Section: 4.7.7.3b 128-bit packet-switched GSM integrity key (in A/Gb mode and only if MS supports integrity protection)
Content: The ME and the network may derive and store a 128-bit packet-switched GPRS GSM Kint from an established UMTS security context. If the GPRS GSM Kint exits, then it is also part of the UMTS security context. The ME with a USIM in use shall compute a new GPRS GSM Kint using the GPRS UMTS ciphering key and the GPRS UMTS integrity key from an established UMTS security context as specified in 3GPP TS 43.020[ Security related network functions ]  [13]. The new GPRS GSM Kint shall be stored only in the ME. The ME shall overwrite the existing GPRS GSM Kint with the new GPRS GSM Kint. The ME shall delete the GPRS GSM Kint at MS 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 Kint when in A/Gb mode a GIA integrityg algorithm that requires a 128-bit integrity key is taken into use. The network shall compute the GPRS GSM Kint using the GPRS UMTS integrity key and the GPRS UMTS ciphering key from an established UMTS security context as specified in 3GPP TS 43.020[ Security related network functions ]  [13]only when in A/Gb mode a GIA integrity algorithm that requires a 128-bit integrity key is to be used. At MS power off, the information in the Integrity Algorithm IE shall be stored in a non-volatile memory in the ME together with the IMSI from the USIM. The information stored in the Integrity 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 Integrity Algorithm IE.