1 <html><head><meta http-equiv="Content-Type" content="text/html; charset=ISO-8859-1"><title>Chapter 13. LanMan and NT Password Encryption</title><link rel="stylesheet" href="samba.css" type="text/css"><meta name="generator" content="DocBook XSL Stylesheets V1.68.1"><link rel="start" href="index.html" title="SAMBA Developers Guide"><link rel="up" href="pt03.html" title="Part III. Samba Subsystems"><link rel="prev" href="wins.html" title="Chapter 12. Samba WINS Internals"><link rel="next" href="pt04.html" title="Part IV. Debugging and tracing"></head><body bgcolor="white" text="black" link="#0000FF" vlink="#840084" alink="#0000FF"><div class="navheader"><table width="100%" summary="Navigation header"><tr><th colspan="3" align="center">Chapter 13. LanMan and NT Password Encryption</th></tr><tr><td width="20%" align="left"><a accesskey="p" href="wins.html">Prev</a> </td><th width="60%" align="center">Part III. Samba Subsystems</th><td width="20%" align="right"> <a accesskey="n" href="pt04.html">Next</a></td></tr></table><hr></div><div class="chapter" lang="en"><div class="titlepage"><div><div><h2 class="title"><a name="pwencrypt"></a>Chapter 13. LanMan and NT Password Encryption</h2></div><div><div class="author"><h3 class="author"><span class="firstname">Jeremy</span> <span class="surname">Allison</span></h3><div class="affiliation"><span class="orgname">Samba Team<br></span><div class="address"><p><br>
2 <code class="email"><<a href="mailto:samba@samba.org">samba@samba.org</a>></code><br>
3 </p></div></div></div></div><div><p class="pubdate">19 Apr 1999</p></div></div></div><div class="toc"><p><b>Table of Contents</b></p><dl><dt><span class="sect1"><a href="pwencrypt.html#id2526965">Introduction</a></span></dt><dt><span class="sect1"><a href="pwencrypt.html#id2526986">How does it work?</a></span></dt><dt><span class="sect1"><a href="pwencrypt.html#id2527070">The smbpasswd file</a></span></dt></dl></div><div class="sect1" lang="en"><div class="titlepage"><div><div><h2 class="title" style="clear: both"><a name="id2526965"></a>Introduction</h2></div></div></div><p>With the development of LanManager and Windows NT
4 compatible password encryption for Samba, it is now able
5 to validate user connections in exactly the same way as
6 a LanManager or Windows NT server.</p><p>This document describes how the SMB password encryption
7 algorithm works and what issues there are in choosing whether
8 you want to use it. You should read it carefully, especially
9 the part about security and the "PROS and CONS" section.</p></div><div class="sect1" lang="en"><div class="titlepage"><div><div><h2 class="title" style="clear: both"><a name="id2526986"></a>How does it work?</h2></div></div></div><p>LanManager encryption is somewhat similar to UNIX
10 password encryption. The server uses a file containing a
11 hashed value of a user's password. This is created by taking
12 the user's plaintext password, capitalising it, and either
13 truncating to 14 bytes or padding to 14 bytes with null bytes.
14 This 14 byte value is used as two 56 bit DES keys to encrypt
15 a 'magic' eight byte value, forming a 16 byte value which is
16 stored by the server and client. Let this value be known as
17 the "hashed password".</p><p>Windows NT encryption is a higher quality mechanism,
18 consisting of doing an MD4 hash on a Unicode version of the user's
19 password. This also produces a 16 byte hash value that is
20 non-reversible.</p><p>When a client (LanManager, Windows for WorkGroups, Windows
21 95 or Windows NT) wishes to mount a Samba drive (or use a Samba
22 resource), it first requests a connection and negotiates the
23 protocol that the client and server will use. In the reply to this
24 request the Samba server generates and appends an 8 byte, random
25 value - this is stored in the Samba server after the reply is sent
26 and is known as the "challenge". The challenge is different for
27 every client connection.</p><p>The client then uses the hashed password (16 byte values
28 described above), appended with 5 null bytes, as three 56 bit
29 DES keys, each of which is used to encrypt the challenge 8 byte
30 value, forming a 24 byte value known as the "response".</p><p>In the SMB call SMBsessionsetupX (when user level security
31 is selected) or the call SMBtconX (when share level security is
32 selected), the 24 byte response is returned by the client to the
33 Samba server. For Windows NT protocol levels the above calculation
34 is done on both hashes of the user's password and both responses are
35 returned in the SMB call, giving two 24 byte values.</p><p>The Samba server then reproduces the above calculation, using
36 its own stored value of the 16 byte hashed password (read from the
37 <code class="filename">smbpasswd</code> file - described later) and the challenge
38 value that it kept from the negotiate protocol reply. It then checks
39 to see if the 24 byte value it calculates matches the 24 byte value
40 returned to it from the client.</p><p>If these values match exactly, then the client knew the
41 correct password (or the 16 byte hashed value - see security note
42 below) and is thus allowed access. If not, then the client did not
43 know the correct password and is denied access.</p><p>Note that the Samba server never knows or stores the cleartext
44 of the user's password - just the 16 byte hashed values derived from
45 it. Also note that the cleartext password or 16 byte hashed values
46 are never transmitted over the network - thus increasing security.</p></div><div class="sect1" lang="en"><div class="titlepage"><div><div><h2 class="title" style="clear: both"><a name="id2527070"></a>The smbpasswd file</h2></div></div></div><a name="SMBPASSWDFILEFORMAT"></a><p>In order for Samba to participate in the above protocol
47 it must be able to look up the 16 byte hashed values given a user name.
48 Unfortunately, as the UNIX password value is also a one way hash
49 function (ie. it is impossible to retrieve the cleartext of the user's
50 password given the UNIX hash of it), a separate password file
51 containing this 16 byte value must be kept. To minimise problems with
52 these two password files, getting out of sync, the UNIX <code class="filename">
53 /etc/passwd</code> and the <code class="filename">smbpasswd</code> file,
54 a utility, <span><strong class="command">mksmbpasswd.sh</strong></span>, is provided to generate
55 a smbpasswd file from a UNIX <code class="filename">/etc/passwd</code> file.
56 </p><p>To generate the smbpasswd file from your <code class="filename">/etc/passwd
57 </code> file use the following command:</p><p><code class="prompt">$ </code><strong class="userinput"><code>cat /etc/passwd | mksmbpasswd.sh
58 > /usr/local/samba/private/smbpasswd</code></strong></p><p>If you are running on a system that uses NIS, use</p><p><code class="prompt">$ </code><strong class="userinput"><code>ypcat passwd | mksmbpasswd.sh
59 > /usr/local/samba/private/smbpasswd</code></strong></p><p>The <span><strong class="command">mksmbpasswd.sh</strong></span> program is found in
60 the Samba source directory. By default, the smbpasswd file is
61 stored in :</p><p><code class="filename">/usr/local/samba/private/smbpasswd</code></p><p>The owner of the <code class="filename">/usr/local/samba/private/</code>
62 directory should be set to root, and the permissions on it should
63 be set to 0500 (<span><strong class="command">chmod 500 /usr/local/samba/private</strong></span>).
64 </p><p>Likewise, the smbpasswd file inside the private directory should
65 be owned by root and the permissions on is should be set to 0600
66 (<span><strong class="command">chmod 600 smbpasswd</strong></span>).</p><p>The format of the smbpasswd file is (The line has been
67 wrapped here. It should appear as one entry per line in
68 your smbpasswd file.)</p><pre class="programlisting">
69 username:uid:XXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXX:XXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXX:
70 [Account type]:LCT-<last-change-time>:Long name
71 </pre><p>Although only the <em class="replaceable"><code>username</code></em>,
72 <em class="replaceable"><code>uid</code></em>, <em class="replaceable"><code>
73 XXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXX</code></em>,
74 [<em class="replaceable"><code>Account type</code></em>] and <em class="replaceable"><code>
75 last-change-time</code></em> sections are significant
76 and are looked at in the Samba code.</p><p>It is <span class="emphasis"><em>VITALLY</em></span> important that there by 32
77 'X' characters between the two ':' characters in the XXX sections -
78 the smbpasswd and Samba code will fail to validate any entries that
79 do not have 32 characters between ':' characters. The first XXX
80 section is for the Lanman password hash, the second is for the
81 Windows NT version.</p><p>When the password file is created all users have password entries
82 consisting of 32 'X' characters. By default this disallows any access
83 as this user. When a user has a password set, the 'X' characters change
84 to 32 ascii hexadecimal digits (0-9, A-F). These are an ascii
85 representation of the 16 byte hashed value of a user's password.</p><p>To set a user to have no password (not recommended), edit the file
86 using vi, and replace the first 11 characters with the ascii text
87 <code class="constant">"NO PASSWORD"</code> (minus the quotes).</p><p>For example, to clear the password for user bob, his smbpasswd file
88 entry would look like :</p><pre class="programlisting">
89 bob:100:NO PASSWORDXXXXXXXXXXXXXXXXXXXXX:XXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXX:
90 [U ]:LCT-00000000:Bob's full name:/bobhome:/bobshell
91 </pre><p>If you are allowing users to use the smbpasswd command to set
92 their own passwords, you may want to give users NO PASSWORD initially
93 so they do not have to enter a previous password when changing to their
94 new password (not recommended). In order for you to allow this the
95 <span><strong class="command">smbpasswd</strong></span> program must be able to connect to the
96 <span><strong class="command">smbd</strong></span> daemon as that user with no password. Enable this
97 by adding the line :</p><p><span><strong class="command">null passwords = yes</strong></span></p><p>to the [global] section of the smb.conf file (this is why
98 the above scenario is not recommended). Preferably, allocate your
99 users a default password to begin with, so you do not have
100 to enable this on your server.</p><p><span class="emphasis"><em>Note : </em></span>This file should be protected very
101 carefully. Anyone with access to this file can (with enough knowledge of
102 the protocols) gain access to your SMB server. The file is thus more
103 sensitive than a normal unix <code class="filename">/etc/passwd</code> file.</p></div></div><div class="navfooter"><hr><table width="100%" summary="Navigation footer"><tr><td width="40%" align="left"><a accesskey="p" href="wins.html">Prev</a> </td><td width="20%" align="center"><a accesskey="u" href="pt03.html">Up</a></td><td width="40%" align="right"> <a accesskey="n" href="pt04.html">Next</a></td></tr><tr><td width="40%" align="left" valign="top">Chapter 12. Samba WINS Internals </td><td width="20%" align="center"><a accesskey="h" href="index.html">Home</a></td><td width="40%" align="right" valign="top"> Part IV. Debugging and tracing</td></tr></table></div></body></html>
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