Section: 6.8.7.2 GSM security context
Content: A GSM security context in GSM BSS can be either: - Established for a UMTS subscriber A GSM security context for a UMTS subscriber is established in case the user has a ME not capable of UMTS AKA, where intersystem change to UTRAN is not possible, or in case the user has a R99+ ME but the SGSN is R98-, where intersystem change to UTRAN implies a change to a R99+ SGSN. As result, in case of intersystem change to a UTRAN controlled by another R99+ SGSN, the initial R98- SGSN sends the 64-bit GSM cipher key Kc agreed during the latest GSM AKA procedure to the new SGSN controlling the target RNC. Since the new R99+ SGSN has no indication of whether the subscriber is GSM or UMTS, a R99+ SGSN shall perform a new UMTS AKA when receiving the 64-bit Kc from a R98- SGSN. A UMTS security context using fresh quintets is then established between the R99+ SGSN and the USIM. The new SGSN becomes the new anchor point for the service. At the user side, new keys shall be agreed during the new UMTS AKA initiated by the R99+ SGSN. - Established for a GSM subscriber Handover from GSM BSS to UTRAN for GSM subscriber is only possible with R99+ ME. At the network side, three cases are distinguished: a) In case of an intersystem change to a UTRAN controlled by the same SGSN, the SGSN derives UMTS cipher/integrity keys CK and IK from the 64-bit GSM cipher key Kc (using the conversion functions c4 and c5) agreed during the latest GSM AKA procedure and sends them to the target RNC. b) In case of an intersystem change from a R99+ SGSN to a UTRAN controlled by another SGSN, the initial SGSN sends the 64-bit GSM cipher key Kc agreed during the latest GSM AKA procedure to the (new) SGSN controlling the target RNC. The new SGSN becomes the new anchor point for the service. The new SGSN stores the 64-bit GSM cipher key Kc and derives the UMTS cipher/integrity keys CK and IK which are then forwarded to the target RNC. c) In case of an intersystem change from an R98-SGSN to a UTRAN controlled by another SGSN, the initial SGSN sends the 64-bit GSM cipher key Kc agreed during the latest GSM AKA procedure to the (new) SGSN controlling the target RNC. The new SGSN becomes the new anchor point for the service. To ensure use of UMTS keys for a possible UMTS subscriber (superfluous in this case), a R99+ SGSN will perform a new AKA when a R99+ ME is coming from a R98-SGSN. At the user side, in all cases, the ME derives the UMTS cipher/integrity keys CK and IK from the GSM cipher key Kc (using the conversion functions c4 and c5) received from the SIM during the latest GSM AKA procedure and applies them. In case c) these keys will be over-written with a new CK, IK pair due to the new AKA.