However, users can voluntarily give up the privacy of their coins if they want law enforcement to trace stolen funds. For this, a user would need to reveal his unique cryptographic key to, say, the police, and then the police can see when these stolen coins are being spent at a restaurant, a store or other kind of merchant, because merchants, unlike individual users, would be known to the system. So the police would be able to find where that merchant is, go there and arrest the thieves, Chaum said.
