Renowned New York University economist Nouriel Roubini attacked the legitimacy of the bitcoin in a series of tweets Sunday.

"So bitcoin isn't a currency. It is btw [by the way] a Ponzi game and a conduit for criminal/illegal activities. And it isn't safe, given hacking of it," wrote the man nicknamed Dr. Doom for his bearish economic forecasts.

Roubini scoffs at the notion that the bitcoin is a real currency. "Bitcoin isn't a store of value, as little wealth is in Bitcoin and no assets in it. Also given price volatility, it is a lousy store of value," he tweeted.

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"Apart from a base 4 criminal activities, bitcoin is not a currency. . . . Bitcoin is not a unit of account, as no price of goods and services is set in bitcoin."

Gold enthusiasts might be surprised to see that Roubini lumps them together with bitcoin advocates. "BTC [Bitcoin] bugs like gold bugs are fanatics who speak of BTC in cult-like religious ways. Like gold bugs, they have paranoid conspiracy views on the $."

To be sure, the bitcoin has risen 14 percent to $621.54 during the last two weeks, according to CoinDesk.

Not everyone agrees with Roubini.

"Bitcoin works really well," Matthew Green, a Johns Hopkins University cryptographer who is working to create another digital currency, told The Washington Post. "All this craziness around bitcoin isn't around bitcoin itself. It's around the people."

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