1 <chapter id="pwencrypt">
6 <firstname>Jeremy</firstname><surname>Allison</surname>
8 <orgname>Samba Team</orgname>
10 <email>samba@samba.org</email>
15 <pubdate>19 Apr 1999</pubdate>
18 <title>LanMan and NT Password Encryption</title>
21 <title>Introduction</title>
23 <para>With the development of LanManager and Windows NT
24 compatible password encryption for Samba, it is now able
25 to validate user connections in exactly the same way as
26 a LanManager or Windows NT server.</para>
28 <para>This document describes how the SMB password encryption
29 algorithm works and what issues there are in choosing whether
30 you want to use it. You should read it carefully, especially
31 the part about security and the "PROS and CONS" section.</para>
36 <title>How does it work?</title>
38 <para>LanManager encryption is somewhat similar to UNIX
39 password encryption. The server uses a file containing a
40 hashed value of a user's password. This is created by taking
41 the user's plaintext password, capitalising it, and either
42 truncating to 14 bytes or padding to 14 bytes with null bytes.
43 This 14 byte value is used as two 56 bit DES keys to encrypt
44 a 'magic' eight byte value, forming a 16 byte value which is
45 stored by the server and client. Let this value be known as
46 the "hashed password".</para>
48 <para>Windows NT encryption is a higher quality mechanism,
49 consisting of doing an MD4 hash on a Unicode version of the user's
50 password. This also produces a 16 byte hash value that is
51 non-reversible.</para>
53 <para>When a client (LanManager, Windows for WorkGroups, Windows
54 95 or Windows NT) wishes to mount a Samba drive (or use a Samba
55 resource), it first requests a connection and negotiates the
56 protocol that the client and server will use. In the reply to this
57 request the Samba server generates and appends an 8 byte, random
58 value - this is stored in the Samba server after the reply is sent
59 and is known as the "challenge". The challenge is different for
60 every client connection.</para>
62 <para>The client then uses the hashed password (16 byte values
63 described above), appended with 5 null bytes, as three 56 bit
64 DES keys, each of which is used to encrypt the challenge 8 byte
65 value, forming a 24 byte value known as the "response".</para>
67 <para>In the SMB call SMBsessionsetupX (when user level security
68 is selected) or the call SMBtconX (when share level security is
69 selected), the 24 byte response is returned by the client to the
70 Samba server. For Windows NT protocol levels the above calculation
71 is done on both hashes of the user's password and both responses are
72 returned in the SMB call, giving two 24 byte values.</para>
74 <para>The Samba server then reproduces the above calculation, using
75 its own stored value of the 16 byte hashed password (read from the
76 <filename>smbpasswd</filename> file - described later) and the challenge
77 value that it kept from the negotiate protocol reply. It then checks
78 to see if the 24 byte value it calculates matches the 24 byte value
79 returned to it from the client.</para>
81 <para>If these values match exactly, then the client knew the
82 correct password (or the 16 byte hashed value - see security note
83 below) and is thus allowed access. If not, then the client did not
84 know the correct password and is denied access.</para>
86 <para>Note that the Samba server never knows or stores the cleartext
87 of the user's password - just the 16 byte hashed values derived from
88 it. Also note that the cleartext password or 16 byte hashed values
89 are never transmitted over the network - thus increasing security.</para>
93 <title>The smbpasswd file</title>
94 <anchor id="SMBPASSWDFILEFORMAT"/>
95 <para>In order for Samba to participate in the above protocol
96 it must be able to look up the 16 byte hashed values given a user name.
97 Unfortunately, as the UNIX password value is also a one way hash
98 function (ie. it is impossible to retrieve the cleartext of the user's
99 password given the UNIX hash of it), a separate password file
100 containing this 16 byte value must be kept. To minimise problems with
101 these two password files, getting out of sync, the UNIX <filename>
102 /etc/passwd</filename> and the <filename>smbpasswd</filename> file,
103 a utility, <command>mksmbpasswd.sh</command>, is provided to generate
104 a smbpasswd file from a UNIX <filename>/etc/passwd</filename> file.
108 <para>To generate the smbpasswd file from your <filename>/etc/passwd
109 </filename> file use the following command:</para>
111 <para><prompt>$ </prompt><userinput>cat /etc/passwd | mksmbpasswd.sh
112 > /usr/local/samba/private/smbpasswd</userinput></para>
114 <para>If you are running on a system that uses NIS, use</para>
116 <para><prompt>$ </prompt><userinput>ypcat passwd | mksmbpasswd.sh
117 > /usr/local/samba/private/smbpasswd</userinput></para>
119 <para>The <command>mksmbpasswd.sh</command> program is found in
120 the Samba source directory. By default, the smbpasswd file is
123 <para><filename>/usr/local/samba/private/smbpasswd</filename></para>
125 <para>The owner of the <filename>/usr/local/samba/private/</filename>
126 directory should be set to root, and the permissions on it should
127 be set to 0500 (<command>chmod 500 /usr/local/samba/private</command>).
130 <para>Likewise, the smbpasswd file inside the private directory should
131 be owned by root and the permissions on is should be set to 0600
132 (<command>chmod 600 smbpasswd</command>).</para>
135 <para>The format of the smbpasswd file is (The line has been
136 wrapped here. It should appear as one entry per line in
137 your smbpasswd file.)</para>
139 <para><programlisting>
140 username:uid:XXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXX:XXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXX:
141 [Account type]:LCT-<last-change-time>:Long name
142 </programlisting></para>
144 <para>Although only the <replaceable>username</replaceable>,
145 <replaceable>uid</replaceable>, <replaceable>
146 XXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXX</replaceable>,
147 [<replaceable>Account type</replaceable>] and <replaceable>
148 last-change-time</replaceable> sections are significant
149 and are looked at in the Samba code.</para>
151 <para>It is <emphasis>VITALLY</emphasis> important that there by 32
152 'X' characters between the two ':' characters in the XXX sections -
153 the smbpasswd and Samba code will fail to validate any entries that
154 do not have 32 characters between ':' characters. The first XXX
155 section is for the Lanman password hash, the second is for the
156 Windows NT version.</para>
158 <para>When the password file is created all users have password entries
159 consisting of 32 'X' characters. By default this disallows any access
160 as this user. When a user has a password set, the 'X' characters change
161 to 32 ascii hexadecimal digits (0-9, A-F). These are an ascii
162 representation of the 16 byte hashed value of a user's password.</para>
164 <para>To set a user to have no password (not recommended), edit the file
165 using vi, and replace the first 11 characters with the ascii text
166 <constant>"NO PASSWORD"</constant> (minus the quotes).</para>
168 <para>For example, to clear the password for user bob, his smbpasswd file
169 entry would look like :</para>
171 <para><programlisting>
172 bob:100:NO PASSWORDXXXXXXXXXXXXXXXXXXXXX:XXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXX:
173 [U ]:LCT-00000000:Bob's full name:/bobhome:/bobshell
174 </programlisting></para>
176 <para>If you are allowing users to use the smbpasswd command to set
177 their own passwords, you may want to give users NO PASSWORD initially
178 so they do not have to enter a previous password when changing to their
179 new password (not recommended). In order for you to allow this the
180 <command>smbpasswd</command> program must be able to connect to the
181 <command>smbd</command> daemon as that user with no password. Enable this
182 by adding the line :</para>
184 <para><command>null passwords = yes</command></para>
186 <para>to the [global] section of the smb.conf file (this is why
187 the above scenario is not recommended). Preferably, allocate your
188 users a default password to begin with, so you do not have
189 to enable this on your server.</para>
191 <para><emphasis>Note : </emphasis>This file should be protected very
192 carefully. Anyone with access to this file can (with enough knowledge of
193 the protocols) gain access to your SMB server. The file is thus more
194 sensitive than a normal unix <filename>/etc/passwd</filename> file.</para>