Hackers allegedly stole $400 million in bitcoins. Here’s how to catch them.

February 28

(Photo by btckeychain)

On Friday, what had been the world's leading Bitcoin exchange declared bankrutpcy, claiming that hackers had exploited a technical issue called "transaction malleability" to steal 750,000 bitcoins.

Bitcoin transactions are sometimes described as being anonymous, but that's not quite accurate. Every Bitcoin transaction ever made is stored in a shared transaction ledger called the blockchain. The problem is that it's not always easy to tie addresses in the blockchain with the identities of individuals.

Sarah Meiklejohn, a computer scientist at the University of California, San Diego, has done groundbreaking research on this problem. In a Friday interview, she discussed the techniques the authorities might use to identify the alleged hackers — and to determine whether hacking actually occurred in the first place. The transcript has been edited for length and clarity.

Timothy B. Lee: What are the chances of identifying the people who allegedly took the Bitcoins?

Sarah Meiklejohn: Using the clustering techniques that we developed, you'd want to look for transactions that represented withdrawals from Mt. Gox -- transactions where the input was Mt. Gox and the output was whoever. In order to take advantage of this transaction malleability thing, you'd look for two transactions that have the same input and the same address as the output and that the amount was the same. Basically all the transaction details were the same, the two transactions were done fairly close together in time.

That would be a way to exploit this transaction malleability flaw. User could do a withdrawal, change the transaction hash, use a different transaction, and then say, "Hey, my transaction never went through." Mt. Gox would say, "That hash is not in the blockchain," and all the details would be the same.

So, that lets you identify transactions that were likely stolen by hackers. Could you follow the stolen bitcoins to figure out who has them now?

That would depend on how sophisticated this alleged thief was. [To identify the thief], you'd try to apply the tracking techniques we developed. If the thief was not very smart, they might have just withdrawn to a different exchange. You could try to subpoena that exchange and figure out who they were. If they used other techniques, you might be able to track them, but it would depend on how sophisticated the user was.

What kind of things might a hacker have done to cover his or her tracks?

One thing is they could have mixed all those bitcoins. They could have withdrawn them to an address they own, mixed them up, and then withdrawn them from the system. They might have been very successful in this.

The other thing is they could have done this multiple years ago, at which point there's basically nothing they can do. If they cashed out two years ago, then it's difficult to go after them. A lot of exchanges that were active two years ago might have shut down. It was sort of a different landscape two years ago.

$400 million is a lot of bitcoins. Wouldn't it be hard to cash them all out without being detected?

If that user, if they exist, were trying to cash out all of the bitcoins they allegedly stole all at once, that would be basically impossible. That's a huge number of bitcoins. But if you spread it out over a couple of years, it becomes easier.

Mt. Gox claims that hackers exploited this transaction malleability issue to steal 750,000 bitcoins from them. Do you believe them?

People knew about this bug for a while. It is possible that someone who did know about this bug — which was a lot of people in the Bitcoin community — were aware of this, this kind of gap. If it's a malicious party who noticed this -- noticed Mt. Gox software was not well-written -- it is possible that someone could have been taking advantage of this for years.

But I'm very skeptical of Mt. Gox's claims. I think they might have mismanaged their funds. It seems a little too convenient, the timing of the whole thing. The fact is Mt. Gox has been having serious problems for up to a year now, first with basically being unable to withdraw dollars. Users were complaining that they could not withdraw dollars from Mt. Gox. They've been having that problem for close to a year now. They've been having a lot of problems. Seems a little too convenient to point the finger at transaction malleability.

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