Which authentication protocol is the most commonly used?

The correct choice highlights Kerberos as the most commonly used authentication protocol, particularly in environments where security and ticket-based authentication are crucial. Developed by MIT, Kerberos utilizes a system of tickets to allow nodes to communicate securely over a non-secure network and authenticate users without transmitting passwords directly.

Kerberos is widely employed in enterprise networks, especially in Microsoft environments, where it facilitates secure communication between clients and servers. Its ability to verify both users and services makes it a foundational component of many authentication systems, enhancing security by minimizing the risk of password theft and replay attacks.

While the other protocols like RADIUS, LDAP, and NTLM also play significant roles in authentication, they serve different purposes or are typically used in specific contexts. RADIUS is often used for remote authentication, LDAP is primarily a directory access protocol, and NTLM is an older authentication protocol that is less secure than Kerberos. All of these protocols have their uses, but Kerberos stands out for its robust security measures and widespread adoption in modern networks, solidifying its position as the most commonly used authentication protocol.