Section: 4.7.7a Authentication and ciphering procedure used for UMTS authentication challenge.
Content: The purpose of the authentication and ciphering procedure is fivefold (see 3GPP TS 33.102[ 3G security; Security architecture ]  [5a] and 3GPP TS 43.020[ Security related network functions ]  [13]): - to permit the network to check whether the identity provided by the MS is acceptable or not; - to provide parameters enabling the MS to calculate a new GPRS UMTS ciphering key and a new GPRS UMTS integrity key; - to let the network set the GSM ciphering mode (ciphering /no ciphering) and GSM ciphering algorithm; - to permit the mobile station to authenticate the network; and - to let the network set GSM integrity protection and GSM integrity algorithm (for control plane and optionally for user plane). In Iu mode, and in the case of a UMTS authentication challenge, the authentication and ciphering procedure can be used for authentication only. The cases in which the authentication and ciphering procedure shall be used are defined in 3GPP TS 33.102[ 3G security; Security architecture ]  [5a], 3GPP TS 43.020[ Security related network functions ]  [13] and 3GPP TS 42.009[ Security aspects ]  [5]. The authentication and ciphering procedure is always initiated and controlled by the network. However, in the case of a UMTS authentication challenge, there is the possibility for the MS to reject the network. The MS shall support the UMTS authentication challenge, if a USIM is inserted. The authentication and ciphering procedure can be used for any combination of the following: - authentication; - setting of the GSM ciphering mode and the GSM ciphering algorithm; and - setting of GSM integrity protection and the GSM integrity algorithm (for control plane and optionally for user plane). NOTE: Setting of GSM integrity protection and the GSM integrity algorithm in the authentication and ciphering procedure is only applicable for an MS and a network supporting integrity protection in A/Gb mode. In A/Gb mode, the network should not send any user data during the authentication and ciphering procedure. 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 GPRS UMTS ciphering key, the GPRS UMTS integrity key, the GPRS GSM ciphering key and the GPRS 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 GPRS GSM Kc128 as part of the UMTS security context as described in the subclause 4.7.7.3a. Furthermore, in A/Gb mode, if integrity protection is used, both the MS and the network shall derive and store a GPRS GSM Kint as part of the UMTS security context as described in the subclause 4.7.7.3b.