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<?xml version="1.0" encoding="iso-8859-1"?>
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<!DOCTYPE chapter PUBLIC "-//Samba-Team//DTD DocBook V4.2-Based Variant V1.0//EN" "http://www.samba.org/samba/DTD/samba-doc">
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<chapter id="pwencrypt">
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<firstname>Jeremy</firstname><surname>Allison</surname>
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<orgname>Samba Team</orgname>
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<email>samba@samba.org</email>
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<pubdate>19 Apr 1999</pubdate>
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<title>LanMan and NT Password Encryption</title>
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<title>Introduction</title>
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<para>With the development of LanManager and Windows NT
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compatible password encryption for Samba, it is now able
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to validate user connections in exactly the same way as
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a LanManager or Windows NT server.</para>
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<para>This document describes how the SMB password encryption
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algorithm works and what issues there are in choosing whether
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you want to use it. You should read it carefully, especially
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the part about security and the "PROS and CONS" section.</para>
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<title>How does it work?</title>
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<para>LanManager encryption is somewhat similar to UNIX
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password encryption. The server uses a file containing a
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hashed value of a user's password. This is created by taking
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the user's plaintext password, capitalising it, and either
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truncating to 14 bytes or padding to 14 bytes with null bytes.
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This 14 byte value is used as two 56 bit DES keys to encrypt
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a 'magic' eight byte value, forming a 16 byte value which is
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stored by the server and client. Let this value be known as
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the "hashed password".</para>
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<para>Windows NT encryption is a higher quality mechanism,
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consisting of doing an MD4 hash on a Unicode version of the user's
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password. This also produces a 16 byte hash value that is
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non-reversible.</para>
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<para>When a client (LanManager, Windows for WorkGroups, Windows
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95 or Windows NT) wishes to mount a Samba drive (or use a Samba
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resource), it first requests a connection and negotiates the
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protocol that the client and server will use. In the reply to this
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request the Samba server generates and appends an 8 byte, random
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value - this is stored in the Samba server after the reply is sent
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and is known as the "challenge". The challenge is different for
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every client connection.</para>
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<para>The client then uses the hashed password (16 byte values
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described above), appended with 5 null bytes, as three 56 bit
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DES keys, each of which is used to encrypt the challenge 8 byte
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value, forming a 24 byte value known as the "response".</para>
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<para>In the SMB call SMBsessionsetupX (when user level security
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is selected) or the call SMBtconX (when share level security is
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selected), the 24 byte response is returned by the client to the
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Samba server. For Windows NT protocol levels the above calculation
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is done on both hashes of the user's password and both responses are
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returned in the SMB call, giving two 24 byte values.</para>
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<para>The Samba server then reproduces the above calculation, using
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its own stored value of the 16 byte hashed password (read from the
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<filename>smbpasswd</filename> file - described later) and the challenge
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value that it kept from the negotiate protocol reply. It then checks
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to see if the 24 byte value it calculates matches the 24 byte value
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returned to it from the client.</para>
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<para>If these values match exactly, then the client knew the
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correct password (or the 16 byte hashed value - see security note
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below) and is thus allowed access. If not, then the client did not
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know the correct password and is denied access.</para>
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<para>Note that the Samba server never knows or stores the cleartext
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of the user's password - just the 16 byte hashed values derived from
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it. Also note that the cleartext password or 16 byte hashed values
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are never transmitted over the network - thus increasing security.</para>
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<title>The smbpasswd file</title>
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<anchor id="SMBPASSWDFILEFORMAT"/>
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<para>In order for Samba to participate in the above protocol
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it must be able to look up the 16 byte hashed values given a user name.
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Unfortunately, as the UNIX password value is also a one way hash
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function (ie. it is impossible to retrieve the cleartext of the user's
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password given the UNIX hash of it), a separate password file
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containing this 16 byte value must be kept. To minimise problems with
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these two password files, getting out of sync, the UNIX <filename>
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/etc/passwd</filename> and the <filename>smbpasswd</filename> file,
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a utility, <command>mksmbpasswd.sh</command>, is provided to generate
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a smbpasswd file from a UNIX <filename>/etc/passwd</filename> file.
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<para>To generate the smbpasswd file from your <filename>/etc/passwd
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</filename> file use the following command:</para>
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<para><prompt>$ </prompt><userinput>cat /etc/passwd | mksmbpasswd.sh
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> /usr/local/samba/private/smbpasswd</userinput></para>
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<para>If you are running on a system that uses NIS, use</para>
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<para><prompt>$ </prompt><userinput>ypcat passwd | mksmbpasswd.sh
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> /usr/local/samba/private/smbpasswd</userinput></para>
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<para>The <command>mksmbpasswd.sh</command> program is found in
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the Samba source directory. By default, the smbpasswd file is
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<para><filename>/usr/local/samba/private/smbpasswd</filename></para>
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<para>The owner of the <filename>/usr/local/samba/private/</filename>
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directory should be set to root, and the permissions on it should
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be set to 0500 (<command>chmod 500 /usr/local/samba/private</command>).
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<para>Likewise, the smbpasswd file inside the private directory should
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be owned by root and the permissions on is should be set to 0600
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(<command>chmod 600 smbpasswd</command>).</para>
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<para>The format of the smbpasswd file is (The line has been
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wrapped here. It should appear as one entry per line in
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your smbpasswd file.)</para>
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<para><programlisting>
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username:uid:XXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXX:XXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXX:
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[Account type]:LCT-<last-change-time>:Long name
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</programlisting></para>
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<para>Although only the <replaceable>username</replaceable>,
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<replaceable>uid</replaceable>, <replaceable>
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XXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXX</replaceable>,
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[<replaceable>Account type</replaceable>] and <replaceable>
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last-change-time</replaceable> sections are significant
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and are looked at in the Samba code.</para>
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<para>It is <emphasis>VITALLY</emphasis> important that there by 32
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'X' characters between the two ':' characters in the XXX sections -
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the smbpasswd and Samba code will fail to validate any entries that
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do not have 32 characters between ':' characters. The first XXX
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section is for the Lanman password hash, the second is for the
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Windows NT version.</para>
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<para>When the password file is created all users have password entries
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consisting of 32 'X' characters. By default this disallows any access
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as this user. When a user has a password set, the 'X' characters change
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to 32 ascii hexadecimal digits (0-9, A-F). These are an ascii
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representation of the 16 byte hashed value of a user's password.</para>
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<para>To set a user to have no password (not recommended), edit the file
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using vi, and replace the first 11 characters with the ascii text
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<constant>"NO PASSWORD"</constant> (minus the quotes).</para>
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<para>For example, to clear the password for user bob, his smbpasswd file
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entry would look like :</para>
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<para><programlisting>
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bob:100:NO PASSWORDXXXXXXXXXXXXXXXXXXXXX:XXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXX:
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[U ]:LCT-00000000:Bob's full name:/bobhome:/bobshell
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</programlisting></para>
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<para>If you are allowing users to use the smbpasswd command to set
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their own passwords, you may want to give users NO PASSWORD initially
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so they do not have to enter a previous password when changing to their
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new password (not recommended). In order for you to allow this the
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<command>smbpasswd</command> program must be able to connect to the
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<command>smbd</command> daemon as that user with no password. Enable this
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by adding the line :</para>
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<para><command>null passwords = yes</command></para>
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<para>to the [global] section of the smb.conf file (this is why
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the above scenario is not recommended). Preferably, allocate your
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users a default password to begin with, so you do not have
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to enable this on your server.</para>
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<para><emphasis>Note : </emphasis>This file should be protected very
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carefully. Anyone with access to this file can (with enough knowledge of
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the protocols) gain access to your SMB server. The file is thus more
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sensitive than a normal unix <filename>/etc/passwd</filename> file.</para>