Guidelines on Cell Phone Forensics
application support using a Software Development Kit (SDK) for C++ or another language.
Characteristics of a wide range of past, current, and future cell phones can be found on
manufacturer and vendor Web sites, as well as product review sites.
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2.3 Identity Module Characteristics
Subscriber Identity Modules are synonymous with mobile phones and devices that interoperate
with GSM cellular networks. Under the GSM framework, a cellular phone is referred to as a
Mobile Station and is partitioned into two distinct components: the Subscriber Identity Module
(SIM) and the Mobile Equipment (ME). As the name implies, a SIM is a removable
component that contains essential information about the subscriber. The ME, the remaining
radio handset portion, cannot function fully without one. The SIM’s main function entails
authenticating the user of the cell phone to the network to gain access to subscribed services.
The SIM also provides storage for personal information, such as phone book entries and text
messages, as well as service-related information.
The SIM-ME partitioning of a cell phone stipulated in the GSM standards has brought about a
form of portability. Moving a SIM between compatible cell phones automatically transfers
with it the subscriber’s identity and the associated information and capabilities. In contrast,
present-day CDMA phones do not employ a SIM. Analogous SIM functionality is instead
directly incorporated within the device. While SIMs are most widely used in GSM systems,
comparable modules are also used in iDEN phones and UMTS user equipment (i.e., a USIM).
Because of the flexibility a SIM offers GSM phone users to port their identity, personal
information, and service between devices, eventually all cellular phones are expected to
include (U)SIM-like capability. For example, requirements for a Removable User Identity
Module (R-UIM), as an extension of SIM capabilities, have been specified for cellular
environments conforming to TIA/EIA/IS-95-A and -B specifications, which include Wideband
Spread Spectrum based CDMA [3GP02].
At its core, a (U)SIM is a special type of smart card that typically contains a processor and
between 16 to 128 KB of persistent electronically erasable, programmable read only memory
(EEPROM). It also includes RAM for program execution and ROM for the operating system,
user authentication and data encryption algorithms, and other applications. The (U)SIM’s
hierarchically organized file system resides in persistent memory and stores such things as
names and phone number entries, text messages, and network service settings. Depending on
the phone used, some information on the (U)SIM may coexist in the memory of the phone.
Information may also reside entirely in the memory of the phone instead of available memory
reserved for it in the file system of the (U)SIM [Wil05, Jan06].
The (U)SIM operating system controls access to elements of the file system [3GP05a].
Actions such are reading or updating can be permitted or denied unconditionally, or allowed
conditionally with certain access rights. Rights are assigned to a subscriber through 4-8 digit
Personal Identification Number (PIN) codes. PINs protect core (U)SIM subscriber-related
data and certain optional data. PIN codes can be modified by the subscriber, and their function
disabled or enabled. A preset number of attempts, usually three, are allowed for providing the
correct PIN code to the (U)SIM before further attempts are blocked completely, rendering
communications inoperative. Only by providing a correct PIN Unblocking Key (PUK) can the
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For example, specifications and product reviews for many current cell phones can be found at http://www.cnet.com.
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