A subscriber identity module (SIM) on a removable SIM card securely stores the service-subscriber key (IMSI) used to identify a subscriber on mobile telephony devices (such as computers and mobile phones). The SIM card allows users to change phones by simply removing the SIM card from one mobile phone and inserting it into another mobile phone or broadband telephony device. A SIM card contains its unique serial number, international unique number of the mobile user (IMSI), security authentication and ciphering information, temporary information related to the local network (also temporary local id that has been issued to the user), a list of the services the user has access to and two passwords (PIN for usual use and PUK for unlocking).

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