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diff --git a/docs/devel/encryption.xml b/docs/devel/encryption.xml new file mode 100644 index 0000000000..56a1d10c9c --- /dev/null +++ b/docs/devel/encryption.xml @@ -0,0 +1,197 @@ +<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> |