How SIM Cards Work
The information stored on a SIM (Subscriber Identity Module) card is unique to the cell phone company, the subscriber and the cell phone handset that the SIM card is associated with. According to Southern Polytechnic State University's research on cell phone forensics, SIM cards store "essential information about the subscriber," like user contacts, text messages and a network authentication program. Using the the SIM card's embedded authentication program, the SIM card identifies the cell phone user and handset to the cell service provider's network.
SIM cards are like micro-computers and contain many of the same elements or components that one would find in a computer or PDA. SIM cards are composed of a CPU (Central Processing Unit) to execute programs and commands, a ROM unit for "program memory," a RAM unit for "working memory," an EPROM for "data memory" and a "Serial communication module" for communicating with the cell service provider's network.
GSM Networks and Authentication
Cell phone handsets use SIM cards to communicate with GSM (Global System for Mobile) networks. Important updates can be downloaded to the SIM card without the need for any user assistance--alternatively, SIM cards allow users to send and manage personal data remotely on the subscriber's associate GSM network. But before a call or data can be processed over the subscriber's GSM network, the SIM card must confirm the cell subscriber's data over an "Authentication Center" (AC). The AC sends the SIM card a randomly generated number. According to Sheng He of Ruhr-University at Bochum, both the AC and the SIM card uses a special algorithm to translate the randomly generated number and "If [the] number sent by mobile matches number calculated by AC, then subscriber is authenticated".