In United States v. New York Telephone Co. the Supreme Court first ruled that an ordinary search warrant sufficed to authorize law enforcement use of a pen register. Additionally, the Court ruled that a federal district judge possessed the authority to include within the search warrant an order compelling a telephone company to assist the government by installing a pen register on the targeted telephone.
In Smith v. Maryland, the Court considered the legality of pen register surveillance conducted by a telephone company pursuant to an informal police request, that is, without a warrant or other court order. In holding such pen register use lawful and the evidence obtained admissible against the defendant at his robbery trial, the Court held that no "search" had occurred and thus no right to privacy protected by the fourth amendment was invaded.
Although the result in Smith is supportable under the particular facts of the case, the Court's reasoning poses significant problems of both statutory and constitutional dimensions. As will be demonstrated below, the majority opinion relied on unsupportable factual assumptions, effectively nullified congressional intent, and introduced an absurd anomaly into federal law. Ironically, however, while the Smith decision has unsettling implications for the right to privacy, it is likely to have little if any impact upon law enforcement use of pen registers.
Clifford S. Fishman, Pen Registers and Privacy: Risks, Expectations, and the Nullification of Congressional Intent, 29 CATH. U. L. REV. 557 (1980).