Section: 4.7.7.8 Handling of keys at intersystem change from Iu mode to A/Gb mode
Content: At an inter-system change from Iu 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 key for ciphering in A/Gb mode, depends on the current GSM/UMTS security context stored in the MS and the network. The ME shall handle the GPRS GSM ciphering key and a potential GPRS GSM Kc128 according to table 4.7.7.8.1. In the case of an established GSM security context, before any initial GMM message is sent in the new cell in A/Gb mode, the GMM layer in the MS shall notify the LLC layer if ciphering shall be used or not. If ciphering shall be used, then the GPRS GSM ciphering key and the applicable GEA ciphering algorithm according to the stored Ciphering Algorithm IE in the MS shall also be indicated to the LLC layer (see 3GPP TS 44.064[ Mobile Station - Serving GPRS Support Node (MS-SGSN); Logical Link Control (LLC) Layer Specification ]  [78a]). In the case of an established UMTS security context, before any initial GMM message is sent in the new cell in A/Gb mode, the GMM layer in the MS shall notify the LLC layer if ciphering shall be used or not. If ciphering shall be used, then the GPRS GSM ciphering key or GPRS GSM Kc128 and the applicable GEA ciphering algorithm according to the stored Ciphering Algorithm IE in the MS shall also be indicated to the LLC layer (see 3GPP TS 44.064[ Mobile Station - Serving GPRS Support Node (MS-SGSN); Logical Link Control (LLC) Layer Specification ]  [78a]). If the network has selected a GEA-algorithm that requires a 128-bit ciphering key, then the ME shall apply a GPRS GSM Kc128 derived from the GPRS UMTS ciphering key and the GPRS UMTS integrity key of the established UTMS security context as specified in 3GPP TS 33.102[ 3G security; Security architecture ]  [5a]. Table 4.7.7.8.1/3GPP TS 24.008[ Mobile radio interface Layer 3 specification; Core network protocols; Stage 3 ] : Inter-system change from Iu mode to A/Gb mode NOTE: A USIM with UMTS security context, passes the GPRS UMTS ciphering key, the GPRS UMTS integrity key and the derived GPRS GSM ciphering key to the ME independent on the current radio access being UTRAN or GERAN.