FILE - In this April 3, 2013 file photo, Mike Caldwell, 35, rolls a stack of bitcoin tokens at his shop in Sandy, Utah. Just as miners dig up precious metals, Bitcoins are mined digitally. Developed at the start of 2009 by an anonymous computer programmer, Bitcoins have been associated with the drug trade and tax avoidance. Because there is a finite supply of just 21 million Bitcoins, the expectation among its devotees is that the currency will continue to appreciate. (AP Photo/Rick Bowmer)