Those who currently use bitcoins to buy drugs, weapons or hire assassins also have something else to worry about. All transactions leave a permanent record that's traceable to specific wallets. Even though Bitcoins are technically anonymous, if they ever pull that money out in their name at a legitimate Bitcoin bank or exchange, law enforcement has a direct connection to a real identity.
That's why Weaver says bitcoins are essentially "prosecution futures."
Bitcoin tracing experts are already cropping up -- and they're teaming up with law enforcement too. Sarah Meiklejohn is a computer scientist and graduate student at the University of California in San Diego, where she explores how people are spending their bitcoins.
She said people spending bitcoins in black markets don't realize "every bitcoin is by nature a marked bill." And it's not as forgiving as actual cash. If police catch you buying something illegal in person, you get criminal charges for that single act. Bitcoin's permanent, public ledger gives prosecutors a much longer memory.
"if you get caught buying drugs with bitcoins and they look at your transaction history, you get caught on every transaction you've ever made," she said. "It's much more serious."