uest Authentication EAP Success EAP Success EAPOL RADIUS Access-Accept Figure 6.5. Common exchanges in the authentication procedure The authenticator continues the procedure by sending the EAP-Request message containing the EAP-Method Identity message. The supplicant provides its identity by replying with an EAP Response/EAP-Method Identity message. This message is transmitted by the authenticator to the authentication server in a RADIUS Access-Request message. The next series of operations depend on the authentication method chosen. After the authentication phase exchanges, the authentication server transmits to the supplicant: - the EAP Success message if it is authenticated, in which case the authenticator authorizes traffic from the supplicant; - the EAP Failure message in the opposite case, and access to the network remains prohibited. The EAP Success (or EAP Failure) message is transmitted in a RADIUS Access-Accept (or Access-Reject) message at the interface between the authentication server and the authenticator. 114 Wi-Fi Integration to the 4G Mobile Network 6.2. Key management 6.2.1. Key hierarchy Data protection on the radio interface is based mainly on secret keys. When a security association is established after successful authentication, temporary keys are created: - pairwise transient key (PTK) is derived from the pairwise master key (PMK) for the unicast data; - group transient key (GTK) is derived from the group master key (GMK) for the multicast and broadcast data. These derived keys are regularly updated until the context is closed. The derivation of the PMK uses the HMAC-SHA1 function, the result of which is 384 bits in size for the CCMP (Counter-mode/Cipher block chaining MAC (Message Authentication Code) Protocol) and 512 bits in size for the TKIP (Temporal Key Integrity Protocol). The PTK, derived from the PMK, is obtained using the MAC addresses of the authenticator (AA) and the supplicant (SPA), and random numbers (Anonce and Snonce) exchanged during the four-way handshake procedure. HMAC-