16 December 1998

CRYPTO-GRAM
December 15, 1998
by Bruce Schneier
President
Counterpane Systems
schneier@counterpane.com
http://www.counterpane.com
A free monthly newsletter providing summaries, analyses, insights, and
commentaries on cryptography and computer security.
Back issues are available at http://www.counterpane.com. To subscribe or
unsubscribe, see below.
Copyright (c) 1998 by Bruce Schneier
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In this issue:
The Fallacy of Cracking Contests
How to Recognize Plaintext
News
The Doghouse: Iomega Zip Disks
Counterpane Systems News
Final Report from the Commerce Department Technical
Advisory Committee on Key Recovery
Comments From Readers
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The Fallacy of Cracking Contests
You see them all the time: "Company X offers $1,000,000 to anyone who can
break through their firewall/crack their algorithm/make a fraudulent
transaction using their protocol/do whatever." These are cracking
contests, and they're supposed to show how strong and secure the target of
the contests are. The logic goes something like this: We offered a prize
to break the target, and no one did. This means that the target is secure.
It doesn't.
Contests are a terrible way to demonstrate security. A
product/system/protocol/algorithm that has survived a contest unbroken is
not obviously more trustworthy than one that has not been the subject of a
contest. The best products/systems/protocols/algorithms available today
have not been the subjects of any contests, and probably never will be.
Contests generally don't produce useful data. There are three basic
reasons why this is so.
1. The contests are generally unfair.
Cryptanalysis assumes that the attacker knows everything except the secret.
He has access to the algorithms and protocols, the source code,
everything. He knows the ciphertext and the plaintext. He may even know
something about the key.
And a cryptanalytic result can be anything. It can be a complete break: a
result that breaks the security in a reasonable amount of time. It can be
a theoretical break: a result that doesn't work "operationally," but still
shows that the security isn't as good as advertised. It can be anything in
between.
Most cryptanalysis contests have arbitrary rules. They define what the
attacker has to work with, and how a successful break looks. Jaws
Technologies provided a ciphertext file and, without explaining how their
algorithm worked, offered a prize to anyone who could recover the
plaintext. This isn't how real cryptanalysis works; if no one wins the
contest, it means nothing.
Most contests don't disclose the algorithm. And since most cryptanalysts
don't have the skills for reverse-engineering (I find it tedious and
boring), they never bother analyzing the systems. This is why COMP128,
CMEA, ORYX, the Firewire cipher, the DVD cipher, and the Netscape PRNG were
all broken within months of their disclosure (despite the fact that some of
them have been widely deployed for many years); once the algorithm is
revealed, it's easy to see the flaw, but it might take years before someone
bothers to reverse-engineer the algorithm and publish it. Contests don't
help.
(Of course, the above paragraph does not hold true for the military. There
are countless examples successful reverse-engineering--VENONA, PURPLE--in
the "real" world. But the academic world doesn't work that way,
fortunately or unfortunately.)
Unfair contests aren't new. Back in the mid-1980s, the authors of an
encryption algorithm called FEAL issued a contest. They provided a
ciphertext file, and offered a prize to the first person to recover the
plaintext. The algorithm has been repeatedly broken by cryptographers,
through differential and then linear cryptanalysis and by other statistical
attacks. Everyone agrees that the algorithm was badly flawed. Still, no
one won the contest.
2. The analysis is not controlled.
Contests are random tests. Do ten people, each working 100 hours to win
the contest, count as 1000 hours of analysis? Or did they all try the same
things? Are they even competent analysts, or are they just random people
who heard about the contest and wanted to try their luck? Just because no
one wins a contest doesn't mean the target is secure...it just means that
no one won.
3. Contest prizes are rarely good incentives.
Cryptanalysis of an algorithm, protocol, or system can be a lot of work.
People who are good at it are going to do the work for a variety of
reasons--money, prestige, boredom--but trying to win a contest is rarely
one of them. Contests are viewed in the community with skepticism: most
companies that sponsor contests are not known, and people don't believe
that they will judge the results fairly. And trying to win a contest is no
sure thing: someone could beat you, leaving you nothing to show for your
efforts. Cryptanalysts are much better off analyzing systems where they
are being paid for their analysis work, or systems for which they can
publish a paper explaining their results.
Just look at the economics. Taken at a conservative $125 an hour for a
competent cryptanalyst, a $10K prize pays for two weeks of work, not enough
time to even dig through the code. A $100K prize might be worth a look,
but reverse-engineering the product is boring and that's still not enough
time to do a thorough job. A prize of $1M starts to become interesting,
but most companies can't afford to offer that. And the cryptanalyst has no
guarantee of getting paid: he may not find anything, he may get beaten to
the attack and lose out to someone else, or the company might not even pay.
Why should a cryptanalyst donate his time (and good name) to the company's
publicity campaign?
Cryptanalysis contests are generally nothing more than a publicity tool.
Sponsoring a contest, even a fair one, is no guarantee that people will
analyze the target. Surviving a contest is no guarantee that there are no
flaws in the target.
The true measure of trustworthiness is how much analysis has been done, not
whether there was a contest. And analysis is a slow and painful process.
People trust cryptographic algorithms (DES, RSA), protocols (Kerberos), and
systems (PGP, IPSec) not because of contests, but because all have been
subjected to years (decades, even) of peer review and analysis. And they
have been analyzed not because of some elusive prize, but because they were
either interesting or widely deployed. The analysis of the fifteen AES
candidates is going to take several years. There isn't a prize in the
world that's going to make the best cryptanalysts drop what they're doing
and examine the offerings of Meganet Corporation or RPK Security Inc., two
companies that recently offered cracking prizes. It's much more
interesting to find flaws in Java, or Windows NT, or cellular telephone
security.
The above three reasons are generalizations. There are exceptions, but
they are few and far between. The RSA challenges, both their factoring
challenges and their symmetric brute-force challenges, are fair and good
contests. These contests are successful not because the prize money is an
incentive to factor numbers or build brute-force cracking machines, but
because researchers are already interested in factoring and brute-force
cracking. The contests simply provide a spotlight for what was already an
interesting endeavor. The AES contest, although more a competition than a
cryptanalysis contest, is also fair
Our Twofish cryptanalysis contest offers a $10K prize for the best negative
comments on Twofish that aren't written by the authors. There are no
arbitrary definitions of what a winning analysis is. There is no
ciphertext to break or keys to recover. We are simply rewarding the most
successful cryptanalysis research result, whatever it may be and however
successful it is (or is not). Again, the contest is fair because 1) the
algorithm is completely specified, 2) there are no arbitrary definition of
what winning means, and 3) the algorithm is public domain.
Contests, if implemented correctly, can provide useful information and
reward particular areas of research. But they are not useful metrics to
judge security. I can offer $10K to the first person who successfully
breaks into my home and steals a book off my shelf. If no one does so
before the contest ends, that doesn't mean my home is secure. Maybe no one
with any burgling ability heard about my contest. Maybe they were too busy
doing other things. Maybe they weren't able to break into my home, but
they figured out how to forge the real-estate title to put the property in
their name. Maybe they did break into my home, but took a look around and
decided to come back when there was something more valuable than a $10,000
prize at stake. The contest proved nothing.
Gene Spafford has written against hacking contests.
http://www.itd.nrl.navy.mil/ITD/5540/ieee/cipher/old-issues/issue9602
Matt Blaze has too, but I can't find a good URL.
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How to Recognize Plaintext
A brute-force cracking machine tries every possible key until it finds the
right one. If the machine has a chunk of ciphertext and decrypts it with
one key after the other, how does it know when it found the correct plaintext?
It seems obvious to me, but I get this question often enough to address it
in these pages. The machine knows that it found the plaintext because it
looks like plaintext.
Plaintext tends to look like plaintext. It's an English-language message,
or a data file from a computer application (programs like Microsoft Word
have large known headers; even PK-ZIP files have known headers), or a
database in a reasonable format. When you look at a decrypted file, it
looks like something understandable. When you look at a ciphertext file,
or a file decrypted with the wrong key, it looks like gibberish.
In the 1940s, Claude Shannon invented a concept called the unicity
distance. Among other things, the unicity distance measures the amount of
ciphertext required such that there is only one reasonable plaintext. This
number depends both on the characteristics of the plaintext and the key
length of the encryption algorithm.
For example, RC4 encrypts data in bytes. Imagine a single ASCII letter as
plaintext. There are 26 possible plaintexts out of 256 possible
decryptions. Any random key, when used to decrypt the ciphertext, has a
26/256 chance of producing a valid plaintext. The analyst has no way to
tell the wrong plaintext from the correct plaintext.
Now imagine a 1K e-mail message. The analyst tries random keys, and
eventually a plaintext emerges that looks like an e-mail message: words,
phrases, sentences, grammar. The odds are infinitesimal that this is not
the correct plaintext.
Everything else is in the middle. The unicity distance determines when you
can think like the second example instead of the first.
For a standard English message, the unicity distance is K/6.8, where K is
the key length. (The 6.8 is a measure of the redundancy of English in
ASCII. For other plaintexts it will be more or less, but not that much
more or less.) For DES, the unicity distance is 8.2 bytes. For 128-bit
ciphers, it is about 19 bytes.
This means that if you are trying to brute-force DES you need two
ciphertext blocks. (DES's block length is 8 bytes.) Decrypt the first
ciphertext block with one key after another. If the resulting plaintext
looks like English, then decrypt the second block with the same key. If
the second plaintext block also looks like English, you've found the
correct key.
The unicity distance grows as the redundancy of the plaintext shrinks. For
compressed files, the redundancy might be 2.5, or three blocks of DES
ciphertext. For a 256-bit-key cipher, that would be 105 plaintext bytes.
If the plaintext is a random key, the redundancy is zero and the unicity
distance reaches infinity: it is impossible to recognize the correct
plaintext from an incorrect plaintext.
But that's a special case. Most of the time, it is easy to recognize
plaintext.
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News
Okay, I finally got the story right about Network Associates Inc. and the
Key Recovery Alliance. (Last month I pointed to a Wired News story that
they quietly rejoined.) The story is wrong. They never left the KRA.
Since its inception, Trusted Information Systems was a big mover and shaker
in the KRA. When NAI bought TIS in May 1998, TIS's membership transferred
to NAI. NAI resigned the leadership posts that TIS had held in the
Alliance and stopped attending its meetings, but never left the KRA. So,
NAI is a member of the KRA, and has been since it bought TIS.
http://www.wired.com/news/print_version/technology/story/16219.html
A federal judge has issued a temporary restraining order (TRO) against
enforcement of the Child Online Protection Act (COPA). (That's the
so-called CDA II.) This is a big deal, and a cause for celebration. The
judge's decision is at:
http://www.aclu.org/court/acluvrenoII_order.html
The full text of the plaintiffs' brief is available at:
http://www.epic.org/free_speech/copa/tro_brief.html
To follow this story, subscribe to EPIC Alert:
http://www.epic.org/alert/subscribe.html
This is a fascinating web security hole:
http://www.securexpert.com/framespoof/index.html
Attorneys for Liquid Audio, a company promoting secure music distribution
on the net, pressured someone to remove information on how to defeat copy
protection. You'd think they still taught the First Amendment in law school.
http://www.mp3.com/news/122.html
A good C++ library: NTL provides data structures and algorithms for
manipulating signed, arbitrary-length integers; and for vectors, matrices,
and polynomials over the integers and finite fields. Version 3.1b has just
been released.
http://www.cs.wisc.edu/~shoup/ntl/
The Clinton Administration isn't satisfied with trying to destroy personal
privacy in the U.S.; for years they've been taking their arguments abroad.
Now, according to the administration, the 33 Wassenaar Arrangement
signatories have agreed to implement the same bizarre export controls on
mass market encryption software as the U.S. has. Now I don't know if this
is true--the administration has lied before about the international
reception of its policies--but if it is, it's a major step backwards. I'm
not sure why the administration believes that making sure that sensitive
information is encrypted poorly so that criminals can read it will help the
world, but I'm sure they've got it figured out.
http://www.nytimes.com/library/tech/98/12/biztech/articles/04encrypt.html
http://jya.com/wass-au.htm
http://jya.com/wass-de2.htm
http://biz.yahoo.com/rf/981203/3l.html
For the record, the Wassenaar Arrangement countries are: Argentina,
Australia, Austria, Belgium, Bulgaria, Canada, Czech Republic, Denmark,
Finland, France, Germany, Greece, Hungary, Ireland, Italy, Japan, Republic
of Korea, Luxembourg, The Netherlands, New Zealand, Norway, Poland,
Portugal, Romania, Russia, Slovakia, Spain, Sweden, Switzerland, Turkey,
Ukraine, United Kingdom, United States. Some of these countries are
presently the major sources for the international distribution of
cryptographic software.
http://www.wassenaar.org/
Visit the Free Crypto website and send a message of support.
http://www.freecrypto.org
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The Doghouse: Iomega Zip Disks
The following instructions describe a hack around the read/write protection
on IOMEGA ZIP-100 disks. This isn't my work; I received it anonymously.
If anyone knows who discovered this, please have him or her contact me.
The password protection feature involves having a password and security
flag stored on the ZIP disk (in the boot sector?). This password and
security status are read by the firmware in the ZIP drive and it refuses to
allow access to a disk it believes to be read/write protected.
The ZIP drive supports a "power down" feature that literally stops the
drive spinning (and parks the drive's read/write head?) after 15 minutes of
inactivity. Drives automatically restart as needed.
The firmware does not currently (28 May 1998) notice a disk change via the
manual disk eject hole on the back of the device. If, after spin-down, a
known protected disk is manually ejected and a different protected disk is
then inserted, the password for the first disk is still considered current
by the firmware, and is thus valid for the second disk. The second disk
can then be "unprotected" by using the password of the first disk.
To implement this hack, perform the following steps:
1. You need a new blank ZIP-100 disk. Call this disk the KNOWN disk.
2. Insert the KNOWN disk into the ZIP drive.
3. Give the KNOWN disk a read/write password using the IOMEGA toolset.
Remember the password.
4. Using the IOMEGA toolset startup preferences, set the SLEEP TIME of the
ZIP drive to 1 minute.
5. Let the drive, with the KNOWN disk still inserted, spin down. (You can
hear the obvious difference in noise output.)
6. Take an unfolded paper clip.
7. Poke it into the small hole at the back of the ZIP drive, and manually
eject the KNOWN disk. (The hole is located at the back of the drive casing
above the printer or second SCSI connector.)
8. Insert the UNKNOWN disk.
9. The drive may spin up to speed again or may remain silent.
10. Using the IOMEGA toolset, REMOVE the protection on this disk. Use the
KNOWN PASSWORD from step (3) above.
11. Using the normal eject button on the front of the device, eject the
UNKNOWN disk.
12. Reinsert the UNKNOWN disk a second time (to prepare for directory and
file access).
13. Double-click on the disk drive icon in your Explorer/File Manager window.
Done.
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Counterpane Systems News
There is another Twofish technical report (#3) on the website. This one is
called "Improved Twofish Implementations," and gives better performance
numbers for 32-bit computers, smart cards, and hardware.
http://www.counterpane.com/twofish-speed.html
There is also a Twofish implementation in Delphi:
http://www.hertreg.ac.uk/ss/d_crypto.html
And finally, we have published a paper that compares the performance of all
fifteen AES candidates on 32-bit processors, smart cards, and hardware.
http://www.counterpane.com/AES-performance.html
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Final Report from the Commerce Department
Technical Advisory Committee on Key Recovery
The Technical Advisory Committee to Develop a Federal Information
Processing Standard for the Federal Key Management Infrastructure was
established by the Department of Commerce in July 1996. The Committee,
which was formally chartered on July 24, 1996, held its first meeting on
December 5-6, 1996. The goal, near as I can tell, was to get industry to
agree on rules for key recovery.
Its final meeting was held November 17-19, 1998. The meeting was more of a
fizzle than a finale. Although the committee concluded all of its work,
their final document continues to undergo editing and will continue to do
so for the next several weeks. The document is far more coherent than it
was at the end of their June "finale"; however, it has still not been
thoroughly reviewed.
Here are some points which may be of interest.
Although it may be described as such, the committee's final document was
hardly the product of "a 22-member commission." Participation dwindled to
such an extent that members who were not present at the final meeting were
asked to resign so that a quorum could be achieved. By the end of the
final meeting, the fewer than half a dozen remaining participants were
still making substantive changes to the document. Changes continued to be
made by the chair and other individuals even after the conclusion of the
committee's final meeting.
The commission did not "conclude" anything. The committee never voted on
or addressed any issues about whether key recovery made sense, where it
could/should be deployed, or even if it is possible, safe, or makes
economic sense.
The commission did not make any recommendations. The work of the committee
is a matter of public record and will presumably be continued by NIST.
However, the committee did not recommend that its work go forward.
The document produced by the committee does not give a blueprint for how to
do key recovery. It lists over two hundred things that key recovery
products should and should not do without ever saying how to do them.
There is no reason to believe that the committee's document is consistent
or complete. As an example, most of the document's 200+ requirements are
contained in a section that was completely rewritten by a single member
shortly before the final meeting. During the last day of the final meeting
and before this section had been reviewed by what remained of the
committee, this individual left saying that since the group had already
dropped below quorum, his presence was no longer necessary. The section
was later reviewed only superficially by the few remaining members.
The document was written at least as much by NIST and NSA as by the
committee. Although the only voting members of the committee were from the
private sector, this was certainly not a product of (or at the initiative
of) the private sector.
You can read the document yourself at:
http://csrc.nist.gov/tacdfipsfkmi/
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Comments from Readers
>>>From Chris Smith <smith@interlog.com>:
You write about Canada's new crypto policy. However, the new policy is not,
as you have described, about "export policy," but about domestic policy.
This is about cryptography in Canada. My understanding is that Canada will
continue to abide by the Wassenaar Arrangement, which currently limits the
export of strong encryption.
>>>From George Foot <georgefoot@oxted.demon.co.uk>:
Schneier wrote:
"The electronic world moves too fast for this cycle. A serious flaw in an
electronic commerce system could bankrupt a company in days. Today's
systems must anticipate future attacks. Any successful electronic commerce
system is likely to remain in use for ten years or more. It must be able
to withstand the future: smarter attackers, more computational power, and
greater incentives to subvert a widespread system. There won't be time to
upgrade them in the field."
I read every issue of the CRYPTO-GRAM with great interest. If I may as a
simple subscriber be permitted an observation, I would say that there is a
great need to learn from the warning in the above paragraph.
But what principles should be adopted to guard against the danger which is
predicted? My criterion is simplicity. Discard the complex algorithms
which at every convolution open a crack through which their inner working
may be examined exhaustively without limit of time.
Choose instead a simple system which has unique parameters for every two
stations desiring to intercommunicate and for every message passed, which
publishes no keys and which has no association whatever with a third party.
>>>From Illuminatus Primus <vermont@gate.net>:
I see that the main reason automation poses a new risk is because the
system increasingly depends on dumb (comparatively speaking) components,
rather than humans. A small, isolated program will usually not recognize
when cash flow trends suddenly change because small, isolated programs will
not teach themselves how to recognize fraud. However, humans sometimes
unreasonably expect them to. The cure is for more people to spend the time
translating warning systems that humans are accustomed to into
machine-friendly language. For example, Visa warns me every time a sharp
change in my spending patterns is noticed. It's almost like I have a
friendly banker keeping a watch over my money, but in reality it's just a
few checks completed in a fraction of a second by a computer somewhere.
Automation also brings a great benefit: new holes in the system can be
squashed in very little time. Centralized systems benefit the moment that
the hole is repaired, and decentralized systems benefit the moment that the
fix propagates. I think a good argument could be made about how the
current human-dependent systems might actually be worse off than the
digital ones. For example, look at the problems involved with introducing
new US currency to stop counterfeiting. Humans often times can take longer
to adapt to a new situation than the time it takes to transmit a new piece
of authentication code across the world millions of times.
Perhaps there is a larger issue underneath all of this: the need for
individual nodes to become more intelligent. If my credit card was
actually a small terminal that allowed me to clear requests to charge to
it, perhaps fraud against my card would be more difficult to accomplish.
Perhaps if cars had meters at the gasoline intake, gas stations couldn't
fraudulently overcharge people so easily. And perhaps, if merchants
exchanged digital money instead of paper money, counterfeiting wouldn't
take decades to stamp out.
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CRYPTO-GRAM is a free monthly newsletter providing summaries, analyses,
insights, and commentaries on cryptography and computer security.
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Please feel free to forward CRYPTO-GRAM to colleagues and friends who will
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it is reprinted in its entirety.
CRYPTO-GRAM is written by Bruce Schneier. Schneier is president of
Counterpane Systems, the author of "Applied Cryptography," and an inventor
of the Blowfish, Twofish, and Yarrow algorithms. He served on the board of
the International Association for Cryptologic Research, EPIC, and VTW. He
is a frequent writer and lecturer on cryptography.
Counterpane Systems is a six-person consulting firm specializing in
cryptography and computer security. Counterpane provides expert consulting
in: design and analysis, implementation and testing, threat modeling,
product research and forecasting, classes and training, intellectual
property, and export consulting. Contracts range from short-term design
evaluations and expert opinions to multi-year development efforts.
http://www.counterpane.com/
Copyright (c) 1998 by Bruce Schneier