Roviaro (D) was indicted when he sold heroin to 'John Doe,' an undercover government employee/informer. After D's arrest, he was confronted with Doe at the police station. Doe denied he knew or had ever seen D. Before trial, D moved for a bill of particulars, requesting, among other things, information regarding the identity of John Doe. The government objected on the grounds that John Doe was an informer, and his identity was privileged. The motion was denied. During the trial, government witnesses testified as to D's transaction with Doe, and during cross-examination, D sought repeatedly to learn Doe's identity. The court did not allow this cross-examination, and Doe was neither produced nor identified. D was convicted and appeals.