selects one of the authentication vectors and uses it for mutual authentication with the UE and shares the same authentication key (KASME). Mutual authentication is the process in which a network and a user authenti- cate each other. In LTE networks, since the identification of the user's serving network is required when generating authentication vectors, authentication of the network by the user is performed in addition to authentication of the user by the network. Access security man- agement entity (ASME) is an entity that receives top-level key(s) from the HSS to be used in an AN. In EPS, the MME serves as the ASME and KASME is used as the top-level key in the AN. The MME conducts mutual authentication with the UE on behalf of the HSS using KASME. Once mutually authenticated, the UE and MME share the same KASME as an authentication key. USIM/AuC MeNB/UE SeNB/UE CK, IK SCG counter S-KeNB UE/HSS KUPenc KASME UE/MME Key derivation function (KDF) UP-enc-alg, Alg-ID (User-plane encryption Trunc algorithm) KNAS enc KNAS int Truncation UE/eNB KUPenc KUP enc K RRC enc K RRC int KenB* Figure 1.89 Key derivation in LTE and dual connectivity [56,57]. 5G Network Architecture 187 NAS security is designed to securely deliver signaling messages between the UE and the MME over the radio link and to perform integrity protection/verification as well as cipher- ing of NAS signaling messages. Different keys are used for integrity verification and ciphering. While integrity verification is a mandatory function, ciphering is an optional function. The NAS security keys, such as integrity key (KNASint) and ciphering key (KNASenc), are derived by the UEs and the MMEs from KASME (see Fig. 1.89). In Fig. 1.89, next hop (NH) key is used by the UE and eNB in the derivation of Kenb* for provisioning forward security. The NH is derived by UE and MME from KASME and KeNB when the security context is established, or from KASME and the previous NH. The NH chaining count (NCC) is a counter related to NH, that is, the number of key chaining that