In Smith v. Maryland, the Supreme Court made clear the important difference between extrinsic information used to route a communication and the communicated content itself.25 In Smith, the Supreme Court found no Fourth Amendment violation from the government’s warrantless use of a pen register.26 Distinguishing its holding in Katz v. United States27 that warrantless wiretapping violated the Fourth Amendment, the Supreme Court explained that “a pen register differs significantly from the listening device employed in Katz, for pen registers do not acquire the contents of communications.”28 Rather, the Court explained, pen registers “disclose only the telephone numbers that have been dialed—a means of establishing communication. Neither the purport of any communication between the caller and the recipient of the call, their identities, nor whether the call was even completed is disclosed by pen registers.”29 Smith’s differentiation between the “means of establishing communication” and the “purport of a[] communication”30 looms large in federal surveillance law. 25 442 U.S. 735 (1979). 26 Id. at 745-46. 27 389 U.S. 347 (1967). 28 Id. at 741 (emphasis in original). 29 Id. (quoting United States v. New York Tel. Co., 434 U.S. 159, 167 (1977)). 30 Id. 18