A pen register is an electronic device which records all outgoing non-content information regarding telephone calls and Internet communications. Non-content information includes telephone numbers dialed and the length of calls, as well as the identities of an email message's sender and recipient.
As defined in 18 U.S.C. §3127(3):
|“||[T]he term "pen register" means a device or process which records or decodes dialing, routing, addressing, or signaling information transmitted by an instrument or facility from which a wire or electronic communication is transmitted, provided, however, that such information shall not include the contents of any communication, but such term does not include any device or process used by a provider or customer of a wire or electronic communication service for billing, or recording as an incident to billing, for communications services provided by such provider or any device or process used by a provider or customer of a wire communication service for cost accounting or other like purposes in the ordinary course of its business.||”|
The term "pen register" is an antiquated term. It stems from the manner in which the digits in a phone number were recorded when telephones used pulse dialing technology, which has since been replaced by touch-tone technology.
The term still applies to the recovery and recording of the dialing information that addresses a call to and from an intercept subject. Authority for initiating a pen register or trap and trace surveillance in the United States is found in 18 U.S.C. §3123.
Law enforcement agents are not required to obtain search warrants before employing pen registers. The Electronic Communications Privacy Act of 1986 requires the U.S. Attorney General to "annually report to Congress on the number of pen register orders and orders for trap and trace devices applied for by law enforcement agencies of the Department of Justice."
Prior to 2001, FISA allowed law enforcement officers to collect incoming and outgoing numbers on a telephone line. The USA PATRIOT Act expanded the law to permit the capture of comparable information related to other forms of communication including the Internet, electronic mail, web surfing, and all other forms of electronic communications.