OPIE - One-time Passwords In Everything

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.

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.

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 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.

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.

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 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");

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.

opie(4), opiekeys(5), opieaccess(5), opiekey(1), opieinfo(1), opiepasswd(1), opielogin(1), opieftpd(8) 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.

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.

OPIE is discussed on the Bellcore "S/Key Users" mailing list. To join, send an email request to: skey-users-request@thumper.bellcore.com