Section: 6.8.8.1 UMTS security context
Content: A UMTS security context is only established for UMTS subscribers. At the network side, two cases are distinguished: a) In case of a PS intra SGSN Handover, the SGSN derives the 64-bit GSM cipher key Kc from the UMTS cipher/integrity keys CK and IK agreed during the latest UMTS AKA procedure (using the conversion function c3) and applies it if the selected GEA ciphering algorithm requires a 64-bit key. If the selected GEA ciphering algorithm requires a 128-bit key, the SGSN shall apply the 128-bit GSM cipher key Kc128 derived from the CK and IK agreed during the latest UMTS AKA. b) In case of a PS inter SGSN handover, the initial SGSN sends the UMTS cipher/integrity keys CK and IK agreed during the latest UMTS AKA procedure to the new SGSN. The new SGSN stores the keys, derives the 64-bit GSM cipher key Kc and applies the latter if the selected GEA ciphering algorithm requires a 64-bit key. If the selected GEA ciphering algorithm requires a 128-bit key, the SGSN shall apply the 128-bit GSM cipher key Kc128 derived from the CK and IK agreed during the latest UMTS AKA. The new SGSN becomes the new anchor point for the service. At the user side, in all cases, the ME applies the derived GSM cipher key Kc received from the USIM during the latest UMTS AKA procedure if the selected GEA ciphering algorithm requires a 64-bit key. If the selected GEA ciphering algorithm requires a 128-bit key, the ME shall apply the derived 128-bit GSM cipher key Kc128 from the key set agreed during the latest UMTS AKA.