The volatile rise-and-fall of Bitcoin has prompted lots of stories explaining why the online virtual currency is a classic bubble.The technicalities of the Bitcoin system are complex, but to make this online currency more successful than previous versions, the designers overcame two key challenges. First, to prevent counterfeiting, they attached a history of transactions to each currency unit — but allowed users to keep their transactions nearly anonymous. Counterfeiting is hard because fake Bitcoins would need an authenticated history to pass muster.