05-10-2012, 03:17 PM
General Characteristics of the Subscriber Identity Module File System
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It used to be you could ask the question “who has a mobile phone?” in a group of people
and relatively few hands would go up, now the question is more along the lines of “Who
doesn’t have a mobile phone?” or even “Who has more than one mobile phone?”. Indeed
one need only take a trip to the local shopping mall to see literally hundreds of people
with tiny electronic devices stuck to their ears or the blinking blue lights on headsets to
realize that mobile phones are an inseparable part of modern society.
This paper seeks to give an overview of the general file system structure of the subscriber
identity module-the smart card that is present in GSM mobile phones. The paper will
discuss the layout of the file structure and the some of the various commands used by the
mobile equipment to access this file system.
What is a Subscriber Identity Module?
A subscriber identity module or SIM is a smart card that is designed to fit into mobile
phone. It provides the identification of a user to a network, allowing him or her to access
such services as telephony, email, internet and text messaging. The SIM card contains a
microcomputer as well as a certain amount of memory to process commands (Random
Access Memory or RAM), and to store user files (Electronically Erasable Programmable
Read Only Memory or EEPROM). The SIM also contains an amount of Read Only
Memory (ROM) which stores the cards operating system.
When the SIM card is activated the microcomputer loads the operating system from
ROM into the RAM of the card and processes commands as requested by the mobile
equipment (ME) or card access device (CAD).
SIM Memory Structure
The SIM memory structure is composed of directories that can be said to be roughly
analogous to the directories of a traditional computer hard disk drive. These directories
are spelled out in detail in GSM 11.11 and 11.14. The file system may be comprised of
the following basic forms: a master file (MF), a directory file (DF) and an elementary file
(EF).
The Master File
The Master File or MF is the root of the file system. It is analogous to the root directory
or “/” in the Linux file system; there is only one MF. An MF may contain one Dedicated
File (DF) or many DFs and it may or may not contain one or many Elementary Files
(EF).
The master file can be identified by the 2 byte file identifier (and indeed any file can be
identified by this 2 byte sequence) of 3F00. This identifier is reserved only for the MF.
It should be noted that there is one EF directly beneath the MF in the hierarchy. This EF
identified by the marker EFMF1 in the graphic above is the Integrated Circuit Card
Identity (EF ICCID). This EF contains the unique serial number of each individual SIM
card and can be used to personalize the ME to the SIM1. The use of the EF ICCID is open
to the individual manufacturer and operator. The EF ICCID is the electronic version of the
SIM Serial number recorded on the face of the SIM card body. The SIM Serial Number
and EF ICCID can be used by an examiner to identify the origin of the SIM where evidence
was obtained2.
Dedicated Files
The term Dedicated File is perhaps a bit confusing since the Dedicated File is more akin
to a container or a sub directory rather than an actual file in the traditional sense. A
dedicated file can also be identified by a two byte identifier. This identifier is assigned by
the DF or the MF that contains it. The DF can also be referenced by a name that is
between one and sixteen bytes long. The naming conventions for the DF name can be
found in the ISO 7816-5 specification3. Two Dedicated Files of interest are the DFGSM
and the DFTELECOM.