NTLM

Key takeaways

NTLM belongs to computer science; place it in that map before memorising details.
Learn the definition first, then one example that makes the definition concrete.
Connect NTLM to a quantity you can measure, compute or draw — that is where exam questions come from.
Reproduce the core statement of NTLM from memory before moving on to harder problems.

Reference excerpt

In a Windows network, NT (New Technology) LAN Manager (NTLM) is a suite of Microsoft security protocols intended to provide 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, which 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.

Protocol 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.

First, the client establishes a network path to the server and sends a NEGOTIATE_MESSAGE advertising its capabilities. Next, the server responds with CHALLENGE_MESSAGE which is used to establish the identity of the client. Finally, the client responds to the challenge with an AUTHENTICATE_MESSAGE. 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 through 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 characters 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).

NTLMv1 The server authenticates the client by sending an 8-byte random number, the challenge. The client performs an operation involving the challenge and a secret shared between client and server, specifically one of the two password hashes described above. The client returns the 24-byte result of the computation. In fact, in NTLMv1 the computations are usually made using both hashes and both 24-byte results are sent. The server verifies that the client has computed the correct result, and from this infers possession of the secret, and hence the authenticity of the client. Both the hashes produce 16-byte quantities. Five bytes of zeros are appended to obtain 21 bytes. The 21 bytes are separated in three 7-byte (56-bit) quantities. Each of these 56-bit quantities is used as a key to DES encrypt the 64-bit challenge. The three encryptions of the challenge are reunited to form the 24-byte response. Both the response using the LM hash and the NT hash are returned as the response, but this is configurable.

NTLMv2 NTLMv2, introduced in Windows NT 4.0 SP4 (and natively supported in Windows 2000), is a challenge-response authentication protocol. It is intended as a cryptographically strengthened replacement for NTLMv1, enhancing NTLM security by hardening the protocol against many spoofing attacks and adding the ability for a server to authenticate to the client. NTLMv2 sends two responses to an 8-byte server challenge. Each response contains a 16-byte HMAC-MD5 hash of the server challenge, a fully/partially randomly generated client challenge, and an HMAC-MD5 hash of the user's password and other identifying information. The two responses differ in the format of the client challenge. The shorter response uses an 8-byte random value for this challenge. In order to verify the response, the server must receive as part of the response the client challenge. For this shorter response, the 8-byte client challenge appended to the 16-byte response makes a 24-byte package which is consistent with the 24-byte response format of the previous NTLMv1 protocol. In certain non-official documentation (e.g. DCE/RPC Over SMB, Leighton) this response is termed LMv2. The second response sent by NTLMv2 uses a variable-length client challenge which includes (1) the current time in NT Time format, (2) an 8-byte random value (CC2 in the box below), (3) the domain name and (4) some standard format stuff. The response must include a copy of this client challenge, and is therefore variable length. In non-official documentation, this response is termed NTv2. Both LMv2 and NTv2 hash the client and server challenge with the NT hash of the user's password and other identifying information. The exact formula is to begin with the NT hash, which is stored in the SAM or AD, and continue to hash in, using HMAC-MD5, the username and domain name. In the box below, X stands for the fixed contents of a formatting field.

SC = 8-byte server challenge, random CC = 8-byte client challenge, random CC* = (X, time, CC2, domain name) v2-Hash = HMAC-MD5(NT-Hash, user name, domain name) LMv2 = HMAC-MD5(v2-Hash, SC, CC) NTv2 = HMAC-MD5(v2-Hash, SC, CC*) response = LMv2 | CC | NTv2 | CC*

… excerpt ends here. Continue reading the full article.

Worked examples

Example 1 — a first encounter with NTLM

Start with the simplest possible case. Write down what NTLM claims or describes in one sentence, then invent the smallest concrete situation in which that sentence is true. In computer science, the smallest case is usually a single object, a single equation or a single measurement. Check that every symbol or term in your sentence has a meaning in that case.

Example 2 — changing one variable

Take the situation from Example 1 and change exactly one quantity: double it, halve it, or set it to zero. Predict what should happen to NTLM before you calculate. Comparing your prediction with the result is the fastest way to find out whether you understand the idea or only the words.

Example 3 — an exam-style question

Typical questions about NTLM ask you to (a) state it precisely, (b) apply it to given data, and (c) explain a limitation. Practise writing all three answers in under five minutes; the third part is what separates a full-mark answer from an average one.

Applications of NTLM

In research: NTLM appears in computer science research whenever the underlying quantities have to be modelled precisely. Papers usually cite it as a starting assumption and then explore where it breaks down.
In technology and industry: Engineering practice reuses NTLM in design rules, simulations and safety margins. Knowing the idea lets you read a specification sheet and understand why the numbers look the way they do.
In the classroom: NTLM is common in secondary-school and first-year university syllabi. It links to neighbouring topics Computer access control protocols, Computer network security, Microsoft Windows security technology, so understanding it makes those chapters shorter.
In everyday life: Look for NTLM outside the textbook — in sport, cooking, traffic, electronics or the sky above you. An example you found yourself is remembered far longer than one you were given.

How to study NTLM in 20 minutes

Read the reference excerpt below once, without taking notes.
Close the page and write down what NTLM means in your own words.
Compare your version with the excerpt and mark what you missed.
Work through the three examples above with pen and paper.
Explain NTLM out loud to somebody else — or to Teacher Smith in the lgStudy chat.

Frequently asked questions

What is NTLM in simple terms?

In a Windows network, NT (New Technology) LAN Manager (NTLM) is a suite of Microsoft security protocols intended to provide authentication, integrity, and confidentiality to users. NTLM is the successor to the authentication protocol in Microsoft LAN Manager (LANMAN), an older Microsoft product.

Why does NTLM matter?

Because it connects several computer science ideas at once: it gives you a definition you can apply, a quantity you can calculate, and a way to check whether a result is plausible.

How should I study NTLM?

Read the excerpt, restate it from memory, then work through the examples and applications listed on this page. The five-step study plan above takes about twenty minutes.

What does this page cover?

It gives you a compact reference excerpt plus original lgStudy explanations, examples, applications and study material on NTLM.