4G mobile network; - the authenticator is represented by the trusted Wi-Fi access that controls the access of the supplicant to the 4G mobile network; - the authentication server is represented by the AAA (Authentication, Authorization and Accounting) server, which authenticates the supplicant and authorizes access to the 4G mobile network. EAP-AKA' messages are carried between the trusted Wi-Fi access and the AAA server in DIAMETER messages: - DER (Diameter-EAP-Request) message is transmitted by the trusted Wi-Fi access; - DEA (Diameter-EAP-Answer) is transmitted by the AAA server. 6.3.2. Mutual authentication procedure The procedure of mutual authentication, in the case of a trusted Wi-Fi access, is part of the procedure of attachment of the mobile. At the end of the association phase with the trusted Wi-Fi access, the mobile transmits the EAPOL-Start message, which triggers the mutual authentication procedure based on the EAP-AKA' method (Figure 6.8). Mutual Authentication Trusted Wi-Fi server access EAP Request EAP-Method Identity EAP Response EAP-Method Identity EAP Response EAP-Method Identity DIAMETER AIR DIAMETER AIA EAP Request EAP-Method AKA' Challenge EAP Request EAP-Method AKA' Challenge EAP Response EAP-Method AKA' Challenge EAP Response EAP-Method AKA' Challenge EAP Success EAP Success Figure 6.8. Mutual authentication procedure 1) Trusted Wi-Fi access sends the EAP Request message containing the EAP-Method Identity message. 2) The mobile transmits the EAP Response/EAP-Method Identity message containing, at the first authentication, the network access identifier (NAI) constructed from the international mobile subscriber identity (IMSI) of the mobile. 3) Wi-Fi access completes the EAP Response/EAP-Method Identity message, including the access network parameters (type, identity) and transfers it to the AAA server in a DER DIAMETER message. 4) The AAA server asks the HSS entity the cryptographic data of the mobile in the AIR (Authentication-Information-Request) DIAMETER message. The HSS entity generate