samba-mirror/docs-xml/Samba-Developers-Guide/encryption.xml

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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">
<chapter id="pwencrypt">


<chapterinfo>
	<author>
		<firstname>Jeremy</firstname><surname>Allison</surname>
		<affiliation>
			<orgname>Samba Team</orgname>
			<address>
				<email>samba@lists.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
	&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>