events? Were services shut down? Were announcements made? Were groups formed?

Ultimate Outcome

What was the end result? Was any investigation done? Were any individuals prosecuted? Was there a lawsuit? Was any tracing done?

Total Amount Recovered

There do not appear to have been any funds recovered in this case.

What funds were recovered? What funds were reimbursed for those affected users?

Ongoing Developments

What parts of this case are still remaining to be concluded?

Prevention Policies

The contest showed that brute forcing a seed phrase can be done practically with only 8 words instead of the full 12. Never share part of your seed phrase with anyone else unless you intend to give them all your funds, as this can be used in a brute force attack.