Contributor: Jeremy Allison Updated: June 27, 1997 Note: Please refer to WinNT.txt also Subject: LanManager / Samba Password Encryption. ============================================================================ 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. 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. How does it work ? ------------------ LanManager encryption is somewhat similar to UNIX password encryption. The server uses a file containing a hashed value of a users password. This is created by taking the users paintext 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*. Windows NT encryption is a higher quality mechanism, consisting of doing an MD4 hash on a Unicode version of the users password. This also produces a 16 byte hash value that is non-reversible. 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. 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*. 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 users password and both responses are returned in the SMB call, giving two 24 byte values. The Samba server then reproduces the above calculation, using it's own stored value of the 16 byte hashed password (read from the smbpasswd 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. If these values match exactly, then the client knew the correct password (or the 16 byte hashed value - see security note below) and is this allowed access. If not then the client did not know the correct password and is denied access. Note that the Samba server never knows or stores the cleartext of the users 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. IMPORTANT NOTE ABOUT SECURITY ----------------------------- The unix and SMB password encryption techniques seem similar on the surface. This similarity is, however, only skin deep. The unix scheme typically sends clear text passwords over the nextwork when logging in. This is bad. The SMB encryption scheme never sends the cleartext password over the network but it does store the 16 byte hashed values on disk. This is also bad. Why? Because the 16 byte hashed values are a "password equivalent". You cannot derive the users password from them, but they could potentially be used in a modified client to gain access to a server. This would require considerable technical knowledge on behalf of the attacker but is perfectly possible. You should thus treat the smbpasswd file as though it contained the cleartext passwords of all your users. Its contents must be kept secret, and the file should be protected accordingly. Ideally we would like a password scheme which neither requires plain text passwords on the net or on disk. Unfortunately this is not available as Samba is stuck with being compatible with other SMB systems (WinNT, WfWg, Win95 etc). PROS AND CONS ------------- There are advantages and disadvantages to both schemes. Advantages of SMB Encryption: ----------------------------- - plain text passwords are not passed across the network. Someone using a network sniffer cannot just record passwords going to the SMB server. - WinNT doesn't like talking to a server that isn't using SMB encrypted passwords. It will refuse to browse the server if the server is also in user level security mode. It will insist on promting the user for the password on each connection, which is very annoying. The only things you can do to stop this is to use SMB encryption. Advantages of non-encrypted passwords: -------------------------------------- - plain text passwords are not kept on disk. - uses same password file as other unix services such as login and ftp - you are probably already using other services (such as telnet and ftp) which send plain text passwords over the net, so not sending them for SMB isn't such a big deal. Note that Windows NT 4.0 Service pack 3 changed the default for permissible authentication so that plaintext passwords are *never* sent over the wire. The solution to this is either to switch to encrypted passwords with Samba or edit the Windows NT registry to re-enable plaintext passwords. See the document WinNT.txt for details on how to do this. The smbpasswd file. ------------------- 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 users password given the UNIX hash of it) then 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 /etc/passwd and the smbpasswd file, a utility, mksmbpasswd.sh, is provided to generate a smbpasswd file from a UNIX /etc/passwd file. To generate the smbpasswd file from your /etc/passwd file use the following command :- cat /etc/passwd | mksmbpasswd.sh >/usr/local/samba/private/smbpasswd If you are running on a system that uses NIS, use ypcat passwd | mksmbpasswd.sh >/usr/local/samba/private/smbpasswd The mksmbpasswd.sh program is found in the Samba source directory. By default, the smbpasswd file is stored in :- /usr/local/samba/private/smbpasswd The owner of the /usr/local/samba/private directory should be set to root, and the permissions on it should be set to :- r-x------ The command chmod 500 /usr/local/samba/private will do the trick. Likewise, the smbpasswd file inside the private directory should be owned by root and the permissions on is should be set to rw------- by the command :- chmod 600 smbpasswd. The format of the smbpasswd file is username:uid:XXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXX:XXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXX:Long name:user home dir:user shell Although only the username, uid, and XXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXX sections are significant and are looked at in the Samba code. It is *VITALLY* 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. 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 users password. To set a user to have no password (not recommended), edit the file using vi, and replace the first 11 characters with the asci text NO PASSWORD Eg. To clear the password for user bob, his smbpasswd file entry would look like : bob:100:NO PASSWORDXXXXXXXXXXXXXXXXXXXXX:XXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXX:Bob's full name:/bobhome:/bobshell 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). Note : 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 /etc/passwd file. The smbpasswd Command. ---------------------- The smbpasswd command maintains the two 32 byte password fields in the smbpasswd file. If you wish to make it similar to the unix passwd or yppasswd programs, install it in /usr/local/samba/bin (or your main Samba binary directory) and make it setuid root. Note that if you do not do this then the root user will have to set all users passwords. To set up smbpasswd as setuid root, change to the Samba binary install directory and then type (as root) : chown root smbpasswd chmod 4555 smbpasswd If smbpasswd is installed as setuid root then you would use it as follows. smbpasswd Old SMB password: New SMB Password: < type new value > Repeat New SMB Password: < re-type new value > If the old value does not match the current value stored for that user, or the two new values do not match each other, then the password will not be changed. If invoked by an ordinary user it will only allow the user to change his or her own Samba password. If run by the root user smbpasswd may take an optional argument, specifying the user name whose SMB password you wish to change. Note that when run as root smbpasswd does not prompt for or check the old password value, thus allowing root to set passwords for users who have forgotten their passwords. smbpasswd is designed to work in the same way and be familiar to UNIX users who use the passwd or yppasswd commands. NOTE. As smbpasswd is designed to be installed as setuid root I would appreciate it if everyone examined the source code to look for potential security flaws. A setuid program, if not written properly can be an open door to a system cracker. Please help make this program secure by reporting all problems to me (the author, Jeremy Allison). My email address is :- jallison@whistle.com Setting up Samba to support LanManager Encryption. -------------------------------------------------- This is a very brief description on how to setup samba to support password encryption. More complete instructions will probably be added later. 1) compile and install samba as usual 2) if your system can't compile the module getsmbpass.c then remove the -DSMBGETPASS define from the Makefile. 3) enable encrypted passwords in smb.conf by adding the line "encrypt passwords = yes" in the [global] section 4) create the initial smbpasswd password file in the place you specified in the Makefile. A simple way to do this based on your existing Makefile (assuming it is in a reasonably standard format) is like this: cat /etc/passwd | mksmbpasswd.sh > /usr/local/samba/private/smbpasswd Change ownership of private and smbpasswd to root. chown -R root /usr/local/samba/private Set the correct permissions on /usr/local/samba/private chmod 500 /usr/local/samba/private Set the correct permissions on /usr/local/samba/private/smbpasswd chmod 600 /usr/local/samba/private/smbpasswd note that the mksmbpasswd.sh script is in the samba source directory. If this fails then you will find that you will need entries that look like this: # SMB password file. tridge:148:XXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXX:XXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXX:Andrew Tridgell:/home/tridge:/bin/tcsh note that the uid and username fields must be right. Also, you must get the number of X's right (there should be 32). If you wish, install the smbpasswd program as suid root. chown root /usr/local/samba/bin/smbpasswd chmod 4555 /usr/local/samba/bin/smbpasswd 5) set the passwords for users using the smbpasswd command. For example, as root you could do "smbpasswd tridge" 6) try it out! Note that you can test things using smbclient, as it also now supports encryption. ============================================================================== Footnote: Please refer to WinNT.txt also