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1.3 Overview

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NT LAN Manager (NTLM) is the name of a family of security protocols. NTLM is used by application protocols to authenticate remote users and, optionally, to provide session security when requested by the application.

NTLM is a challenge-response style authentication protocol. This means that to authenticate a user, the server sends a challenge to the client. The client then sends back a response that is a function of the challenge, the user's password, and possibly other information. Computing the correct response requires knowledge of the user's password. The server (or another party trusted by the server) can validate the response by consulting an account database to get the user's password and computing the proper response for that challenge.

NTLM has messages and a state machine, like other protocols, but it does not have a network protocol stack layer. The NTLM protocols are embedded protocols. Rather, NTLM is implemented as a subroutine package that creates, reads and manipulates NTLM packets - but communicates those packets as byte arrays between the NTLM code and the caller. The caller is typically the code of some other protocol - one that has a defined layer in the network stack. Unlike stand-alone application protocols such as [MS-SMB] or HTTP, NTLM messages are embedded in the packets of an application protocol that requires authentication of a user. The application protocol semantics determine how and when the NTLM messages are encoded, framed, and transported from the client to the server and vice versa. See section 4 for an example of how NTLM messages are embedded in the SMB Version 1.0 Protocol as specified in [MS-SMB].

The NTLM implementation also differs from normal protocol implementations, in that the best way to implement it is as a function library called by some other protocol implementation (the application protocol), rather than as a layer in a network protocol stack. For more information about GSS-API calls, see section 3.4.6. The NTLM function library receives parameters from the application protocol caller and returns an authentication message that the caller places into fields of its own messages as it chooses. Nevertheless, if one looks at just the NTLM messages apart from the application protocol in which they are embedded, there is an NTLM protocol and that is what is specified by this document.

There are two major variants of the NTLM authentication protocol: the connection-oriented transport protocol variant and the connectionless protocol variant. In the connectionless (datagram) variant:

  • NTLM does not use the internal sequence number maintained by the NTLM implementation. Instead, it uses a sequence number passed in by the protocol implementation in which NTLM is embedded.

  • Keys for session security are established at client initialization time (while in connection-oriented mode they are established only at the end of authentication exchange), and session security can be used as soon as the session keys are established.

  • It is not possible to send a NEGOTIATE_MESSAGE message (section 2.2.1.1).

Each of these variants has three versions: LM, NTLMv1, and NTLMv2. The message flow for all three is the same; the only differences are the function used to compute various response fields from the challenge, and which response fields are set. <2>

In addition to authentication, the NTLM protocol optionally provides for session security—specifically message integrity and confidentiality through signing and sealing functions in NTLM.