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authorcvs2svn Import User <samba-bugs@samba.org>2002-11-07 19:45:56 +0000
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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><anchor id="SMBPASSWDFILEFORMAT">The smbpasswd file</title>
+
+ <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
+ &gt; /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
+ &gt; /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-&lt;last-change-time&gt;: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>