Authentication

Kerberos Security Support Provider

The Kerberos authentication protocol is implemented as a security support provider (SSP) — a dynamic-link library (DLL) that is supplied with the operating system. Windows 2000 also includes an SSP for NTLM authentication. By default, both the Kerberos protocol and the NTLM protocol are loaded by the LSA on a computer that is running Windows 2000 when the system starts. In Windows 2000 domains, either of these SSPs can be used to authenticate network logons and client/server connections. Which SSP is used depends on the capabilities of the computer on the other side of the connection. The Kerberos SSP is always the first choice.

System services and transport-level applications have access to SSPs through the Microsoft Security Support Provider Interface (SSPI). The SSPI is a Microsoft® Win32® interface with methods for enumerating the SSPs available on a computer, selecting one, and then using it to obtain an authenticated connection. The methods provided in the SSPI are generic, black-box routines that developers can use without knowing the details of a particular protocol. For example, when a client/server connection is authenticated, the application on the client's side of the connection sends credentials to the server using the SSPI method InitializeSecurityContext. If the Kerberos SSP has been selected, InitializeSecurityContext generates a KRB_AP_REQ message from the client. The application on the server's side of the connection responds with the SSPI method AcceptSecurityContext, which generates a KRB_AP_REP message from the server. After the connection has been authenticated, the LSA on the server uses information from the client's session ticket to build an access token. The server then invokes the SSPI method ImpersonateSecurityContext to attach the access token to an impersonation thread for the service.

After the LSA establishes the security context for an interactive user, another instance of the Kerberos SSP might be loaded by a process running in the user's security context to support message signing and sealing.

All distributed services in Windows 2000 domains use the SSPI for authentication. Thus, all domain services support the Kerberos protocol. Services that use the Kerberos protocol for authentication include:

Print spooler services.
Common Internet File System/Server Message Block (CIFS/SMB) remote file access.
Lightweight Directory Access Protocol (LDAP) queries to Active Directory.
Distributed file system management and referrals.
Internet Protocol Security (IPSec) host-to-host security authority authentication.
Reservation requests for network Quality of Service.
Intranet authentication to Internet Information Services (IIS).
Remote server or workstation management using authenticated remote procedure call (RPC).
Certificate requests to the Certificate Services for domain users and computers.