SIM File System

The file system used on a SIM is identical to that laid down by ISO 7816 for smart cards, and is fairly basic. Anyone familiar with common desktop computer systems should be aware of the following differences:

 

Ø       All files are identified (named) in 4 bytes

Ø       Directories are of fixed size, and known as "dedicated"

Ø       Files must be 'selected' before you can do anything with them

 

Files on a SIM come in a variety of types:

 

File Type	Description
Transparent	Random access file that can contain anything.
Linear fixed file	Records of fixed length; can be navigated by moving to 'next', 'last', etc. Examples include an address book function.
Cyclic	Fixed length records, where selecting the 'next' of the last one leads to the first. Examples include last numbers dialed.
Variable length records	Not supported in GSM.
Incremental	Very useful files, but very small. A single byte that can be incremented or decremented by one. Useful because control can be very granular: one PIN to decrement, one to increment. Examples include pay-as-you-go applications.

 

There are already many files on a typical SIM, and indeed, to conform to the GSM protocol you need a number of files. The following tree structure shows the files required for GSM compatibility:

 

Some Useful Files

Ø       2FE2 - Unique integrated circuit number

Ø       6F05 - Language choice

Ø       6F07 - Unique subscriber number

Ø       6F3C - SMS messages (176 length fixed linear)

Ø       6F44 - Last number dialed

Creating Files

When creating new files, it's important to avoid namespace problems, and the best process is certainly to create your own directory off the root (which is always known as 3F00, for arcane reasons) and place all your data there. Creating files is just a matter of sending the correct commands to the SIM:

 

F0	Create LF /DA01
E0
00
00
11
FF
FF
00	Block Size
80
DA	File ID
01
02	Type
00	Valid Action
44	Access Conditions
F1
44
01	Status
04	Bytes
11	Access Keys
11
11
20	Optional (in this case, record length)

 

This example creates a linear fixed file, with a fixed length of 32 bytes. The directory will already have been selected. Note the complexity of "Access Conditions" and "Access Keys" - in this example, only one PIN is required, and it allows full read/write access.

 

However, files are generally written to the SIM during the manufacturing process, for security reasons. These files generally make up a "Soft Mask" that cannot be changed, ever. SIMs also hold a "Hard Mask", which contains the OS and system files. The "Hard Mask" is not normally available, even to developers.

 

Details of all the available commands can be found in ISO 7816, or the programming manual for your particular SIM.

PIN Security

Access to files may be controlled in an extremely granular manner, using only the ISO 7816 required controls. Files may be read (or write) only, may allow read or write after the presentation of a particular PIN, or never allow any kind of external access. This gives an enormous amount of security, with very little programming.

 

GSM PINs should generally be numerical (given the restrictions of the keypad), but ISO 7816 allows any combination of bytes to be used.

 

Once a PIN has been presented, it remains valid for the duration of the session. This may be until power down, or until a different directory is selected. (This will be the same thing in most cases, but not always.) This means that a user can be asked for a PIN, which is then presented to the SIM, which allows use until power down without storing the PIN on the handset (which wouldn't be very secure).