Chapter 13. LanMan and NT Password Encryption
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## Chapter 13. LanMan and NT Password Encryption

### Jeremy Allison

Samba Team

`<samba@samba.org>`

19 Apr 1999

## Introduction

With the development of LanManager and Windows NT compatible password encryption for Samba, it is now able to validate user connections in exactly the same way as a LanManager or Windows NT server.

This document describes how the SMB password encryption algorithm works and what issues there are in choosing whether you want to use it. You should read it carefully, especially the part about security and the "PROS and CONS" section.

## How does it work?

LanManager encryption is somewhat similar to UNIX password encryption. The server uses a file containing a hashed value of a user's password. This is created by taking the user's plaintext password, capitalising it, and either truncating to 14 bytes or padding to 14 bytes with null bytes. This 14 byte value is used as two 56 bit DES keys to encrypt a 'magic' eight byte value, forming a 16 byte value which is stored by the server and client. Let this value be known as the "hashed password".

Windows NT encryption is a higher quality mechanism, consisting of doing an MD4 hash on a Unicode version of the user's password. This also produces a 16 byte hash value that is non-reversible.

When a client (LanManager, Windows for WorkGroups, Windows 95 or Windows NT) wishes to mount a Samba drive (or use a Samba resource), it first requests a connection and negotiates the protocol that the client and server will use. In the reply to this request the Samba server generates and appends an 8 byte, random value - this is stored in the Samba server after the reply is sent and is known as the "challenge". The challenge is different for every client connection.

The client then uses the hashed password (16 byte values described above), appended with 5 null bytes, as three 56 bit DES keys, each of which is used to encrypt the challenge 8 byte value, forming a 24 byte value known as the "response".

In the SMB call SMBsessionsetupX (when user level security is selected) or the call SMBtconX (when share level security is selected), the 24 byte response is returned by the client to the Samba server. For Windows NT protocol levels the above calculation is done on both hashes of the user's password and both responses are returned in the SMB call, giving two 24 byte values.

The Samba server then reproduces the above calculation, using its own stored value of the 16 byte hashed password (read from the `smbpasswd` file - described later) and the challenge value that it kept from the negotiate protocol reply. It then checks to see if the 24 byte value it calculates matches the 24 byte value returned to it from the client.

If these values match exactly, then the client knew the correct password (or the 16 byte hashed value - see security note below) and is thus allowed access. If not, then the client did not know the correct password and is denied access.

Note that the Samba server never knows or stores the cleartext of the user's password - just the 16 byte hashed values derived from it. Also note that the cleartext password or 16 byte hashed values are never transmitted over the network - thus increasing security.

## The smbpasswd file

In order for Samba to participate in the above protocol it must be able to look up the 16 byte hashed values given a user name. Unfortunately, as the UNIX password value is also a one way hash function (ie. it is impossible to retrieve the cleartext of the user's password given the UNIX hash of it), a separate password file containing this 16 byte value must be kept. To minimise problems with these two password files, getting out of sync, the UNIX `/etc/passwd` and the `smbpasswd` file, a utility, `mksmbpasswd`, is provided to generate a smbpasswd file from a UNIX `/etc/passwd` file.

To generate the smbpasswd file from your `/etc/passwd` file use the following command:

`$` `cat /etc/passwd | mksmbpasswd > /usr/local/samba/private/smbpasswd`

If you are running on a system that uses NIS, use

`$` `ypcat passwd | mksmbpasswd > /usr/local/samba/private/smbpasswd`

The `mksmbpasswd` program is found in the Samba source directory. By default, the smbpasswd file is stored in :

`/usr/local/samba/private/smbpasswd`

The owner of the `/usr/local/samba/private/` directory should be set to root, and the permissions on it should be set to 0500 (`chmod 500 /usr/local/samba/private`).

Likewise, the smbpasswd file inside the private directory should be owned by root and the permissions on is should be set to 0600 (`chmod 600 smbpasswd`).

The format of the smbpasswd file is (The line has been wrapped here. It should appear as one entry per line in your smbpasswd file.)

    username:uid:XXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXX:XXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXX:
    	[Account type]:LCT-<last-change-time>:Long name

Although only the `username`, `uid`, `XXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXX`, [`Account type`] and `last-change-time` sections are significant and are looked at in the Samba code.

It is VITALLY important that there by 32 'X' characters between the two ':' characters in the XXX sections - the smbpasswd and Samba code will fail to validate any entries that do not have 32 characters between ':' characters. The first XXX section is for the Lanman password hash, the second is for the Windows NT version.

When the password file is created all users have password entries consisting of 32 'X' characters. By default this disallows any access as this user. When a user has a password set, the 'X' characters change to 32 ascii hexadecimal digits (0-9, A-F). These are an ascii representation of the 16 byte hashed value of a user's password.

To set a user to have no password (not recommended), edit the file using vi, and replace the first 11 characters with the ascii text `"NO PASSWORD"` (minus the quotes).

For example, to clear the password for user bob, his smbpasswd file entry would look like :

    bob:100:NO PASSWORDXXXXXXXXXXXXXXXXXXXXX:XXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXX:
    	[U          ]:LCT-00000000:Bob's full name:/bobhome:/bobshell

If you are allowing users to use the smbpasswd command to set their own passwords, you may want to give users NO PASSWORD initially so they do not have to enter a previous password when changing to their new password (not recommended). In order for you to allow this the `smbpasswd` program must be able to connect to the `smbd` daemon as that user with no password. Enable this by adding the line :

`null passwords = yes`

to the [global] section of the smb.conf file (this is why the above scenario is not recommended). Preferably, allocate your users a default password to begin with, so you do not have to enable this on your server.

Note : This file should be protected very carefully. Anyone with access to this file can (with enough knowledge of the protocols) gain access to your SMB server. The file is thus more sensitive than a normal unix `/etc/passwd` file.

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