practice

Doi: 10.1145/1592761.1592776

Article development led by queue.acm.org

encoded in bits. Voices are digitized shortly after they leave the speaker’s lips, carried over an IP network as packets, and returned to analog format for presentation to the listener’s ears.

BY WhitfieLD Diffie anD susan LanDau
communications
surveillance:

Although big changes in telephony have given rise to equally big changes in wiretapping, the essentials remain the same. The interception and exploitation of communications has three basic components: accessing the signal, collecting the signal, and exfiltrating the signal. Access may come through alligator clips, a radio, or a computer program. Exfiltration is moving the results to where they can be used. Collection may be merged with exfiltration or may involve recording or listening.

Privacy and
security at Risk

A phone call can be intercepted at various points along its path. The tap can be in the phone itself, through introduction of a bug or malware that covertly exfiltrates the call, often by radio. The tap can be at the junction box, in a phone closet down the hall, on a telephone pole, or on the frame where incoming subscriber lines connect to the telephone company central office.

The development in the 1980s of digital switches and the features they made possible created problems for traditional local-loop wiretapping. Call forwarding in particular, which diverts the call to a different number before it ever reaches the frame, was problematic. To avoid the possibility of being bypassed, the tap must be placed at or above the level of the diversion. Fortunately for wiretappers, digital switches also introduced conferencing, which allowed several people to converse at once. Taps could be implemented by conferencing in a silent additional party. Taps on analog circuits can, in principle, be detected by the power they drain. Digital wiretaps are invisible to the target but require changes in the programming of the switch rather than extra connections to the frame.

In the 1990s the FBI, claiming that advanced switching technology threatened the effectiveness of wiretapping, persuaded Congress to require that telephone companies build wiretap-

PhotograPhS froM iStoCKPhoto.CoM

As the sophistication of wiretapping
technology grows, so too do the risks
it poses to our privacy and security.

We aLL KNo W the scene: It is the basement of an apartment building and the lights are dim. The man is wearing a trench coat and a fedora pulled down low to hide his face. Between the hat and the coat we see headphones, and he appears to be listening intently to the output of a set of alligator clips attached to a phone line. He is a detective eavesdropping on a suspect’s phone calls. This is wiretapping—as it was in the film noir era of 1930s Hollywood. It doesn’t have much to do with modern electronic eavesdropping, which is about bits, packets, switches, and routers.

Scarcely a generation ago, phone calls traveled
through wires between fixed locations, encoded as
fluctuating electric signals. now phones are mobile,
and, through most of their journeys, phone calls are