practice
Doi: 10.1145/1592761.1592776
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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