The EPS AKA produces keying material forming a basis for the user plane, RRC and the NAS ciphering keys as well as RRC and NAS integrity protection keys.
The MME sends to the USIM a random challenge, an authentication token, in addition to the KASME. The KASME key is a base key, from which NAS keys and KeNB keys and H 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 derived the UP and RRC Keys.
When the USIM receives the authentication request, as shown in Figure 14.5, it verifies the freshness of the authentication vector and, if acceptable, computes a response. If the verification fails, the ME responds an authentication reject message indicating cause.
This procedure enables the HSS in the UE's home environment to provide one or authentication vector to the serving network's MME to perform user authentication. The standard recommends that only one EPS authentication vector is fetched due to capability of an elaborate key hierarchy (see below). The authentication ...