Despite its small size, the SIM card is one of the most important parts of a GSM network because it contains all the subscription information of a subscriber. Since it is standardized, a subscriber can use any GSM or UMTS phone by simply inserting the SIM card. Exceptions are phones that contain a ‘SIM lock’ and thus only work with a single SIM card or only with the SIM card of a certain operator. However, this is not a GSM restriction. It was introduced by mobile phone operators to ensure that a subsidized phone is only used with SIM cards of their network.
The most important parameters on the SIM card are the IMSI and the secret key (Ki), which is used for authentication and the generation of ciphering keys (Kc). With a number of tools, which are generally available on the Internet free of charge, it is possible to read out most parameters from the SIM card, except for sensitive parameters that are read protected. Protected parameters can only be accessed with a special unlock code that is not available to the end user.
Astonishingly, a SIM card is much more than just a simple memory card as it contains a complete microcontroller system that can be used for a number of additional purposes. The mobile phone cannot access the information on the EEPROM directly, but has to request the information from the SIM’s CPU. Therefore, direct access to sensitive information is prohibited.
Furthermore, the microcontroller system on the SIM can also execute programs which the network operator may have installed on the SIM card. This is done via the SIM application toolkit (SAT) interface, which is specified in 3GPP TS. With the SAT interface, programs on the SIM card can access functionalities of the mobile phone such as waiting for user input, or can be used to show text messages and menu entries on the display. Many mobile network operators use this functionality to put an operator-specific menu item into the overall menu structure of the mobile phone’s graphical user interface. In the menu created by the SIM card program, the subscriber can, for example, request a current news overview. When the subscriber enters the menu, all user input via the keypad is forwarded by the mobile phone to the SIM card. The program on the SIM card in this example would react to the news request by generating an SMS, which it then instructs the mobile phone to send to the network. The network replies with one or more SMS messages which contain a news overview. The SIM card can then extract the information from the SMS messages and present the content to the subscriber.
The mobile phone can only access the SIM card if the user has typed in the PIN when the phone is started. The mobile phone then uses the PIN to unlock the SIM card. SIM cards of some network operators, however, allow deactivating the password protection and thus the user does not have to type in a PIN code when the mobile phone is switched on. Despite unlocking the SIM card with the PIN, the mobile phone is still restricted to only being able to read or write certain files. Thus, it is not possible for example to read or write the file which contains the secret key Ki even after unlocking the SIM card with the PIN.
The most important parameters on the SIM card are the IMSI and the secret key (Ki), which is used for authentication and the generation of ciphering keys (Kc). With a number of tools, which are generally available on the Internet free of charge, it is possible to read out most parameters from the SIM card, except for sensitive parameters that are read protected. Protected parameters can only be accessed with a special unlock code that is not available to the end user.
Astonishingly, a SIM card is much more than just a simple memory card as it contains a complete microcontroller system that can be used for a number of additional purposes. The mobile phone cannot access the information on the EEPROM directly, but has to request the information from the SIM’s CPU. Therefore, direct access to sensitive information is prohibited.
Furthermore, the microcontroller system on the SIM can also execute programs which the network operator may have installed on the SIM card. This is done via the SIM application toolkit (SAT) interface, which is specified in 3GPP TS. With the SAT interface, programs on the SIM card can access functionalities of the mobile phone such as waiting for user input, or can be used to show text messages and menu entries on the display. Many mobile network operators use this functionality to put an operator-specific menu item into the overall menu structure of the mobile phone’s graphical user interface. In the menu created by the SIM card program, the subscriber can, for example, request a current news overview. When the subscriber enters the menu, all user input via the keypad is forwarded by the mobile phone to the SIM card. The program on the SIM card in this example would react to the news request by generating an SMS, which it then instructs the mobile phone to send to the network. The network replies with one or more SMS messages which contain a news overview. The SIM card can then extract the information from the SMS messages and present the content to the subscriber.
The mobile phone can only access the SIM card if the user has typed in the PIN when the phone is started. The mobile phone then uses the PIN to unlock the SIM card. SIM cards of some network operators, however, allow deactivating the password protection and thus the user does not have to type in a PIN code when the mobile phone is switched on. Despite unlocking the SIM card with the PIN, the mobile phone is still restricted to only being able to read or write certain files. Thus, it is not possible for example to read or write the file which contains the secret key Ki even after unlocking the SIM card with the PIN.
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