Section: 4.3.2.3a 128-bit circuit-switched GSM ciphering key
Content: The ME and the network may derive and store a 128-bit circuit-switched GSM ciphering key or GSM Kc128 from an established UMTS security context. If the GSM Kc128 exists, then it is also part of the UMTS security context. The ME with a USIM in use shall compute a new GSM Kc128 using the UMTS ciphering key and the UMTS integrity key from an established UMTS security context as specified in 3GPP TS 33.102[ 3G security; Security architecture ]  [5a]. The new GSM Kc128 shall be stored only in the ME. The ME shall overwrite the existing GSM Kc128 with the new GSM Kc128. The ME shall delete the GSM Kc128 at switch off, when the USIM is disabled as well as under the conditions identified in the subclause 4.1.2.2 and 4.3.2.4. The ME with a USIM in use shall apply the GSM Kc128 when in A/Gb mode an A5 ciphering algorithm that requires a 128-bit ciphering key is taken into use. The network shall compute the GSM Kc128 using the UMTS integrity key and the 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 an A5 ciphering algorithm that requires a 128-bit ciphering key is to be used.