NTLM

In a Windows network, NT LAN Manager (NTLM) is a suite of Microsoft security protocols that provides authentication, integrity, and confidentiality to users. NTLM is the successor to the authentication protocol in Microsoft LAN Manager (LANMAN), an older Microsoft product. The NTLM protocol suite is implemented in a Security Support Provider, which combines the LAN Manager authentication protocol, NTLMv1, NTLMv2 and NTLM2 Session protocols in a single package. Whether these protocols are used or can be used on a system is governed by Group Policy settings, for which different versions of Windows have different default settings. NTLM passwords are considered weak because they can be brute-forced very easily with modern hardware.

NTLM is a challenge-response authentication protocol which uses three messages to authenticate a client in a connection oriented environment (connectionless is similar), and a fourth additional message if integrity is desired.

The NTLM protocol uses one or both of two hashed password values, both of which are also stored on the server (or domain controller), and which though a lack of salting are password equivalent, meaning that if you grab the hash value from the server, you can authenticate without knowing the actual password. The two are the LM Hash (a DES-based function applied to the first 14 chars of the password converted to the traditional 8 bit PC charset for the language), and the NT Hash (MD4 of the little endian UTF-16 Unicode password). Both hash values are 16 bytes (128 bits) each.

The NTLM protocol also uses one of two one way functions, depending on the NTLM version. NT LanMan and NTLM version 1 use the DES based LanMan one way function (LMOWF), while NTLMv2 uses the NT MD4 based one way function (NTOWF).

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