Security And Encryption
motivations for security in cellular telecommunications systems are to secure
conversations and signaling data from interception as well as to prevent cellular
telephone fraud. With the older analog-based cellular telephone systems such as
the Advanced Mobile Phone System (AMPS) and the Total Access Communication System
(TACS), it is a relatively simple matter for the radio hobbyist to intercept cellular
telephone conversations with a police scanner. A well-publicized case involved
a potentially embarrassing cellular telephone conversation with a member of the
British royal family being recorded and released to the media. Another security
consideration with cellular telecommunications systems involves identification
credentials such as the Electronic Serial Number (ESN), which are transmitted
"in the clear" in analog systems. With more complicated equipment, it
is possible to receive the ESN and use it to commit cellular telephone fraud by
"cloning" another cellular phone and placing calls with it. Estimates
for cellular fraud in the U.S. in 1993 are as high as $500 million. The procedure
wherein the Mobile Station (MS) registers its location with the system is also
vulnerable to interception and permits the subscriber's location to be monitored
even when a call is not in progress, as evidenced by the recent highly-publicized
police pursuit of a famous U.S. athlete.
security and authentication mechanisms incorporated in GSM make it the most secure
mobile communication standard currently available, particularly in comparison
to the analog systems described above. Part of the enhanced security of GSM is
due to the fact that it is a digital system utilizing a speech coding algorithm,
Gaussian Minimum Shift Keying (GMSK) digital modulation, slow frequency hopping,
and Time Division Multiple Access (TDMA) time slot architecture. To intercept
and reconstruct this signal would require more highly specialized and expensive
equipment than a police scanner to perform the reception, synchronization, and
decoding of the signal. In addition, the authentication and encryption capabilities
discussed in this paper ensure the security of GSM cellular telephone conversations
and subscriber identification credentials against even the determined eavesdropper.
GSM (group special mobile or general
system for mobile communications) is the Pan-European standard for digital cellular
communications. The Group Special Mobile was established in 1982 within the European
Conference of Post and Telecommunication Administrations (CEPT). A Further important
step in the history of GSM as a standard for a digital mobile cellular communications
was the signing of a GSM Memorandum of Understanding (MoU) in 1987 in which 18
nations committed themselves to implement cellular networks based on the GSM specifications.
In 1991 the first GSM based networks commenced operations. GSM provides enhanced
features over older analog-based systems, which are summarized below:
Mobility: The subscriber has the advantage of a Pan-European system allowing
him to communicate from everywhere and to be called in any area served by a GSM
cellular network using the same assigned telephone number, even outside his home
location. The calling party does not need to be informed about the called person's
location because the GSM networks are responsible for the location tasks. With
his personal chipcard he can use a telephone in a rental car, for example, even
outside his home location. This mobility feature is preferred by many business
people who constantly need to be in touch with their headquarters.
Capacity and Optimal Spectrum Allocation: The former analog-based cellular
networks had to combat capacity problems, particularly in metropolitan areas.
Through a more efficient utilization of the assigned frequency bandwidth and smaller
cell sizes, the GSM System is capable of serving a greater number of subscribers.
The optimal use of the available spectrum is achieved through the application
Frequency Division Multiple Access (FDMA), Time Division Multiple Access (TDMA),
efficient half-rate and full-rate speech coding, and the Gaussian Minimum Shift
Keying (GMSK) modulation scheme.
The security methods standardized for the GSM System make it the most secure
cellular telecommunications standard currently available. Although the confidentiality
of a call and anonymity of the GSM subscriber is only guaranteed on the radio
channel, this is a major step in achieving end-to- end security. The subscriber's
anonymity is ensured through the use of temporary identification numbers. The
confidentiality of the communication itself on the radio link is performed by
the application of encryption algorithms and frequency hopping which could only
be realized using digital systems and signaling.
The list of services available to GSM subscribers typically includes the following:
voice communication, facsimile, voice mail, short message transmission, data transmission
and supplemental services such as call forwarding.
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