Section: B.5 Derivation of Kc128
Content: This input string is used when there is a need to derive Kc128 from CK and IK. The key Kc128 is used as input to the GSM A5 and GEA ciphering algorithms which requires 128-bit keys. Kc128 shall only be derived by the MS and the network when in UMTS security context. Kc128 shall not be derived by the MS or the network when in GSM security context. This implies that GSM A5 using Kc128 and GEA using Kc128 can only be selected by the network (see TS 43.020[ Security related network functions ]  [36]) when the UE and network are in UMTS security context as there is otherwise no key which the ciphering algorithms can use. - FC = 0x32 The Key input is the concatenation of CK and IK (i.e., CK || IK). No input parameters (Pi, Li) are used by this function. The KDF returns a 256-bit output, where the 128 most significant bits are identified with Kc128 .