Section: 4.7.7.3a 128-bit packet-switched GSM ciphering key
Content: The ME and the network may derive and store a 128-bit packet-switched GSM key or GPRS GSM Kc128 from an established UMTS security context. If the GPRS GSM Kc128 exits, then it is also part of the UMTS security context. The ME with a USIM in use shall compute a new GPRS GSM Kc128 using the GPRS UMTS ciphering key and the GPRS UMTS integrity key from an established UMTS security context as specified in 3GPP TS 33.102[ 3G security; Security architecture ]  [5a]. The new GPRS GSM Kc128 shall be stored only in the ME. The ME shall overwrite the existing GPRS GSM Kc128 with the new GPRS GSM Kc128. The ME shall delete the GPRS GSM Kc128 at switch off, when the USIM is disabled as well as under the conditions identified in the subclause 4.1.3.2 and 4.7.7.4. The ME with a USIM in use shall apply the GPRS GSM Kc128 when in A/Gb mode a GEA ciphering algorithm that requires a 128-bit ciphering key is taken into use. The network shall compute the GPRS GSM Kc128 using the GPRS UMTS integrity key and the GPRS UMTS ciphering key from an established UMTS security context as specified in 3GPP TS 33.102[ 3G security; Security architecture ]  [5a] only when in A/Gb mode a GEA ciphering algorithm that requires a 128-bit ciphering key is to be used. In A/Gb mode, if the MS supports integrity protection, the information in the Ciphering Algorithm IE together with the IMSI from the USIM shall be stored in a non-volatile memory in the ME at MS power off. The information stored in the Ciphering Algorithm IE can only be used if the IMSI from the USIM matches the IMSI stored in the ME non-volatile memory at MS power on; otherwise the MS shall delete the Ciphering Algorithm IE.