Section: 6.8.2.3 VLR/SGSN
Content: The R99+ VLR/SGSN shall perform GSM AKA using a triplet that is either: a) retrieved from the local database, b) provided by the HLR/AuC, or c) provided by the previously visited VLR/SGSN. NOTE: All triplets are originally provided by the HLR/AuC. GSM AKA results in the establishment of a GSM security context; the GSM cipher key Kc and the cipher key sequence number CKSN are stored in the VLR/SGSN. When the user is attached to a UTRAN, the R99+ VLR/SGSN derives the UMTS cipher/integrity keys from the GSM cipher key using the following conversion functions: a) c4: CK[UMTS] = Kc || Kc; b) c5: IK[UMTS] = Kc1 xor Kc2 || Kc || Kc1 xor Kc2; whereby in c5, Kci are both 32 bits long and Kc = Kc1 || Kc2. The UMTS cipher/integrity keys are then sent to the RNC where the ciphering and integrity algorithms are allocated. When the user is attached to a GSM BSS and the user receives service from an MSC/VLR, the cipher key Kc is sent to the BSC (and forwarded to the BTS). When the user receives service from an SGSN, the cipher key Kc is applied in the SGSN itself.