it would have to be enacted by assembly as statutory solitude law.

A pen register is similar to a trap and trace device. A trap and trace device would show what numbers had called a specific telephone, i.e. all incoming phone numbers. A pen register rather would show what numbers a phone had called, i.e. all outgoing phone numbers. The two terms are often used in concert, especially in the context of Internet communications. They are often jointly referred to as "Pen Register or Trap and Trace devices," to reflect the fact that the same program will probably do both functions in the modern era, and the distinction is not that important. The term 'pen register' is often used to describe both pen registers and trap and trace devices.

In Katz v. United States (1967), the United States Supreme Court recognized its "reasonable expectation of privacy" test. It overturned Olmstead v. United States and held that wiretaps were unconstitutional searches, because there was a reasonable expectation that the announcement would be private. The government was then required to get a warrant to execute a wiretap.
Ten years later the superlative Court held that a pen register is not a search because the "petitioner voluntarily conveyed numerical information to the telephone company." Smith v. Maryland, 442 U.S. 735, 744 (1979). Since the defendant had disclosed the dialed numbers to the telephone business so they could connect his call, he did not have a reasonable expectation of privacy in the numbers he dialed. The court did not differentiate between disclosing the numbers to a human worker or just the routine equipment used by the telephone company.
The Smith decision left pen registers completely outside constitutional protection. If there was to be any privacy protection, it would have to be enacted by assembly as statutory solitude law.

A device or process which records or decodes routing.

A device or process which records or decodes routing, addressing, or signalling information transmitted by an tool or facility from which a wire or electronic announcement is transmitted, make available, 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 announcement 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 client of a wire communication service for cost accounting or other like purposes in the ordinary course of its commerce.
This is the current definition of a Pen Register, as amended by way of the 2001 USA PATRIOT Act. The original statutory definition of a pen register was created in 1984 as part of the Electronic Communications Privacy Act, which defined a "Pen Register" as:
A device which proceedings or decodes electronic or other impulses which identify the numbers called or otherwise transmitted on the telephone line to which such device is dedicated.


Almost private: Pen Registers, Packet Sniffers, and
Privacy at the Margin
The word pen record originally referred to a device for recording telegraph signals on a strip of paper. Samuel F B Morse's 1840 telegraph patent described such a register as consisting of level holding an armature on one end, opposite an electromagnet with a fountin pen, pensil or other marking tool on the other end, and a clockwork mechanism to advance a paper recording tape under the marker.
The term cable register came to be a generic term for such a recording machine in the later 19th century. See for example, Frank Wood's Telegraph Register. Where the record was made in ink with a pen, the term pen register emerged. By the end of the 19th century, pen registers were widely used to record pulsed electrical signals in many contexts. For example, one fire-alarm scheme used a "double pen-register", and another used a "single or multiple pen register".
As pulse dialing came into use for telephone exchange, pen registers had obvious applications as diagnostic instruments for recording sequences of phone dial pulses. With the passage of time, any tool that could be used for this purpose came to be defined as a pen register.

Request of the Espionage Act of 1917

Request of the Espionage Act of 1917 has since its
original passage during World War I.  Adapting to fit the need to
protect national security during wartime and peacetime, the Act
applies to both American citizens and foreign nationals and
extends beyond the physical borders of the United States.  The
advent of computer technology, and perhaps even more
highly, the Internet, has created newer and faster modes of
intelligence acquisition, transfer, theft, communication, and
publication.  Courts have consistently apparent a line between the
person who leaks intelligence that threatens national security and
the person or institution that publishes the leaked intelligence that
threatens national security.  The equilibrium between national security
and freedom of the press has been respected, but when modern
technologies blur the line between leaking and publishing
classified data the balance can no longer be so maintain. The recent
Wick Leaks argument has highlighted the impact
of technology on application of the Espionage Act to publication of
sensitive government information.  As a foreign nationwide, Assuage
could still be subject to prosecution if the U.S. government could
show he acted as a communicator of dangerous national security
information rather than as a traditional associate of the pres.