Section: 4.7.7.7 Use of established security contexts
Content: In A/Gb mode, in the case of an established GSM security context, the GPRS GSM ciphering key shall be taken into use by the MS before the AUTHENTICATION AND CIPHERING RESPONSE message is transmitted. In A/Gb mode, in the case of an established UMTS security context, and if the network indicates in the AUTHENTICATION AND CIPHERING REQUEST message to the MS that a GEA ciphering algorithm that requires a 64-bit ciphering key shall be taken into use, then the GPRS GSM ciphering key shall be taken into use by the MS before the AUTHENTICATION AND CIPHERING RESPONSE message is transmitted. The network shall derive a GPRS GSM ciphering key from the GPRS UMTS ciphering key and the GPRS UMTS integrity key, by using the conversion function named "c3" defined in 3GPP TS 33.102[ 3G security; Security architecture ]  [5a]. In A/Gb mode, in the case of an established UMTS security context, and if the network indicates in the AUTHENTICATION AND CIPHERING REQUEST message to the MS that a GEA ciphering algorithm that requires a 128-bit ciphering key shall be taken into use, then the MS shall take the following actions: - if authentication is not requested and a GEA ciphering algorithm that requires 64-bit ciphering key is in use, the MS shall take into use the GPRS GSM Kc128 derived by the ME from the GPRS UMTS ciphering key and GPRS UMTS integrity key of the established UMTS security context in use (see 3GPP TS 33.102[ 3G security; Security architecture ]  [5a]) before the AUTHENTICATION AND CIPHERING RESPONSE message is transmitted;. - if authentication is not requested and a GEA ciphering algorithm that requires 128-bit ciphering key is in use, the GPRS GSM Kc128 of the established UMTS security context in use still applies; otherwise, the MS shall take into use the GPRS GSM Kc128 derived by the ME from the GPRS UMTS ciphering key and the GPRS UMTS integrity key provided by the USIM during the latest successful authentication procedure (see subclause 4.7.7.3a) before the AUTHENTICATION AND CIPHERING RESPONSE message is transmitted. In A/Gb mode, in the case of an established UMTS security context, and if the network indicates in the AUTHENTICATION AND CIPHERING REQUEST message to the MS that a GEA ciphering algorithm that requires a 128-bit ciphering key shall be taken into use, then the network shall derive a GPRS GSM Kc128 (see subclause 4.7.7.3a). In A/Gb mode, if an established UMTS security context context is available in the network, if the MS indicates support of integrity protection to the network and the network supports integrity protection, if the network indicates in the AUTHENTICATION AND CIPHERING REQUEST message to the MS that a new GPRS GSM integrity algorithm shall be taken into use but no authentication is requested, then the GPRS GSM Kint of the established UMTS security context in use still applies in the MS and network. In A/Gb mode, in the case of an established UMTS security context, and if the network indicates in the AUTHENTICATION AND CIPHERING REQUEST message to the MS that a GIA integrity protection algorithm that requires a 128-bit integrity key shall be taken into use but no authentication is requested, then the GPRS GSM Kint of the established UMTS security context in use still applies in the MS. In A/Gb mode, if during an ongoing, already ciphering protected RR connection, the network initiates a new Authentication and ciphering procedure, the new GPRS GSM ciphering key or GPRS GSM Kc128 shall be taken into use by the MS before the AUTHENTICATION AND CIPHERING RESPONSE message is transmitted. In case of inter-system change to Iu mode after receipt of the AUTHENTICATION AND CIPHERING REQUEST message, the MS and the network shall take the new keys into use immediately after the inter-system change. In Iu mode, in the case of an established GSM security context, the ME shall derive a GPRS UMTS ciphering key and a GPRS UMTS integrity key from the GPRS GSM ciphering key by using the conversion functions named "c4" and "c5" defined in 3GPP TS 33.102[ 3G security; Security architecture ]  [5a]. The derived GPRS UMTS ciphering key and GPRS UMTS integrity key shall be taken into use by the MS when a valid SECURITY MODE COMMAND message indicating PS domain is received during an RR connection (the definition of a valid SECURITY MODE COMMAND message is given in 3GPP TS 25.331[ None ]  [23c]). The network shall derive a GPRS UMTS ciphering key and a GPRS UMTS integrity key from the GPRS GSM ciphering key by using the conversion functions named "c4" and "c5" defined in 3GPP TS 33.102[ 3G security; Security architecture ]  [5a]. In Iu mode, in the case of an established UMTS security context, the GPRS UMTS ciphering key and the GPRS UMTS integrity key shall be taken into use by the MS when a valid SECURITY MODE COMMAND message indicating PS domain is received during a PS signalling connection (the definition of a valid SECURITY MODE COMMAND message is given in 3GPP TS 25.331[ None ]  [23c]). In Iu mode, if the MS received a valid SECURITY MODE COMMAND message indicating PS domain in Iu mode or a valid AUTHENTICATION AND CIPHERING REQUEST message in A/Gb mode before the network initiates a new authentication and ciphering procedure and establishes a new GSM/UMTS security context, the new GPRS UMTS ciphering key and GPRS UMTS integrity key are taken into use by the MS, when a new valid SECURITY MODE COMMAND message indicating PS domain is received during the PS signalling connection. In case of inter-system change to A/Gb mode, the MS and the network shall take the new keys into use immediately after the inter-system change.