Ubuntu Manpage: OPIE - One-time Passwords In Everything

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NAME

       OPIE - One-time Passwords In Everything

DESCRIPTION

       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 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.

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 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.

OPIE COMPONENTS

       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 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_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");

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.

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:

       skey-users-request@thumper.bellcore.com

                                         January 10, 1995                                 OPIE(4)