Until the introduction of telegraphy in the mid-19th century, investigators, spies, and petty eavesdroppers could only listen at keyholes or under windows. Since then, the developed world’s growing dependence on electronic communications has provided ever increasing opportunities for those who are determined to "overhear" the exchanges of information between others.
Telephones can be tapped, and advances in electronics and computing have made it possible to listen in on conversations that take place within a room rather than by telephone and to screen other forms of communication, such as faxes and e-mails. At the start of the 21st century, the Internet is a major focus of attention for eavesdroppers and security forces alike.
The motives for electronic surveillance are many and varied. Government intelligence agencies monitor the communications of both enemy and friendly states as part of their routine activities, hoping that the information acquired might help forestall unwanted political developments or offer commercially useful information. Similar agencies monitor the communications of persons suspected of terrorist or subversive activities.
Police forces and security companies use electronic surveillance in the forms of telephone taps and closed-circuit television monitoring for the prevention and detection of crime. Private investigators use telephone taps and "bugs"—miniature microphones and cameras—in the hope of capturing information relevant to their investigations. The legality of such acts varies from state to state and from country to country.
Wiretapping
Telephone conversations are relatively easy to intercept, since they take the form of electrical signals that pass through wires for at least part of their transmission. In its simplest form, a telephone tap is nothing more than a secretly in-stalled extension telephone. The tapping point might be in the basement of the building where the telephone is installed, at the local telephone exchange, or anywhere that the telephone lines are exposed so that the eavesdropper can make the connection with ease. The listening device might be a handset or recording device connected directly at the point of tapping or via a radio link or land line to a remote listening site.
The files of the now-defunct Stasi (Staats-sicherheitsdienst)—state security service of the former German Democratic Republic—reveal that some 5,000 telephone taps were permanently maintained in West Berlin. The taps were installed by Stasi agents and almost constantly monitored from Stasi headquarters in East Berlin until the reunification of Germany in 1990.
The computerization of telephone systems has done a great deal to simplify the work of legally authorized telephone tappers. Computerized exchanges can be programmed to monitor selected telephone lines and transmit conversations to a central listening station. With developments in voice recognition, it is possible in principle to monitor calls made by a given person on any telephone in a network, thus making it impossible to evading tapping by using public telephones.
