By default, FreeBSD includes support for OPIE (One-time Passwords In Everything),
which 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 OPIE opiekey(1) program and
accepted by the opiepasswd(1) program
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 opiekey
program (and sometimes the 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.
OPIE secret passwords are not limited to 8 characters like old UNIX passwords, they can be as long as you
like. Passwords of six or seven word long phrases are fairly common. For the most part,
the OPIE system operates completely independently of the UNIX password system.
Besides the password, there are two other pieces of data that are important to 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. OPIE creates the one-time password by concatenating the seed
and the secret password, then applying the 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, OPIE must be
reinitialized.
There are a few programs involved in each system which we will discuss below. The opiekey program accepts an iteration count, a seed, and a secret
password, and generates a one-time password or a consecutive list of one-time passwords.
The opiepasswd program is used to initialize OPIE, and to change
passwords, iteration counts, or seeds; it takes either a secret passphrase, or an
iteration count, seed, and a one-time password. The opieinfo
program will examine the relevant credentials files (/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 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 opiepasswd over an insecure connection, in conjunction with opiekey over a secure connection, to do the same. The third is using
opiekey to log in over an insecure connection. The fourth is
using 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.
To initialize OPIE for the first time, execute the opiepasswd
command:
% 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.
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 opiekey; this might be in the form of 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 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 press Return. Then before entering an
access password, move over to your secure connection and give it the same parameters:
% 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.
Once you have initialized OPIE and 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>
otp-md5 498 gr4269 ext
Password:
As a side note, the 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 opiekey
on. (There are versions of these for DOS, Windows® and
Mac OS® as well.) They need 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:
% 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.
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 opiekey command to generate a number of one-time passwords
beforehand to be printed out and taken with you. For example:
% 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.
OPIE can restrict the use of UNIX passwords based on
the IP address of a login session. The relevant file is /etc/opieaccess, which is present by default. 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.
