Section: 6.6.4.2 CK
Content: The cipher key CK is 128 bits long. There may be one CK for CS connections (CKCS), established between the CS service domain and the user and one CK for PS connections (CKPS) established between the PS service domain and the user. The CK to use for a particular radio bearer is described in 6.6.5. For UMTS subscribers, CK is established during UMTS AKA, as the output of the cipher key derivation function f3, available in the USIM and in HLR/AuC. For GSM subscribers that access the UTRAN, CK is established following GSM AKA and is derived from the GSM cipher key Kc, as described in 8.2. CK is stored in the USIM and a copy is stored in the ME. CK is sent from the USIM to the ME upon request of the ME. The USIM shall send CK under the condition that a valid CK is available. The ME shall trigger a new authentication procedure if the current value of STARTCS or STARTPS in the USIM have reached THRESHOLD. The ME shall delete CK from memory after power-off as well as after removal of the USIM. CK is sent from the HLR/AuC to the VLR/SGSN and stored in the VLR/SGSN as part of the quintet. It is sent from the VLR/SGSN to the RNC in the (RANAP) security mode command. At handover, the CK is transmitted within the network infrastructure from the old RNC to the new RNC, to enable the communication to proceed. The cipher CK remains unchanged at handover, with the exception of SRVCC handover and reverse SRVCC handover.