Section: 6.8.6.1 UMTS security context
Content: A UMTS security context in UTRAN is only established for UMTS subscribers. At the network side, four cases are distinguished: a) In case of an intersystem change to a GSM BSS controlled by the same SGSN, 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. b) In case of an intersystem change to a GSM BSS controlled by another R99+ SGSN, 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. The new SGSN becomes the new anchor point for the service. c) In case of an intersystem change to a GSM BSS controlled by a R98- SGSN, the initial SGSN derives the GSM cipher key Kc from the UMTS cipher/integrity keys CK and IK agreed during the latest UMTS AKA procedure and sends the GSM cipher key Kc to the new SGSN. The new SGSN stores the GSM cipher key Kc and applies it. The new SGSN becomes the new anchor point for the service. d) In case of a handover to another Rel-9+ SGSN, the initial SGSN sends the UMTS cipher/integrity keys CK and IK agreed at the latest UMTS AKA procedure to the new SGSN. The new SGSN derives the 64-bit Kc. The new SGSN stores the keys. If the new SGSN selects a GEA ciphering algorithm requiring a 128-bit key, the new SGSN shall compute Kc128 from the CK/IK and shall apply it. If the new SGSN selects a GEA ciphering algorithm requiring a 64-bit key then Kc shall be applied. The new SGSN becomes the new anchor point for the service. At the user side, in all cases, the ME applies the derived 64-bit 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 derive 128-bit GSM cipher key Kc128 from the CK and IK agreed during the latest UMTS AKA and apply it. In case the current UMTS security context is mapped from an EPS security context and there has been no UMTS AKA run since the current UMTS security context was mapped, the CK , IK and Kc belonging to the mapped UMTS security context shall be considered to be the keys from the latest AKA.