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==Phrack Inc.==
Volume Two, Issue Eleven, Phile #9 of 12
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The following is reprinted from the November 1985 issue of Personal
Communications Technology magazine by permission of the authors and
the publisher, FutureComm Publications Inc., 4005 Williamsburg Ct.,
Fairfax, VA 22032, 703/352-1200.
Copyright 1985 by FutureComm Publications Inc. All rights reserved.
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THE ELECTRONIC SERIAL NUMBER: A CELLULAR 'SIEVE'?
'SPOOFERS' CAN DEFRAUD USERS AND CARRIERS
by Geoffrey S. Goodfellow, Robert N. Jesse, and Andrew H. Lamothe, Jr.
What's the greatest security problem with cellular phones? Is it privacy of
communications? No.
Although privacy is a concern, it will pale beside an even greater problem:
spoofing.
'Spoofing' is the process through which an agent (the 'spoofer') pretends to
be somebody he isn't by proffering false identification, usually with intent
to defraud. This deception, which cannot be protected against using the
current U.S. cellular standards, has the potential to create a serious
problem--unless the industry takes steps to correct some loopholes in the
present cellular standards.
Compared to spoofing, the common security concern of privacy is not so severe.
Most cellular subscribers would, at worst, be irked by having their
conversational privacy violated. A smaller number of users might actually
suffer business or personal harm if their confidential exchanges were
compromised. For them, voice encryption equipment is becoming increasingly
available if they are willing to pay the price for it.
Thus, even though technology is available now to prevent an interloper from
overhearing sensitive conversations, cellular systems cannot--at any
cost--prevent pirates from charging calls to any account. This predicament is
not new to the industry. Even though cellular provides a modern,
sophisticated quality mobile communications service, it is not fundamentally
much safer than older forms of mobile telephony.
History of Spoofing Vulnerability
The earliest form of mobile telephony, unsquelched manual Mobile Telephone
Service (MTS), was vulnerable to interception and eavesdropping. To place a
call, the user listened for a free channel. When he found one, he would key
his microphone to ask for service: 'Operator, this is Mobile 1234; may I
please have 555-7890.' The operator knew to submit a billing ticket for
account number 1234 to pay for the call. So did anybody else listening to the
channel--hence the potential for spoofing and fraud.
Squelched channel MTS hid the problem only slightly because users ordinarily
didn't overhear channels being used by other parties. Fraud was still easy
for those who turned off the squelch long enough to overhear account numbers.
Direct-dial mobile telephone services such as Improved Mobile Telephone
Service (IMTS) obscured the problem a bit more because subscriber
identification was made automatically rather than by spoken exchange between
caller and operator. Each time a user originated a call, the mobile telephone
transmitted its identification number to the serving base station using some
form of Audio Frequency Shift Keying (AFSK), which was not so easy for
eavesdroppers to understand.
Committing fraud under IMTS required modification of the mobile--restrapping
of jumpers in the radio unit, or operating magic keyboard combinations in
later units--to reprogram the unit to transmit an unauthorized identification
number. Some mobile control heads even had convenient thumb wheel switches
installed on them to facilitate easy and frequent ANI (Automatic Number
Identification) changes.
Cellular Evolution
Cellular has evolved considerably from these previous systems. Signaling
between mobile and base stations uses high-speed digital techniques and
involves many different types of digital messages. As before, the cellular
phone contains its own Mobile Identification Number (MIN), which is programmed
by the seller or service shop and can be changed when, for example, the phones
sold to a new user. In addition, the U.S. cellular standard incorporates a
second number, the 'Electronic Serial Number' (ESN), which is intended to
uniquely and permanently identify the mobile unit.
According to the Electronic Industries Association (EIA) Interim Standard
IS-3-B, Cellular System Mobile Station--Land Station Compatibility
Specification (July 1984), 'The serial number is a 32-bit binary number that
uniquely identifies a mobile station to any cellular system. It must be
factory-set and not readily alterable in the field. The circuitry that
provides the serial number must be isolated from fraudulent contact and
tampering. Attempts to change the serial number circuitry should render the
mobile station inoperative.'
The ESN was intended to solve two problems the industry observed with its