Which of the following is not considered a single sign-on (SSO) implementation?

Single sign-on (SSO) implementations allow users to authenticate once and gain access to multiple applications or systems without needing to enter credentials again. The correct answer indicates that RADIUS is not considered an SSO implementation.

RADIUS, which stands for Remote Authentication Dial-In User Service, is primarily an authentication, authorization, and accounting protocol designed for use in network access scenarios. It provides access control for network services but does not provide the kind of seamless user authentication across multiple applications that defines an SSO solution. Instead, it necessitates authentication for each session, thus not fulfilling the key requirement of an SSO system.

In contrast, ADFS (Active Directory Federation Services), Kerberos, and CAS (Central Authentication Service) are all technologies designed to facilitate SSO. ADFS allows for federated identity and single sign-on across applications, Kerberos provides mutual authentication and supports SSO by allowing systems to verify a user's identity without repeatedly asking for credentials, and CAS is a protocol specifically developed for web-based SSO. Each of these options directly supports the SSO concept, making RADIUS distinctively different in its function and purpose.