Section: 4.3.2.8 Handling of keys at intersystem change from A/Gb mode to Iu mode
Content: At inter-system change from A/Gb mode to Iu mode, ciphering and integrity may be started (see 3GPP TS 25.331[ None ]  [23c]) without any new authentication procedure. Deduction of the appropriate security keys for ciphering and integrity check in Iu mode, depends on the current GSM/UMTS security context stored in the MS and the network. The ME shall handle the UMTS ciphering key and the UMTS integrity key according to table 4.3.2.8.1. Table 4.3.2.8.1/3GPP TS 24.008[ Mobile radio interface Layer 3 specification; Core network protocols; Stage 3 ] : Inter-system change from A/Gb mode to Iu mode NOTE: A USIM with UMTS security context, passes the UMTS ciphering key, the UMTS integrity key and the derived GSM ciphering key to the ME independent on the current radio access being UTRAN or GERAN.