14.5. One-time Passwords

S/Key is a one-time password scheme based on a one-way hash function. FreeBSD uses the MD4 hash for compatibility but other systems have used MD5 and DES-MAC. S/Key has been part of the FreeBSD base system since version 1.1.5 and is also used on a growing number of other operating systems. S/Key is a registered trademark of Bell Communications Research, Inc.

From version 5.0 of FreeBSD, S/Key has been replaced with the functionally equivalent OPIE (One-time Passwords In Everything). OPIE uses the MD5 hash by default.

There are three different sorts of passwords which we will discuss below. The first is your usual UNIX® style or Kerberos password; we will call this a UNIX® password. The second sort is the one-time password which is generated by the S/Key key program or the OPIE opiekey(1) program and accepted by the keyinit or opiepasswd(1) programs and the login prompt; we will call this a one-time password. The final sort of password is the secret password which you give to the key/opiekey programs (and sometimes the keyinit/opiepasswd programs) which it uses to generate one-time passwords; we will call it a secret password or just unqualified password.

The secret password does not have anything to do with your UNIX® password; they can be the same but this is not recommended. S/Key and OPIE secret passwords are not limited to 8 characters like old UNIX® passwords[11], they can be as long as you like. Passwords of six or seven word long phrases are fairly common. For the most part, the S/Key or OPIE system operates completely independently of the UNIX® password system.

Besides the password, there are two other pieces of data that are important to S/Key and OPIE. One is what is known as the seed or key, consisting of two letters and five digits. The other is what is called the iteration count, a number between 1 and 100. S/Key creates the one-time password by concatenating the seed and the secret password, then applying the MD4/MD5 hash as many times as specified by the iteration count and turning the result into six short English words. These six English words are your one-time password. The authentication system (primarily PAM) keeps track of the last one-time password used, and the user is authenticated if the hash of the user-provided password is equal to the previous password. Because a one-way hash is used it is impossible to generate future one-time passwords if a successfully used password is captured; the iteration count is decremented after each successful login to keep the user and the login program in sync. When the iteration count gets down to 1, S/Key and OPIE must be reinitialized.

There are three programs involved in each system which we will discuss below. The key and opiekey programs accept an iteration count, a seed, and a secret password, and generate a one-time password or a consecutive list of one-time passwords. The keyinit and opiepasswd programs are used to initialize S/Key and OPIE respectively, and to change passwords, iteration counts, or seeds; they take either a secret passphrase, or an iteration count, seed, and one-time password. The keyinfo and opieinfo programs examine the relevant credentials files (/etc/skeykeys or /etc/opiekeys) and print out the invoking user's current iteration count and seed.

There are four different sorts of operations we will cover. The first is using keyinit or opiepasswd over a secure connection to set up one-time-passwords for the first time, or to change your password or seed. The second operation is using keyinit or opiepasswd over an insecure connection, in conjunction with key or opiekey over a secure connection, to do the same. The third is using key/opiekey to log in over an insecure connection. The fourth is using key or opiekey to generate a number of keys which can be written down or printed out to carry with you when going to some location without secure connections to anywhere.

14.5.1. Secure Connection Initialization

To initialize S/Key for the first time, change your password, or change your seed while logged in over a secure connection (e.g. on the console of a machine or via ssh), use the keyinit command without any parameters while logged in as yourself:

% keyinit
Adding unfurl:
Reminder - Only use this method if you are directly connected.
If you are using telnet or rlogin exit with no password and use keyinit -s.
Enter secret password:
Again secret password:

ID unfurl s/key is 99 to17757
DEFY CLUB PRO NASH LACE SOFT

For OPIE, opiepasswd is used instead:

% opiepasswd -c
[grimreaper] ~ $ opiepasswd -f -c
Adding unfurl:
Only use this method from the console; NEVER from remote. If you are using
telnet, xterm, or a dial-in, type ^C now or exit with no password.
Then run opiepasswd without the -c parameter.
Using MD5 to compute responses.
Enter new secret pass phrase:
Again new secret pass phrase:
ID unfurl OTP key is 499 to4268
MOS MALL GOAT ARM AVID COED

At the Enter new secret pass phrase: or Enter secret password: prompts, you should enter a password or phrase. Remember, this is not the password that you will use to login with, this is used to generate your one-time login keys. The ID line gives the parameters of your particular instance: your login name, the iteration count, and seed. When logging in the system will remember these parameters and present them back to you so you do not have to remember them. The last line gives the particular one-time password which corresponds to those parameters and your secret password; if you were to re-login immediately, this one-time password is the one you would use.

14.5.2. Insecure Connection Initialization

To initialize or change your secret password over an insecure connection, you will need to already have a secure connection to some place where you can run key or opiekey; this might be in the form of a desk accessory on a Macintosh®, or a shell prompt on a machine you trust. You will also need to make up an iteration count (100 is probably a good value), and you may make up your own seed or use a randomly-generated one. Over on the insecure connection (to the machine you are initializing), use the keyinit -s command:

% keyinit -s
Updating unfurl:
Old key: to17758
Reminder you need the 6 English words from the key command.
Enter sequence count from 1 to 9999: 100
Enter new key [default to17759]:
s/key 100 to 17759
s/key access password:
s/key access password:CURE MIKE BANE HIM RACY GORE

For OPIE, you need to use opiepasswd:

% opiepasswd

Updating unfurl:
You need the response from an OTP generator.
Old secret pass phrase:
        otp-md5 498 to4268 ext
        Response: GAME GAG WELT OUT DOWN CHAT
New secret pass phrase:
        otp-md5 499 to4269
        Response: LINE PAP MILK NELL BUOY TROY

ID mark OTP key is 499 gr4269
LINE PAP MILK NELL BUOY TROY

To accept the default seed (which the keyinit program confusingly calls a key), press Return. Then before entering an access password, move over to your secure connection or S/Key desk accessory, and give it the same parameters:

% key 100 to17759
Reminder - Do not use this program while logged in via telnet or rlogin.
Enter secret password: <secret password>
CURE MIKE BANE HIM RACY GORE

Or for OPIE:

% opiekey 498 to4268
Using the MD5 algorithm to compute response.
Reminder: Don't use opiekey from telnet or dial-in sessions.
Enter secret pass phrase:
GAME GAG WELT OUT DOWN CHAT

Now switch back over to the insecure connection, and copy the one-time password generated over to the relevant program.

14.5.3. Generating a Single One-time Password

Once you have initialized S/Key or OPIE, when you login you will be presented with a prompt like this:

% telnet example.com
Trying 10.0.0.1...
Connected to example.com
Escape character is '^]'.

FreeBSD/i386 (example.com) (ttypa)

login: <username>
s/key 97 fw13894
Password: 

Or for OPIE:

% telnet example.com
Trying 10.0.0.1...
Connected to example.com
Escape character is '^]'.

FreeBSD/i386 (example.com) (ttypa)

login: <username>
otp-md5 498 gr4269 ext
Password: 

As a side note, the S/Key and OPIE prompts have a useful feature (not shown here): if you press Return at the password prompt, the prompter will turn echo on, so you can see what you are typing. This can be extremely useful if you are attempting to type in a password by hand, such as from a printout.

At this point you need to generate your one-time password to answer this login prompt. This must be done on a trusted system that you can run key or opiekey on. (There are versions of these for DOS, Windows® and Mac OS® as well.) They need both the iteration count and the seed as command line options. You can cut-and-paste these right from the login prompt on the machine that you are logging in to.

On the trusted system:

% key 97 fw13894
Reminder - Do not use this program while logged in via telnet or rlogin.
Enter secret password:
WELD LIP ACTS ENDS ME HAAG

For OPIE:

% opiekey 498 to4268
Using the MD5 algorithm to compute response.
Reminder: Don't use opiekey from telnet or dial-in sessions.
Enter secret pass phrase:
GAME GAG WELT OUT DOWN CHAT

Now that you have your one-time password you can continue logging in:

login: <username>
s/key 97 fw13894
Password: <return to enable echo>
s/key 97 fw13894
Password [echo on]: WELD LIP ACTS ENDS ME HAAG
Last login: Tue Mar 21 11:56:41 from 10.0.0.2 ... 

14.5.4. Generating Multiple One-time Passwords

Sometimes you have to go places where you do not have access to a trusted machine or secure connection. In this case, it is possible to use the key and opiekey commands to generate a number of one-time passwords beforehand to be printed out and taken with you. For example:

% key -n 5 30 zz99999
Reminder - Do not use this program while logged in via telnet or rlogin.
Enter secret password: <secret password>
26: SODA RUDE LEA LIND BUDD SILT
27: JILT SPY DUTY GLOW COWL ROT
28: THEM OW COLA RUNT BONG SCOT
29: COT MASH BARR BRIM NAN FLAG
30: CAN KNEE CAST NAME FOLK BILK

Or for OPIE:

% opiekey -n 5 30 zz99999
Using the MD5 algorithm to compute response.
Reminder: Don't use opiekey from telnet or dial-in sessions.
Enter secret pass phrase: <secret password>
26: JOAN BORE FOSS DES NAY QUIT
27: LATE BIAS SLAY FOLK MUCH TRIG
28: SALT TIN ANTI LOON NEAL USE
29: RIO ODIN GO BYE FURY TIC
30: GREW JIVE SAN GIRD BOIL PHI

The -n 5 requests five keys in sequence, the 30 specifies what the last iteration number should be. Note that these are printed out in reverse order of eventual use. If you are really paranoid, you might want to write the results down by hand; otherwise you can cut-and-paste into lpr. Note that each line shows both the iteration count and the one-time password; you may still find it handy to scratch off passwords as you use them.

14.5.5. Restricting Use of UNIX® Passwords

S/Key can place restrictions on the use of UNIX® passwords based on the host name, user name, terminal port, or IP address of a login session. These restrictions can be found in the configuration file /etc/skey.access. The skey.access(5) manual page has more information on the complete format of the file and also details some security cautions to be aware of before depending on this file for security.

If there is no /etc/skey.access file (this is the default on FreeBSD 4.X systems), then all users will be allowed to use UNIX® passwords. If the file exists, however, then all users will be required to use S/Key unless explicitly permitted to do otherwise by configuration statements in the skey.access file. In all cases, UNIX® passwords are permitted on the console.

Here is a sample skey.access configuration file which illustrates the three most common sorts of configuration statements:

permit internet 192.168.0.0 255.255.0.0
permit user fnord
permit port ttyd0

The first line (permit internet) allows users whose IP source address (which is vulnerable to spoofing) matches the specified value and mask, to use UNIX® passwords. This should not be considered a security mechanism, but rather, a means to remind authorized users that they are using an insecure network and need to use S/Key for authentication.

The second line (permit user) allows the specified username, in this case fnord, to use UNIX® passwords at any time. Generally speaking, this should only be used for people who are either unable to use the key program, like those with dumb terminals, or those who are ineducable.

The third line (permit port) allows all users logging in on the specified terminal line to use UNIX® passwords; this would be used for dial-ups.

OPIE can restrict the use of UNIX® passwords based on the IP address of a login session just like S/Key does. The relevant file is /etc/opieaccess, which is present by default on FreeBSD 5.0 and newer systems. Please check opieaccess(5) for more information on this file and which security considerations you should be aware of when using it.

Here is a sample opieaccess file:

permit 192.168.0.0 255.255.0.0

This line allows users whose IP source address (which is vulnerable to spoofing) matches the specified value and mask, to use UNIX® passwords at any time.

If no rules in opieaccess are matched, the default is to deny non-OPIE logins.



[11] Under FreeBSD the standard login password may be up to 128 characters in length.

All FreeBSD documents are available for download at http://ftp.FreeBSD.org/pub/FreeBSD/doc/

Questions that are not answered by the documentation may be sent to <freebsd-questions@FreeBSD.org>.
Send questions about this document to <freebsd-doc@FreeBSD.org>.