Which of the following protocols periodically verifies the identity of the client by using a three-way handshake?

A.
CHAP

B.
PAP

C.
PPP

D.
EAP-TLS

Explanation:
The Challenge-handshake authentication protocol (CHAP) is an authentication scheme used by Point to Point Protocol (PPP) servers to validate the identity of remote clients. CHAP periodically verifies the identity of the client by using a threeway handshake. This happens at the time of establishing the initial link, and may happen again at any timeafterwards. The verification is based on a shared secret (such as the client user’s password).
1. After the completion of the link establishment phase, the authenticator sends a "challenge" message to the peer.
2. The peer responds with a value calculated using a one-way hash function, such as an MD5 checksum hash.
3. The authenticator checks the response against its own calculation of the expected hash value. If the values match, the authenticator acknowledges the authentication; otherwise, it should terminate the connection.
4. At random intervals, the authenticator sends a new challenge to the peer and repeats steps 1 through 3.

Answer option C is incorrect. Point-to-Point Protocol (PPP) is a remote access protocol commonly used to connect to the Internet. PPP supports compression and encryption and can be used to connect to a variety of networks. It can connect to a network running on IPX, TCP/IP, or NetBEUI protocol. PPP supports multi-protocol and dynamic IP assignments. It is the default protocol for the Microsoft Dial-Up adapter. Answer option B is incorrect. A password authentication protocol (PAP) is an authentication protocol that uses a password. PAP is used by Point to Point Protocol to validate users before allowing them access to server resources. Almost all network operating system remote servers support PAP. Working cycle: Client sends username and password. Server sends authentication-ack (if credentials are OK) or authentication-nak. Answer option D is incorrect. EAP-Transport Layer Security (EAP-TLS) is an IETF open standard and is wellsupported among wireless vendors. The security of the TLS protocol is strong, provided the user understands potential warnings about false credentials. It uses PKI to secure communication to a RADIUS authentication server or another type of authentication server.