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 MIL-HDBK-232A
TECHNICAL SURVEILLANCE COUNTERMEASURES SUBCOMMITTEE
AUDIO COUNTERMEASURES WORKING GROUP
TELEPHONE SECURITY PANEL
JUNE 30, 1983
REPORT TO THE COMMUNITY:
COMPUTERIZED TELEPHONE SYSTEMS
It is the policy of the U.S. Intelligence Community that all appropriate
audio security measures are rigorously enforced for any telephone system
which services an area where classified information is discussed. It has
been a matter of concern that appropriate audio security measures for
computer controlled private branch telephone exchanges have not been clearly
defined; the installation and operating standards developed by the Telephone
Security Panel, and provided in this report, are intended to remedy this
deficiency.
A private branch exchange (PBX) operates, essentially, as a private telephone
subnetwork. It ties together an internal group of subscribers into an
independent network and provides external connections to the universal
network by means of trunk lines to a telephone company central office
exchange. A computer controlled private branch exchange (CBX) uses stored
program computer technology to perform the necessary message switching
functions. The resident computer in modern commercial CBX has made it
possible to incorporate a multitude of attractive features for many diverse
applications. There are many features which enhance the basic telephone
service but the applications are not restricted to telephone service; modern
CBX systems provide data processing, word processing, energy consumption
control, communications traffic analysis, and other services in addition to
processing telephone calls. CBX systems were introduced in the nineteen
seventies. There are a great number of manufacturers from many countries
producing them; to a large extent they are utilizing different technologies
and approaches to achieve basically the same objectives. There has been from
introduction, uncertainty as to how the inclusion of CBX systems at sensitive
locations affects the security of those locations. This uncertainty
developed because the CBX properties and features were unfamiliar and because
computer software (rather than easily verifiable hardwire connections) is
used to implement the features and to control the network switching
functions.
It has been determined that the security-related concerns and considerations
which must be accommodated with a CBX are essentially the same as for any
type of telephone system. In fact, a great number of telephone company
central offices use the same kind of stored-pro,-ram computer-controlled
switching to provide multifeature CENTREX service.
All of the protective measures now accepted for central off ice service and
manual PBX systems are fully as effective with CBX systems. These measures
are designed to assure that on-hook audio signals (room conversations picked
up by some microphonic function in a telephone, or other station equipment
such as a console or data terminal, while it is on-line but not actually
being used in a telephone call) cannot become available for clandestine,
unauthorized, intercept. This is accomplished by requiring specified
isolation or disconnect devices on the telephone lines, either on the lines
connecting to the station equipment or on the lines leaving the physical
control zone (PCZ). The isolation/disconnect devices are located within the
PCZ. They prevent audio signals originating at the on-hook station equipment
from being transmitted any further on the telephone lines, thus eliminating
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