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OPIE(4)                FreeBSD Kernel Interfaces Manual                OPIE(4)

       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".

              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".

              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 number 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
       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 feasibly 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.

       The OPIE distribution has been incorporated into three standard client
       programs: login(1), su(1), and ftpd(8),

       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_CHALLENGE_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_CHALLENGE_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.

       ftpd(8) login(1), opie(4), opiekeys(5), opieaccess(5), opiekey(1),
       opieinfo(1), opiepasswd(1),

       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:

       [email protected]

                               January 10, 1995                        OPIE(4)
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