Section: 4.3.2a Authentication procedure used for a UMTS authentication challenge
Content: The purpose of the authentication procedure is fourfold (see 3GPP TS 33.102[ 3G security; Security architecture ]  [5a]): 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 UMTS ciphering key; Third to provide parameters enabling the mobile station to calculate a new UMTS integrity key; Fourth to permit the mobile station to authenticate the network. The cases where the authentication procedure should be used are defined in 3GPP TS 33.102[ 3G security; Security architecture ]  [5a]. The UMTS authentication procedure is always initiated and controlled by the network. However, there is the possibility for the MS to reject the UMTS authentication challenge sent by the network. The MS shall support the UMTS authentication challenge, if a USIM is inserted. A UMTS security context is established in the MS and the network when a UMTS authentication challenge is performed in A/Gb mode or in Iu mode. After a successful UMTS authentication, the UMTS ciphering key, the UMTS integrity key, the GSM ciphering key and the ciphering key sequence number, are stored both in the network and the MS. Furthermore, in A/Gb mode both the ME and the network may derive and store a GSM Kc128 as part of the UMTS security context as described in the subclause 4.3.2.3a.