SIM CARDS

A SIM (Subscriber Identity Module) Card is a small smart card used in GSM mobile phones. The SIM card normally looks like a small plastic package, with a series of metal contact plates embedded onto it. The SIM card has an embedded microcontroller and between 16 to 64Kbytes of memory (for storage of phone numbers, text messages etc). Whereas a standard smart card has a credit card sized form factor - the majority of which is just a card transport holder, a SIM card dispenses with the large holder substrate, and provides a minimal substrate just large enough to contain the smart card electronics.

The photograph to the left shows 2 typical mobile phone SIM cards. Although the contact layout is slightly different between them, the actual positioning of the contacts remains the same, thus enabling standard SIM card readers across the whole range of mobile phones to read different SIM cards.

The SIM Card Reader.

The SIM card reader is composed of the electronics that communicates with the SIM card, and the metal contacts that make the connections with the metal pads on the SIM card. Generally, the electronics is hidden from site, and only the metal contacts of the SIM card reader are seen in a mobile phone.

This photograph shows the SIM card reader on a Nokia 3410 mobile phone. Only the metal contacts that touch the pads on the SIM card are visible. The actual electronics is concealed.

This photograph shows the SIM card reader on a Sony Ericsson P800 mobile phone. In this design, some of the electronics is also visible around the actual metal contacts on the SIM card reader.

SIM Card ID.

Within the memory of the SIM card is the mobile number, which is held in the form of the International Mobile Subscriber Identity (IMSI). The IMSI number is different to the dial up number of the mobile phone, and the mobile phone network translates between the dial up mobile phone number and the IMSI. The IMSI uniquely identifies the mobile phone owner on any GSM network in the world

Note: The IMSI is not the same as the IMEI. For more details on the IMEI, the IMEI information page is available.

SIM Card Power.

Mobile phone SIM cards generally operate at a voltage of 3 Volts. Older SIM cards operated at 5 Volts, and the next generation of mobile phone SIM cards will run at 1.8 volts. 3 volt SIM cards generally work in older, 5 volt mobile phone SIM card slots, but 5 volt SIM cards will not work in the more modern 3 volt mobile phone SIM card slots.

SIM Card Security.

SIM cards frequently use a cryptographic algorithm called COMP128. Research by IBM has shown that the full 128-bit cryptographic keys for the COMP128 algorithm can be extracted, and used for cracking attacks relatively straightforwardly, if the SIM card falls into the possession of a cracker, even for a minute or so. The key security point here, is that SIM card users should not lend mobile phones to others, or leave them unattended.

If this page was useful, you may also want to read:

IMEI Number - International Mobile Equipment Identity Number
IMSI Number - International Mobile Subscriber Identity Number
Smart Cards

Acronym Glossary.