1 .\" opie.4: Overview of the OPIE software.
3 .\" %%% portions-copyright-cmetz-96
4 .\" Portions of this software are Copyright 1996-1999 by Craig Metz, All Rights
5 .\" Reserved. The Inner Net License Version 2 applies to these portions of
7 .\" You should have received a copy of the license with this software. If
8 .\" you didn't get a copy, you may request one from <license@inner.net>.
10 .\" Portions of this software are Copyright 1995 by Randall Atkinson and Dan
11 .\" McDonald, All Rights Reserved. All Rights under this copyright are assigned
12 .\" to the U.S. Naval Research Laboratory (NRL). The NRL Copyright Notice and
13 .\" License Agreement applies to this software.
17 .\" Modified by cmetz for OPIE 2.4. Spelling fixes.
18 .\" Modified by cmetz for OPIE 2.2. Removed MJR DES documentation. Removed
19 .\" references to the old square brackets challenge delimiters.
20 .\" Modified at NRL for OPIE 2.01. Updated UNIX trademark credit.
21 .\" Definition of "seed" written by Neil Haller of Bellcore
22 .\" Written at NRL for OPIE 2.0.
24 .\" $FreeBSD: src/contrib/opie/opie.4,v 1.6.2.4 2002/07/15 14:48:43 des Exp $
25 .\" $DragonFly: src/contrib/opie/opie.4,v 1.2 2003/06/17 04:24:04 dillon Exp $
27 .TH OPIE 4 "January 10, 1995"
29 .B OPIE \- One-time Passwords In Everything
32 OPIE is a package derived from the Bellcore S/Key Version 1 distribution
33 that helps to secure a system against replay attacks (see below). It does so
34 using a secure hash function and a challenge/response system. It provides
40 programs that use OPIE
41 authentication as well as demonstrate how a program might be adapted to use
42 OPIE authentication. OPIE was developed at and for the United States Naval
43 Research Laboratory (NRL). OPIE is derived in part from Berkeley Standard
44 Distribution UNIX and the Bellcore S/Key Version 1 distribution.
46 From the average user's perspective, OPIE is a nuisance that prevents their
47 account from being broken into. The first time a user wishes to use OPIE,
48 (s)he needs to use the
50 command to put an entry for them into
51 the OPIE database. The user can then use OPIE to authenticate themselves
52 with any program that supports it. If no other clients are being used,
53 this means they can use OPIE to
59 log in on a terminal port (like a modem), or switch to another user's
60 account. When they would normally be asked for a password, they will get
61 a challenge from the server. They then need to copy that challenge (or
62 re-type, if they don't have the ability to copy and paste through something
63 like a window system) to their calculator program, enter their password,
64 then copy (or re-type) the response from the calculator as their password.
65 While this will seem cumbersome at first, with some practice, it becomes
71 The name that the system knows you as. For example, "jdoe".
74 A password, usually selected by the user, that is needed to gain access to the
75 system. For example, "SEc1_rt".
78 A packet of information output by a system when it wishes to authenticate a
79 user. In OPIE, this is a three-item group consisting of a hash identifier,
80 a sequence number, and a seed. This
81 information is needed by the OPIE calculator to generate a proper response.
82 For example, "otp-md5 95 wi14321".
85 A packet of information generated from a challenge that is used by a system to
86 authenticate a user. In OPIE, this is a group of six words that is generated by
87 the calculator given the challenge and the secret password. For example,
88 "PUP SOFT ROSE BIAS FLAG END".
91 A piece of information that is used in conjunction with the secret password
92 and sequence number to compute the response. Its purpose is to allow the same
93 secret password to be used for multiple sequences, by changing the seed, or
94 for authentication to multiple machines by using different seeds.
97 A counter used to keep track of key iterations. In OPIE, each time a successful
98 response is received by the system, the sequence number is decremented. For
102 A piece of text that identifies the actual algorithm that needs to be used to
103 generate a proper response. In OPIE, the only two valid hash identifiers are
104 "otp-md4", which selects MD4 hashing, and "otp-md5", which selects MD5.
107 When you use a network terminal program like
109 or even use a modem to log into a
110 computer system, you need a user name and a secret password. Anyone who can
111 provide those to the system is recognized as you because, in theory, only you
112 would have your secret password. Unfortunately, it is now easy to listen in
113 on many computer communications media. From modem communication to many
114 networks, your password is not usually safe over remote links. If a
115 cracker can listen in when you send your password, (s)he then has a copy
116 of your password that can be used at any time in the future to access your
117 account. On more than one occasion, major sites on the Internet have been
118 broken into exactly this way.
120 All an attacker has to
121 do is capture your password once and then replay it to the server when it's
122 asked for. Even if the password is communicated between machines in encoded
123 or encrypted form, as long as a cracker can get in by simply replaying
124 a previously captured communication, you are at risk. Up until very recently,
125 Novell NetWare was vulnerable this way. A cracker couldn't find out what your
126 password actually is, but (s)he didn't need to -- all that was necessary to
127 get into your account was to capture the encrypted password and send that
128 back to the server when asked for it.
130 .SH ONE-TIME PASSWORDS
131 One solution to the problem of replay attacks
132 is to keep changing the way that a password is being encoded so that what is
133 sent over the link to another system can only be used once. If you can do that,
134 then a cracker can replay it as many times as (s)he wants -- it's just not
135 going to get them anywhere. It's important, however, to make sure you encode
136 the password in such a way that the cracker can't use the encoded version to
137 figure out what the password is or what a future encoded password will be.
138 Otherwise, while still an improvement over no encoding or a fixed encoding,
139 you can still be broken into.
141 .SH THE S/KEY ALGORITHM
143 A solution to this whole problem was invented by Lamport in 1981. This
144 technique was implemented by Haller, Karn, and Walden at Bellcore. They
145 created a free software package called "S/Key" that used an algorithm
146 called a cryptographic checksum. A cryptographic checksum is a strong one-way
147 function such that, knowing the result of such a function, an attacker still
148 cannot feasibly determine the input. Further, unlike cyclic redundancy
149 checksums (CRCs), cryptographic checksums have few inputs that result in the
152 In S/Key, what changes is the number of
153 times the password is run through the secure hash. The password is run through
154 the secure hash once, then the output of the hash is run through the secure
155 hash again, that output is run through the secure hash again, and so on until
156 the number of times the password has been run through the secure hash is equal
157 to the desired sequence number. This is much slower than just, say, putting
158 the sequence number in before the password and running that through the secure
159 hash once, but it gains you one significant benefit. The server machine you
160 are trying to connect to has to have some way to determine whether the output
161 of that whole mess is right. If it stores it either without any encoding or
162 with a normal encoding, a cracker could still get at your password. But if it
163 stores it with a secure hash, then how does it account for the response
164 changing every time because the sequence number is changing? Also what if you
165 can never get to the machine any way that can't be listened in on? How do you
166 change your password without sending it over the link?
169 devised by Lamport is to keep in mind that the sequence number is
170 always decrementing by one and that, in the S/Key system, simply by running any
171 response with a sequence number N through the secure hash, you can get the
172 response with a sequence number N+1, but you can't go the other way. At any
173 given time, call the sequence number of the last valid response that the
174 system got N+1 and the sequence number of the response you are giving it N.
175 If the password that generated the response for N is the same as the one for
176 N+1, then you should be able to run the response for N through the secure hash
177 one more time, for a total of N+1 times, and get the same response as you got
178 back for N+1. Once you compare the two and find that they are the same, you
179 subtract one from N so that, now, the key for N that you just verified becomes
180 the new key for N+1 that you can store away to use the next time you need to
181 verify a key. This also means that if you need to change your password but
182 don't have a secure way to access your machine, all the system really needs to
183 have to verify your password is a valid response for one more than the sequence
184 number you want to start with.
186 Just for good measure, each side of
187 all of this uses a seed in conjunction with your password when it actually
188 generates and verifies the responses. This helps to jumble things up a little
189 bit more, just in case. Otherwise, someone with a lot of time and disk space
190 on their hands could generate all the responses for a lot of frequent passwords
191 and defeat the system.
193 This is not, by any means, the best explanation of how the S/Key algorithm
194 works or some of the more minor details. For that, you should go to some of
195 the papers now published on the topic. It is simply a quick-and-dirty
196 introduction to what's going on under the hood.
200 The OPIE distribution has been incorporated into three standard client
207 There are also three programs in the OPIE distribution that are specific to
210 which allows a user to set and change their
213 which allows a user to find out what their current
214 sequence number and seed are, and
216 which is an OPIE key calculator.
218 .SH ADDING OPIE TO OTHER PROGRAMS
220 Adding OPIE authentication to programs other than the ones included as clients
221 in the OPIE distribution isn't very difficult. First, you will need to make
222 sure that the program includes <stdio.h> somewhere. Then, below the other
223 includes such as <stdio.h>, but before variable declarations, you need to
224 include <opie.h>. You need to add a variable of type "struct opie" to your
225 program, you need to make sure that the buffer that you use to get a password
226 from the user is big enough to hold OPIE_RESPONSE_MAX+1 characters, and you
227 need to have a buffer in which to store the challenge string that is big enough
228 to hold OPIE_CHALLENGE_MAX+1 characters.
230 When you are ready to output the challenge string and know the user's name,
231 you would use a call to opiechallenge. Later, to verify the response received,
232 you would use a call to opieverify. For example:
250 /* Always remember the trailing null! */
252 char password[OPIE_RESPONSE_MAX+1];
258 struct opie opiedata;
260 char opieprompt[OPIE_CHALLENGE_MAX+1];
266 opiechallenge(&opiedata, user_name, opieprompt);
272 if (opieverify(&opiedata, password)) {
274 printf("Login incorrect");
276 .SH TERMINAL SECURITY AND OPIE
278 When using OPIE, you need to be careful not to allow your password to be
279 communicated over an insecure channel where someone might be able to listen
280 in and capture it. OPIE can protect you against people who might get your
281 password from snooping on the line, but only if you make sure that the password
282 itself never gets sent over the line. The important thing is to always run the
283 OPIE calculator on whichever machine you are actually using - never on a machine
284 you are connected to by network or by dialup.
286 You need to be careful about the
287 X Window System, because it changes things quite a bit. For instance, if you
288 run an xterm (or your favorite equivalent) on another machine and display it
289 on your machine, you should not run an OPIE calculator in that window. When you
290 type in your secret password, it still gets transmitted over the network to go
291 to the machine the xterm is running on. People with machines such as
292 X terminals that can only run the calculator over the network are in an
293 especially precarious position because they really have no choice. Also, with
294 the X Window System, as with some other window system (NeWS as an example),
295 it is sometimes possible for people to read your keystrokes and capture your
296 password even if you are running the OPIE calculator on your local machine.
297 You should always use the best security mechanism available on your system to
298 protect your X server, be it XDM-AUTHORIZATION-1, XDM-MAGIC-COOKIE-1, or host
299 access control. *Never* just allow any machine to connect to your server
300 because, by doing so, you are allowing any machine to read any of your windows
301 or your keystrokes without you knowing it.
313 Lamport, L. "Password Authentication with Insecure Communication",
314 Communications of the ACM 24.11 (November 1981), pp. 770-772.
316 Haller, N. "The S/KEY One-Time Password System", Proceedings of the ISOC
317 Symposium on Network and Distributed System Security, February 1994,
320 Haller, N. and Atkinson, R, "On Internet Authentication", RFC-1704,
321 DDN Network Information Center, October 1994.
323 Rivest, R. "The MD5 Message Digest Algorithm", RFC-1321,
324 DDN Network Information Center, April 1992.
326 Rivest, R. "The MD4 Message Digest Algorithm", RFC-1320,
327 DDN Network Information Center, April 1992.
330 Bellcore's S/Key was written by Phil Karn, Neil M. Haller, and John S. Walden
331 of Bellcore. OPIE was created at NRL by Randall Atkinson, Dan McDonald, and
334 S/Key is a trademark of Bell Communications Research (Bellcore).
335 UNIX is a trademark of X/Open.
338 OPIE is discussed on the Bellcore "S/Key Users" mailing list. To join,
339 send an email request to:
341 skey-users-request@thumper.bellcore.com
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