Ntlm is an authentication protocol used in various microsoft network protocol implementations and supported by the ntlm security support provider ("ntlmssp"). originally used for authentication and negotiation of secure dce/rpc, ntlm is also used throughout microsoft's systems as an integrated single sign-on mechanism.

ntlm employs a challenge-response mechanism for authentication, in which clients are able to prove their identities without sending a password to the server. it consists of three messages, commonly referred to as type 1 (negotiation), type 2 (challenge) and type 3 (authentication).