Section: 4.7.7b Authentication and ciphering procedure used for GSM authentication challenge
Content: The purpose of the authentication and ciphering procedure is threefold (see 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 GSM ciphering key; and - to let the network set the GSM ciphering mode (ciphering/no ciphering) and GSM ciphering algorithm. The authentication and ciphering procedure can be used for either: - authentication only; - setting of the GSM ciphering mode and the GSM ciphering algorithm only; or - authentication and the setting of the GSM ciphering mode and the GSM ciphering algorithm. The cases in which the authentication and ciphering procedure shall be used are defined in 3GPP TS 42.009[ Security aspects ]  [5]. In A/Gb mode, the authentication and ciphering procedure is always initiated and controlled by the network. It shall be performed in a non ciphered mode because of the following reasons: - the network cannot decipher a ciphered AUTHENTICATION_AND_CIPHERING RESPONSE from an unauthorised MS and put it on the prohibited list; and - to be able to define a specific point in time from which on a new GPRS GSM ciphering key should be used instead of the old one. GSM authentication challenge shall be supported by a ME supporting GERAN or UTRAN. In A/Gb mode, the network should not send any user data during the authentication and ciphering procedure. 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 challenge, the GPRS GSM ciphering key and the GPRS ciphering key sequence number, are stored both in the network and the MS.