Windows Challenge/Response (NTLM) is the authentication protocol used on networks that include systems running the Windows operating system and on stand-alone systems. NTLM credentials are based on data obtained during the interactive logon process and consist of a domain name, a user name, and a one-way hash of the user’s password. It uses an encrypted challenge/response protocol to authenticate a user without sending the user’s password over the wire. Instead, the system requesting authentication must perform a calculation that proves it has access to the secured NTLM credentials.
Interactive NTLM authentication over a network typically involves two systems: a client system, where the user is requesting authentication, and a domain controller, where information related to the user’s password is kept. Noninteractive authentication, which may be required to permit an already logged-on user to access a resource such as a server application, typically involves three systems: a client, a server, and a domain controller that does the authentication calculations on behalf of the server.
The Microsoft Kerberos security package adds greater security than NTLM to systems on a network. Although Microsoft Kerberos is the protocol of choice, NTLM is still supported. It must also be used for logon authentication on stand-alone systems. The Kerberos protocol defines how clients interact with a network authentication service. Clients obtain tickets from the Kerberos Key Distribution Center (KDC), and they present these tickets to servers when connections are established. Kerberos tickets represent the client’s network credentials