On February 10, a Bitcoin exchange called MtGox announced it had lost some 850,000 bitcoins, of which 750,000 belonged to its customers. At the time, bitcoins were trading at $827 apiece, making the value of the loss equivalent to $620 million.
That’s a significant shortfall by anyone’s standards. But MtGox had an explanation. In a press release on that day, it announced it had been the victim of a fraud in which the bitcoins had been stolen by hackers.
The fraud, said the company, was a result of a problem known as a transaction malleability bug. This allows malicious users to transfer bitcoins into their accounts while making MtGox think the transfer had failed. Consequently, MtGox repeated these transactions so that the total amount was transferred twice.
Today, Christian Decker and Roger Wattenhofer at the Swiss Federal Institute of Technology in Zurich cast doubt on this version of events. These guys have been monitoring bitcoin transactions since January 2013 in a way that allows them to detect malleability bug transactions. And they say that the total number of fraudulent transfers in that time is several orders of magnitude smaller than MtGox claims.
Decker and Wattenhofer began monitoring the Bitcoin network in January 2013. They recorded all transactions, as well as those that were blocked, by connecting to around 1000 nodes in the Bitcoin network. That’s about 20 percent of the total.
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