OPIE is a package derived from the Bellcore S/Key Version 1 distribution
that helps to secure a system against replay attacks (see below). It does so
using a secure hash function and a challenge/response system. It provides
replacements for the
login(1) ,su(1) , and
ftpd(8) programs that use OPIE
authentication as well as demonstrate how a program might be adapted to use
OPIE authentication. OPIE was developed at and for the United States Naval
Research Laboratory (NRL). OPIE is derived in part from Berkeley Standard
Distribution UNIX and the Bellcore S/Key Version 1 distribution.
From the average user's perspective, OPIE is a nuisance that prevents their
account from being broken into. The first time a user wishes to use OPIE,
(s)he needs to use the
opiepasswd(1) command to put an entry for them into
the OPIE database. The user can then use OPIE to authenticate themselves
with any program that supports it. If no other clients are being used,
this means they can use OPIE to
telnet, rlogin, or
ftp into the system,
log in on a terminal port (like a modem), or switch to another user's
account. When they would normally be asked for a password, they will get
a challenge from the server. They then need to copy that challenge (or
re-type, if they don't have the ability to copy and paste through something
like a window system) to their calculator program, enter their password,
then copy (or re-type) the response from the calculator as their password.
While this will seem cumbersome at first, with some practice, it becomes
easy.
TERMS
user name
The name that the system knows you as. For example, "jdoe".
secret password
A password, usually selected by the user, that is needed to gain access to the
system. For example, "SEc1_rt".
challenge
A packet of information output by a system when it wishes to authenticate a
user. In OPIE, this is a three-item group consisting of a hash identifier,
a sequence number, and a seed. This
information is needed by the OPIE calculator to generate a proper response.
For example, "otp-md5 95 wi14321".
response
A packet of information generated from a challenge that is used by a system to
authenticate a user. In OPIE, this is a group of six words that is generated by
the calculator given the challenge and the secret password. For example,
"PUP SOFT ROSE BIAS FLAG END".
seed
A piece of information that is used in conjunction with the secret password
and sequence numer to compute the response. Its purpose is to allow the same
secret password to be used for multiple sequences, by changing the seed, or
for authentication to multiple machines by using different seeds.
sequence number
A counter used to keep track of key iterations. In OPIE, each time a successful
response is received by the system, the sequence number is decremented. For
example, "95".
hash identifier
A piece of text that identifies the actual algorithm that needs to be used to
generate a proper response. In OPIE, the only two valid hash identifiers are
"otp-md4", which selects MD4 hashing, and "otp-md5", which selects MD5.
REPLAY ATTACKS
When you use a network terminal program like
telnet(1) or even use a modem to log into a
computer system, you need a user name and a secret password. Anyone who can
provide those to the system is recognized as you because, in theory, only you
would have your secret password. Unfortunately, it is now easy to listen in
on many computer communications media. From modem communication to many
networks, your password is not usually safe over remote links. If a
cracker can listen in when you send your password, (s)he then has a copy
of your password that can be used at any time in the future to access your
account. On more than one occasion, major sites on the Internet have been
broken into exactly this way.
All an attacker has to
do is capture your password once and then replay it to the server when it's
asked for. Even if the password is communicated between machines in encoded
or encrypted form, as long as a cracker can get in by simply replaying
a previously captured communication, you are at risk. Up until very recently,
Novell NetWare was vulnerable this way. A cracker couldn't find out what your
password actually is, but (s)he didn't need to -- all that was necessary to
get into your account was to capture the encrypted password and send that
back to the server when asked for it.
ONE-TIME PASSWORDS
One solution to the problem of replay attacks
is to keep changing the way that a password is being encoded so that what is
sent over the link to another system can only be used once. If you can do that,
then a cracker can replay it as many times as (s)he wants -- it's just not
going to get them anywhere. It's important, however, to make sure you encode
the password in such a way that the cracker can't use the encoded version to
figure out what the password is or what a future encoded password will be.
Otherwise, while still an improvement over no encoding or a fixed encoding,
you can still be broken into.
THE S/KEY ALGORITHM
A solution to this whole problem was invented by Lamport in 1981. This
technique was implemented by Haller, Karn, and Walden at Bellcore. They
They created a free software package called "S/Key" that used an algorithm
called a cryptographic checksum. A cryptographic checksum is a strong one-way
function such that, knowing the result of such a function, an attacker still
cannot feasably determine the input. Further, unlike cyclic redundancy
checksums (CRCs), cryptographic checksums have few inputs that result in the
same output.
In S/Key, what changes is the number of
times the password is run through the secure hash. The password is run through
the secure hash once, then the output of the hash is run through the secure
hash again, that output is run through the secure hash again, and so on until
the number of times the password has been run through the secure hash is equal
to the desired sequence number. This is much slower than just, say, putting
the sequence number in before the password and running that through the secure
hash once, but it gains you one significant benefit. The server machine you
are trying to connect to has to have some way to determine whether the output
of that whole mess is right. If it stores it either without any encoding or
with a normal encoding, a cracker could still get at your password. But if it
stores it with a secure hash, then how does it account for the response
changing every time because the sequence number is changing? Also what if you
can never get to the machine any way that can't be listened in on? How do you
change your password without sending it over the link?
The clever solution
devised by Lamport is to keep in mind that the sequence number is
always decrementing by one and that, in the S/Key system, simply by running any
response with a sequence number N through the secure hash, you can get the
response with a sequence number N+1, but you can't go the other way. At any
given time, call the sequence number of the last valid response that the
system got N+1 and the sequence number of the response you are giving it N.
If the password that generated the response for N is the same as the one for
N+1, then you should be able to run the response for N through the secure hash
one more time, for a total of N+1 times, and get the same response as you got
back for N+1. Once you compare the two and find that they are the same, you
subtract one from N so that, now, the key for N that you just verified becomes
the new key for N+1 that you can store away to use the next time you need to
verify a key. This also means that if you need to change your password but
don't have a secure way to access your machine, all the system really needs to
have to verify your password is a valid response for one more than the sequence
number you want to start with.
Just for good measure, each side of
all of this uses a seed in conjunction with your password when it actually
generates and verifies the responses. This helps to jumble things up a little
bit more, just in case. Otherwise, someone with a lot of time and disk space
on their hands could generate all the responses for a lot of frequent passwords
and defeat the system.
This is not, by any means, the best explanation of how the S/Key algorithm
works or some of the more minor details. For that, you should go to some of
the papers now published on the topic. It is simply a quick-and-dirty
introduction to what's going on under the hood.
OPIE COMPONENTS
Included in the OPIE distribution are three OPIE client programs:
opielogin(1) ,opiesu(1) , and
opieftpd(8) . These three programs are modified versions of the
freely available 4.3BSD Net/2 versions of
login(1) , su(1) , and
ftpd(8) , respectively. Although most of the modifications actually done to them are so
that they will work on as many machines as possible, they also have been
modified to support OPIE for authentication. As you will see from the source,
it is not very difficult to add support for OPIE to other programs.
There are also three programs in the OPIE distribution that are specific to
the OPIE system:
opiepasswd(1) , which allows a user to set and change their
OPIE password,
opieinfo(1) , which allows a user to find out what their current
sequence number and seed are, and
opiekey(1) , which is an OPIE key calculator.
ADDING OPIE TO OTHER PROGRAMS
Adding OPIE authentication to programs other than the ones included as clients
in the OPIE distribution isn't very difficult. First, you will need to make
sure that the program includes <stdio.h> somewhere. Then, below the other
includes such as <stdio.h>, but before variable declarations, you need to
include "opie.h". You need to add a variable of type "struct opie" to your
program, you need to make sure that the buffer that you use to get a password
from the user is big enough to hold OPIE_RESPONSE_MAX+1 characters, and you
need to have a buffer in which to store the challenge string that is big enough
to hold OPIE_PROMPT_MAX+1 characters.
When you are ready to output the challenge string and know the user's name,
you would use a call to opiechallenge. Later, to verify the response received,
you would use a call to opieverify. For example:
#include <stdio.h>
.
.
#include "opie.h"
.
.
char *user_name;
/* Always remember the trailing null! */
char password[OPIE_RESPONSE_MAX+1];
.
.
struct opie opiedata;
char opieprompt[OPIE_PROMPT_MAX+1];
.
.
opiechallenge(&opiedata, user_name, &opieprompt);
.
.
if (opieverify(&opiedata, password)) {
printf("Login incorrect");
TERMINAL SECURITY AND OPIE
When using OPIE, you need to be careful not to allow your password to be
communicated over an insecure channel where someone might be able to listen
in and capture it. OPIE can protect you against people who might get your
password from snooping on the line, but only if you make sure that the password
itself never gets sent over the line. The important thing is to always run the
OPIE calculator on whichever machine you are actually using - never on a machine
you are connected to by network or by dialup.
You need to be careful about the
X Window System, because it changes things quite a bit. For instance, if you
run an xterm (or your favorite equivalent) on another machine and display it
on your machine, you should not run an OPIE calculator in that window. When you
type in your secret password, it still gets transmitted over the network to go
to the machine the xterm is running on. People with machines such as
X terminals that can only run the calculator over the network are in an
especially precarious position because they really have no choice. Also, with
the X Window System, as with some other window system (NeWS as an example),
it is sometimes possible for people to read your keystrokes and capture your
password even if you are running the OPIE calculator on your local machine.
You should always use the best security mechanism available on your system to
protect your X server, be it XDM-AUTHORIZATION-1, XDM-MAGIC-COOKIE-1, or host
access control. *Never* just allow any machine to connect to your server
because, by doing so, you are allowing any machine to read any of your windows
or your keystrokes without you knowing it.
Lamport, L. "Password Authentication with Insecure Communication",
Communications of the ACM 24.11 (November 1981), pp. 770-772.
Haller, N. "The S/KEY One-Time Password System", Proceedings of the ISOC
Symposium on Network and Distributed System Security, February 1994,
San Diego, CA.
Haller, N. and Atkinson, R, "On Internet Authentication", RFC-1704,
DDN Network Information Center, October 1994.
Rivest, R. "The MD5 Message Digest Algorithm", RFC-1321,
DDN Network Information Center, April 1992.
Rivest, R. "The MD4 Message Digest Algorithm", RFC-1320,
DDN Network Information Center, April 1992.
AUTHOR
Bellcore's S/Key was written by Phil Karn, Neil M. Haller, and John S. Walden
of Bellcore. OPIE was created at NRL by Randall Atkinson, Dan McDonald, and
Craig Metz.
S/Key is a trademark of Bell Communications Research (Bellcore).
UNIX is a trademark of X/Open.
CONTACT
OPIE is discussed on the Bellcore "S/Key Users" mailing list. To join,
send an email request to: