What authentication protocol does Windows use by default for Active Directory systems?

Windows uses the Kerberos authentication protocol by default for Active Directory systems due to its effectiveness in providing strong security through mutual authentication. Kerberos is designed to securely authenticate users in a network environment, ensuring that both the user and the services they access are who they claim to be.

The protocol works by issuing "tickets" to users after they authenticate with a central server, which can be reused to access various network services without the need for re-entering credentials. This ticket-based system greatly enhances security by limiting the number of times that passwords are transmitted over the network. It also offers features like encrypted communications and session keys, which further protect user data and credentials.

In the context of Active Directory, which serves as a directory service for managing user accounts, group policies, and other resources, Kerberos integrates seamlessly, providing a robust framework for managing access controls in a Windows domain.

While options such as RADIUS, LDAP, and X.509 are important in various authentication scenarios, they do not serve as the default authentication protocol for Windows Active Directory systems like Kerberos does. RADIUS is often used for network access authentication, LDAP is primarily used for querying and modifying directory services, and X.509 refers to a standard for public key certificates, which is