Ari Juels and Elaine Shi explain to MIT Technology Review that "smart contracts" could be problematic. Together with Ahmed Kosba of University of Maryland, the team, in the provocatively titled paper "The Ring of Gyges: Using smart contracts for crime", show that "criminal smart contracts" can  facilitate leakage of confidential information, theft of cryptographic keys, and various real-world "calling-card" crimes, such as murder, arson, and terrorism.

For example, a criminal organization might put out a smart contract on someone's life: an unpredictable feature of the crime-to-be, such as the time and location of the killing, is provided in a cryptographically sealed form in advance of the crime; the smart contract automatically releases the funds when this proof of responsiblity for the crime is opened by the criminals who perpetrated it.

“In some ways this is the perfect vehicle for criminal acts, because it’s meant to create trust in situations where otherwise it’s difficult to achieve,” says Juels.  “We are optimistic about [these contracts'] beneficial applications, but crime is something that is going to have to be dealt with in an effective way if those benefits are to bear fruit,” says Shi.