Section: 6.8.4.1 UMTS security context
Content: A UMTS security context in UTRAN is only established for a UMTS subscriber with a ME that is capable of UMTS AKA. At the network side, four cases are distinguished: a) In case of a handover to a GSM BSS controlled by the same MSC/VLR, the MSC/VLR derives the 64-bit GSM cipher key Kc from the UMTS cipher/integrity keys CK and IK used before the intersystem handover (using the conversion function c3) and sends the 64-bit Kc to the target BSC (which forwards it to the BTS). If the MSC/VLR is Rel-9+ and MSC/VLR has included a 128-bit GSM ciphering algorithms as a permitted ciphering algorithm, the MSC/VLR shall also derive the 128-bit ciphering key Kc128 and send also this to the target BSC (which forwards it to the BTS). b) In case of a handover to a GSM BSS controlled by another MSC/VLR, depending on the capability of the inter-MSC communication protocol version, the initial MSC/VLR sends to the target MSC/VLR the security keys associated with the allowed security algorithms. If the inter-MSC communication protocol version only allows inclusion of the 64-bit GSM security key Kc, and the initial MSC/VLR includes a 64-bit GSM A5 ciphering algorithm as allowed ciphering algorithm, the initial MSC/VLR derives the 64-bit Kc and sends it to the new MSC/VLR. Otherwise, if the inter-MSC communication protocol version allows inclusion of UMTS security keys, the initial MSC/VLR sends, in addition, the UMTS cipher/integrity keys CK and IK used before the intersystem handover to the new MSC/VLR. If the initial MSC/VLR includes a 128-bit GSM A5 ciphering algorithm as an allowed ciphering algorithm, the initial MSC/VLR shall also calculate a Kc128 from the CK/IK and forward this to the new MSC. The new MSC/VLR stores the key(s) and then forwards them to the target BSC (which forwards them to the BTS). The initial MSC/VLR remains the anchor point throughout the service. At the user side, in either case, the ME applies the derived 64-bit GSM cipher key Kc from the key set which was used before the intersystem handover if the selected GSM ciphering algorithm requires a 64-bit key. If the selected GSM A5 ciphering algorithm requires a 128-bit key, the ME shall apply the derived 128-bit GSM cipher key Kc128 from the key set which was used before the intersystem handover.