Remote Authentication Dial-In User Service (RADIUS) authentication is accomplished by using clear text and shared secret keys. RADIUS is a protocol that provides centralized authentication, authorization, and accounting for remote network access. RADIUS uses User Datagram Protocol (UDP) to communicate between the client and the server. RADIUS authentication uses clear text to send the username and password of the user, but it also uses a shared secret key to encrypt a message authentication code (MAC) that is appended to the packet. The MAC is used to verify the integrity and authenticity of the packet. The shared secret key is only known by the client and the server, and it is never transmitted over the network. The other options are not correct. RADIUS does not use firewall rules, Virtual Private Networks (VPN), or asymmetric encryption keys for authentication. References: Official (ISC)2 CISSP CBK Reference, Fifth Edition, Domain
4: Communication and Network Security, p. 503-504; CISSP All-in-One Exam Guide, Eighth Edition, Chapter
4: Communication and Network Security, p. 228-229.