Final answer:
The default authentication protocol for Windows 2000 Server and later versions is Kerberos, which uses secret-key cryptography for secure client/server applications.
Step-by-step explanation:
The default authentication protocol used by Windows 2000 Server and later versions is Kerberos. Kerberos is a network authentication protocol designed to provide strong authentication for client/server applications by using secret-key cryptography. It has been the default protocol since the release of Windows 2000 Server, providing a more secure alternative to the previously used NTLM protocol.