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<chapter id="pwencrypt">
<chapterinfo>
<author>
<firstname>Jeremy</firstname><surname>Allison</surname>
<affiliation>
<orgname>Samba Team</orgname>
<address>
<email>samba@samba.org</email>
</address>
</affiliation>
</author>
<pubdate>19 Apr 1999</pubdate>
</chapterinfo>
<title>LanMan and NT Password Encryption</title>
<sect1>
<title>Introduction</title>
<para>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.</para>
<para>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.</para>
</sect1>
<sect1>
<title>How does it work?</title>
<para>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".</para>
<para>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.</para>
<para>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.</para>
<para>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".</para>
<para>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.</para>
<para>The Samba server then reproduces the above calculation, using
its own stored value of the 16 byte hashed password (read from the
<filename>smbpasswd</filename> 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.</para>
<para>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.</para>
<para>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.</para>
</sect1>
<sect1>
<title>The smbpasswd file</title>
<anchor id="SMBPASSWDFILEFORMAT"/>
<para>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 <filename>
/etc/passwd</filename> and the <filename>smbpasswd</filename> file,
a utility, <command>mksmbpasswd.sh</command>, is provided to generate
a smbpasswd file from a UNIX <filename>/etc/passwd</filename> file.
</para>
<para>To generate the smbpasswd file from your <filename>/etc/passwd
</filename> file use the following command:</para>
<para><prompt>$ </prompt><userinput>cat /etc/passwd | mksmbpasswd.sh
> /usr/local/samba/private/smbpasswd</userinput></para>
<para>If you are running on a system that uses NIS, use</para>
<para><prompt>$ </prompt><userinput>ypcat passwd | mksmbpasswd.sh
> /usr/local/samba/private/smbpasswd</userinput></para>
<para>The <command>mksmbpasswd.sh</command> program is found in
the Samba source directory. By default, the smbpasswd file is
stored in :</para>
<para><filename>/usr/local/samba/private/smbpasswd</filename></para>
<para>The owner of the <filename>/usr/local/samba/private/</filename>
directory should be set to root, and the permissions on it should
be set to 0500 (<command>chmod 500 /usr/local/samba/private</command>).
</para>
<para>Likewise, the smbpasswd file inside the private directory should
be owned by root and the permissions on is should be set to 0600
(<command>chmod 600 smbpasswd</command>).</para>
<para>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.)</para>
<para><programlisting>
username:uid:XXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXX:XXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXX:
[Account type]:LCT-<last-change-time>:Long name
</programlisting></para>
<para>Although only the <replaceable>username</replaceable>,
<replaceable>uid</replaceable>, <replaceable>
XXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXX</replaceable>,
[<replaceable>Account type</replaceable>] and <replaceable>
last-change-time</replaceable> sections are significant
and are looked at in the Samba code.</para>
<para>It is <emphasis>VITALLY</emphasis> 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.</para>
<para>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.</para>
<para>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
<constant>"NO PASSWORD"</constant> (minus the quotes).</para>
<para>For example, to clear the password for user bob, his smbpasswd file
entry would look like :</para>
<para><programlisting>
bob:100:NO PASSWORDXXXXXXXXXXXXXXXXXXXXX:XXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXX:
[U ]:LCT-00000000:Bob's full name:/bobhome:/bobshell
</programlisting></para>
<para>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
<command>smbpasswd</command> program must be able to connect to the
<command>smbd</command> daemon as that user with no password. Enable this
by adding the line :</para>
<para><command>null passwords = yes</command></para>
<para>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.</para>
<para><emphasis>Note : </emphasis>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 <filename>/etc/passwd</filename> file.</para>
</sect1>
</chapter>
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