NTLM user authentication in Windows
- Password storage in the account database
- User authentication by using the MSV1_0 authentication package
- Pass-through authentication
Password storage in the account databaseUser records are stored in the security accounts manager (SAM) database or in the Active Directory database. Each user account is associated with two passwords: the LAN Manager-compatible password and the Windows password. Each password is encrypted and stored in the SAM database or in the Active Directory database.
The LAN Manager-compatible password is compatible with thepassword that is used by LAN Manager. This password is based on the original equipmentmanufacturer (OEM) character set. This password is not case sensitive and can be up to 14 characters long. The OWFversion of this password is also known as the LAN Manager OWF or ESTD version. This passwordis computed by using DES encryption to encrypt a constant with the clear text password. The LAN Manager OWF password is 16 bytes long.The first 7 bytes of the clear text password are used to compute thefirst 8 bytes of the LAN Manager OWF password. The second 7 bytes ofthe clear text password are used to computer the second 8 bytes of theLAN Manager OWF password.
The Windows password is based on the Unicode character set. This password is casesensitive and can be up to 128 characters long. The OWF version of this password is also known as the Windows OWF password. This password is computed by using the RSA MD-4encryption algorithm. This algorithm computes a 16-byte digest of a variable-length string of clear text password bytes.
Any user account might lack either the LAN Manager password or theWindows password. However, every attempt is made to maintain bothversions of the password. For example, if the user account is ported from a LANManager UAS database by using PortUas, or if the password is changed froma LAN Manager client or from a Windows for Workgroups client, only the LAN Manager versionof the password will exist. If the password is set or changed ona Windows client, and the password has no LAN Manager representation, only the Windowsversion of the password will exist. (The password might have no LAN Manager representation because the password is longer than 14 characters or because the characters cannot berepresented in the OEM character set.) User interfacelimits in Windows do not let Windows passwords exceed 14 characters. Theimplications of this limitation are discussed later in this article.
In Windows 2000 Service Pack 2 and in later versions of Windows, a setting is available that lets you prevent Windows from storing a LAN Manager hash of your password. For more information, click the following article number to view the article in the Microsoft Knowledge Base:
User authentication by using the MSV1_0 authentication packageWindows uses the LsaLogonUser API for all kinds of user authentications. The LsaLogonUser API authenticates users by calling an authentication package. By default, LsaLogonUser calls the MSV1_0 (MSV) authentication package. This package is included with Windows NT. The MSV authentication package stores user records in the SAM database. This package supports pass-through authentication of users in other domains by using the Netlogon service.
Internally, the MSV authentication package is divided into two parts. The first part of the MSV authentication package runs on the computer that is beingconnected to. The second part runs on the computer that contains the user account. When both parts run on the samecomputer, the first part of the MSV authentication package calls thesecond part without involving the Netlogon service. The first part of the MSVauthentication package recognizes that pass-throughauthentication is required because the domain namethat is passed is not its own domain name. When pass-through authentication is required, MSV passes the request tothe Netlogon service. The Netlogon service then routes the request to the Netlogon serviceon the destination computer. In turn, the Netlogon service passes the request to theother part of the MSV authentication package on that computer.
LsaLogonUser supports interactive logons, service logons, and networklogons. In the MSV authentication package, all forms of logon pass the name of theuser account,the name of the domain that contains the user account, and some function of the user's password. The different kinds of logon represent the password differently when they pass it to LsaLogonUser.
For interactive logons, batch logons, and service logons, the logon client is on the computer that is running the first part of the MSV authentication package. In this case, the clear-text password is passed to LsaLogonUser and to the first part of the MSV authentication package. For service logons and batch logons, the Service Control Manager and the Task Scheduler provide a more secure way of storing the account's credentials.
The first part of the MSV authentication package converts the clear-text password both to a LAN Manager OWF password and to a Windows NT OWF password. Then, the first part of the package passes the clear-text password either to the NetLogon service or to the second part of the package. The second part then queries the SAM database for the OWF passwords and makes sure that they are identical.
For network logons, the client that connects to the computer was previously given a 16-byte challenge, or "nonce." If the client is a LAN Manager client, the client computed a 24-byte challenge response by encrypting the 16-byte challenge with the 16-byte LAN Manager OWF password. The LAN Manager client then passes this "LAN Manager Challenge Response" to the server. If the client is a Windows client, a "Windows NT Challenge Response" is computed by using the same algorithm. However, the Windows client uses the 16-byte Windows OWF data instead of the LAN Manager OWF data. The Windows client then passes both the LAN Manager Challenge Response and the Windows NT Challenge Response to the server. In either case, the server authenticates the user by passing all the following to the LsaLogonUser API:
- The domain name
- The user name
- The original challenge
- The LAN Manager Challenge Response
- The optional Windows NT Challenge Response
Note NTLMv2 also lets the client send a challenge together with the use of session keys that help reduce the risk of common attacks.
As mentioned earlier, either version of the password might be missing from the SAM database or from the Active Directory database. Also, either version of the password might be missing from the call to LsaLogonUser. If both the Windows version of password from the SAM database and the Windows version of the password from LsaLogonUser are available, they both are used. Otherwise, the LAN Manager version of the password is used for comparison. This rule helps enforce case sensitivity when network logons occur from Windows to Windows. This rule also allows for backward compatibility.
Pass-through authenticationThe NetLogon service implements pass-through authentication. It performs the following functions:
- Selects the domain to pass the authenticationrequest to.
- Selects the server within the domain.
- Passes the authentication request through to the selected server.
- If the domain name matches the name of the SAM database, the authentication is processed on that computer. On a Windows workstation that is a member of a domain, the name of the SAM database is considered to be the name of the computer. On an Active Directory domain controller, the name of the account database is the name of the domain. On a computer that is not a member of a domain, all logons process requests locally.
- If the specified domain name is trusted by this domain, the authentication request is passed through to the trusted domain. On Active Directory domain controllers, the list of trusted domains is easily available. On a member of a Windows domain, the request is always passed through to the primary domain of the workstation, letting the primary domain determine whether the specified domain is trusted.
- If the domain name specified is not trusted by the domain, the authentication request is processed on the computer being connected to as if the domain name specified were that domain name. NetLogon does not differentiate between a nonexistent domain, an untrusted domain, and an incorrectly typed domain name.
Article ID: 102716 - Last Review: 11/01/2006 07:29:13 - Revision: 2.2
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