the next KeNB* needs to be based on the current KeNB or a fresh NH. The AuC, HSS, and MME entities shown in Figure 8-7 are Authentication Center (i.e., the network entity that can authenticate subscribers in a mobile network), Home Subscriber Service (i.e., the main IMS database which also acts as a database in EPC that combines legacy Home Location Register and AuC functions for circuit-switched and packet-switched domains), and Mobility Management Entity, respectively [8]. In Figure 8-7, CK and IK denote keys that have been agreed during the AKA procedure. The MME invokes the AKA procedures by requesting authentication vectors to the Home Environ- ment (HE), if no unused EPS authentication vectors have been stored. The HE sends an authentication response back to the MME that contains a fresh authentication vector, including a base key named KASME. Consequently, the EPC and the UE share KASME. From KASME, the NAS keys, and indirectly KeNB keys and NH, are derived. The KASME is never transported to an entity outside of the EPC, but KeNB and NH are transported to the eNB from the EPC when the UE transitions to ECM-CONNECTED. From the KeNB, the eNB and UE can derive the UP and RRC keys. The RRC and user-plane keys are refreshed during handover. KenB* is derived by the UE and source eNB from the target Physical Cell Identifier, target frequency, and KeNB, or alternatively from the target Physical Cell Identifier, target frequency, and NH. The KeNB* is then used as a new KeNB for RRC and UP traffic at the target. If the UE transitions to ECM-IDLE state, all keys are deleted in the eNB. The reuse of COUNT is avoided for the same radio bearer identity in RRC_CONNECTED mode without KeNB change, and this is left to eNB implementation. In case of HFN de-synchronization in CHAPTER 8 The IEEE 802. 16m Security Sub-Layer Table 8-1 Security Termination Points in 3GPP LTE [7] Ciphering Integrity Protection NAS Signaling Required and terminated in MME Required and terminated in MME User-Plane Data Required and terminated in eNB