Which protocol is described as a free authentication protocol for client/server applications?

• A. Kerberos
• B. OAuth
• C. TLS
• D. PAP

Answer: (A)

Kerberos is an open, vendor-neutral authentication protocol designed for client/server networks. It uses a trusted Key Distribution Center to issue time‑stamped tickets after a client proves its identity, and the client then presents these tickets to services to prove who it is without sending passwords over the network. This ticket-based system enables mutual authentication and supports single sign-on in many environments, making it a robust choice for authenticating client/server interactions.

OAuth is about granting access to resources (authorization), not primarily about proving identities. TLS secures the communication channel and can perform certificate-based authentication, but it isn’t used as a dedicated, standalone client/server authentication protocol in the same way. PAP is a simple, insecure protocol that transmits passwords in clear text over the link. Therefore, Kerberos best fits as the free authentication protocol for client/server applications.