Section: 4.7.7.10 Handling of keys at intersystem change from S1 mode to Iu mode or A/Gb mode
Content: At an inter-system change from S1 mode to Iu mode, ciphering and integrity may be started (see 3GPP TS 25.331[ None ]  [23c]) without any new authentication and ciphering procedure. At an inter-system change from S1 mode to A/Gb mode, ciphering may be started (see 3GPP TS 44.064[ Mobile Station - Serving GPRS Support Node (MS-SGSN); Logical Link Control (LLC) Layer Specification ]  [78a]) without any new authentication and ciphering procedure. Deduction of the appropriate security keys for ciphering and integrity check in Iu mode or for ciphering in A/Gb mode, depends on the current EPS security context or the UMTS security context for the PS domain stored in the MS and the network. The ME shall handle the GPRS UMTS ciphering key, the GPRS UMTS integrity key, the GPRS GSM ciphering key and a potential GPRS GSM Kc128 according to table 4.7.7.10.1, table 4.7.7.10.2 and table 4.7.7.10.3. Table 4.7.7.10.1/3GPP TS 24.008[ Mobile radio interface Layer 3 specification; Core network protocols; Stage 3 ] : Inter-system change from S1 mode to Iu mode or A/Gb mode in connected mode. NOTE 1: For the case in table 4.7.7.10.1, because of deriving a new UMTS security context for the PS domain, a new GPRS GSM ciphering key needs to be derived from the new derived UMTS security keys (i.e. CK' and IK'). Note that the new GPRS GSM ciphering key is also part of the new UMTS security context for the PS domain, and therefore any old GPRS GSM ciphering key stored in the USIM and in the ME belongs to an old UMTS security context for the PS domain and can no longer be taken into use. Table 4.7.7.10.2/3GPP TS 24.008[ Mobile radio interface Layer 3 specification; Core network protocols; Stage 3 ] : Inter-system change from S1 mode to Iu mode or A/Gb mode in idle mode when the TIN indicates "GUTI". NOTE 2: For the case in table 4.7.7.10.2, because of deriving a new UMTS security context for the PS domain, a new GPRS GSM ciphering key needs to be derived from the new derived UMTS security keys (i.e. CK' and IK'). The new GPRS GSM ciphering key is also part of the new UMTS security context for the PS domain, and therefore any old GPRS GSM ciphering key stored in the USIM and in the ME belongs to an old UMTS security context for the PS domain and can no longer be taken into use. Table 4.7.7.10.3/3GPP TS 24.008[ Mobile radio interface Layer 3 specification; Core network protocols; Stage 3 ] : Inter-system change from S1 mode to Iu mode or A/Gb mode in idle mode when the TIN indicates "RAT-related TMSI" The network shall replace an already established UMTS security context for the PS domain, if any, when a handover from S1mode to Iu mode or from S1mode to A/Gb mode has been completed successfully. If the handover from S1mode to Iu mode or S1mode to A/Gb mode has not been completed successfully, the ME and the network shall delete the new derived UMTS security context for the PS domain. Additionally, the network shall delete the already established UMTS security context for the PS domain, if the CKSN of the already established UMTS security context is equal to the CKSN of the new derived security context for the PS domain.