USIM

USIM cards are subscriber identity modules for 3G mobile telephony. They are the same physical size as normal 2G GSM SIM cards.

The Universal Subscriber Identity Module is an application running on a smartcard (UICC) and is issued by the network operator in charge of the Home Environment (HE). The USIM stores the long-term preshared secret key K, which is shared with the Authentication Center (AuC). The USIM also verifies a sequence number that must be within a range using a window mechanism to avoid replay attacks, and is in charge of generating the session keys CK and IK to be used in the confidentiality and integrity algorithms of the KASUMI block cipher in UMTS.