Section: 4.3.2b Authentication Procedure used for a GSM authentication challenge
Content: The purpose of the authentication procedure is twofold (see 3GPP TS 43.020[ Security related network functions ]  [13]): First to permit the network to check whether the identity provided by the mobile station is acceptable or not; Second to provide parameters enabling the mobile station to calculate a new GSM ciphering key. The cases where the authentication procedure should be used are defined in 3GPP TS 42.009[ Security aspects ]  [5]. The authentication procedure is always initiated and controlled by the network. GSM authentication challenge shall be supported by a ME supporting GERAN or UTRAN. A GSM security context is established in the MS and the network when a GSM authentication challenge is performed in A/Gb mode or in Iu mode. However, in Iu mode the MS shall not accept a GSM authentication challenge, if a USIM is inserted. After a successful GSM authentication, the GSM ciphering key and the ciphering key sequence number, are stored both in the network and the MS.