17 April 1999

CRYPTO-GRAM
April 15, 1999
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
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Copyright (c) 1999 by Bruce Schneier
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In this issue:
Cryptography: The Importance of Not Being Different
News
Counterpane Systems -- Featured Research
Threats Against Smart Cards
Attacking Certificates with Computer Viruses
Counterpane Systems News
Comments from Readers
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Cryptography: The Importance of Not Being Different
Suppose your doctor said, "I realize we have antibiotics that are good at
treating your kind of infection without harmful side effects, and that
there are decades of research to support this treatment. But I'm going to
give you tortilla-chip powder instead, because, uh, it might work." You'd
get a new doctor.
Practicing medicine is difficult. The profession doesn't rush to embrace
new drugs; it takes years of testing before benefits can be proven, dosages
established, and side effects cataloged. A good doctor won't treat a
bacterial infection with a medicine he just invented when proven
antibiotics are available. And a smart patient wants the same drug that
cured the last person, not something different.
Cryptography is difficult, too. It combines mathematics, computer science,
sometimes electrical engineering, and a twisted mindset that can figure out
how to get around rules, break systems, and subvert the designers'
intentions. Even very smart, knowledgeable, experienced people invent bad
cryptography. In the crypto community, people aren't even all that
embarrassed when their algorithms and protocols are broken. That's how
hard it is.
Reusing Secure Components
Building cryptography into products is hard, too. Most cryptography
products on the market are insecure. Some don't work as advertised. Some
are obviously flawed. Others are more subtly flawed. Sometimes people
discover the flaws quickly, while other times it takes years (usually
because no one bothered to look for them). Sometimes a decade goes by
before someone invents new mathematics to break something.
This difficulty is made even more serious for several reasons. First,
flaws can appear anywhere. They can be in the trust model, the system
design, the algorithms and protocols, the implementations, the source code,
the human-computer interface, the procedures, the underlying computer
system. Anywhere.
Second, these flaws cannot be found through normal beta testing. Security
has nothing to do with functionality. A cryptography product can function
normally and be completely insecure. Flaws remain undiscovered until
someone looks for them explicitly.
Third, and most importantly, a single flaw breaks the security of the
entire system. If you think of cryptography as a chain, the system is only
as secure as its weakest link. This means that everything has to be
secure. It's not enough to make the algorithms and protocols perfect if
the implementation has problems. And a great product with a broken
algorithm is useless. And a great algorithm, protocol, and implementation
can be ruined by a flawed random number generator. And if there is a
security flaw in the code, the rest of it doesn't matter.
Given this harsh reality, the most rational design decision is to use as
few links as possible, and as high a percentage of strong links as
possible. Since it is impractical for a system designer (or even a design
team) to analyze a completely new system, a smart designer reuses
components that are generally believed to be secure, and only invents new
cryptography where absolutely necessary.
Trusting the Known
Consider IPSec, the Internet IP security protocol. Beginning in 1992, it
was designed in the open by committee and was the subject of considerable
public scrutiny from the start. Everyone knew it was an important protocol
and people spent a lot of effort trying to get it right. Security
technologies were proposed, broken, and then modified. Versions were
codified and analyzed. The first draft of the standard was published in
1995. Aspects were debated on security merits and on performance, ease of
implementation, upgradability, and use.
In November 1998, the committee published a pile of RFCs -- one in a series
of steps to make IPSec an Internet standard. And it is still being
studied. Cryptographers at the Naval Research Laboratory recently
discovered a minor implementation flaw. The work continues, in public, by
anyone and everyone who is interested.
On the other hand, Microsoft developed its own Point-to-Point Tunneling
Protocol (PPTP) to do much the same thing. They invented their own
authentication protocol, their own hash functions, and their own
key-generation algorithm. Every one of these items was badly flawed. They
used a known encryption algorithm, but they used it in such a way as to
negate its security. They made implementation mistakes that weakened the
system even further. But since they did all this work internally, no one
knew that their PPTP was weak.
Microsoft fielded PPTP in Windows NT and 95, and used it in their virtual
private network (VPN) products. It wasn't until summer of 1998 that
Counterpane Systems published a paper describing the flaws we found.
Microsoft quickly posted a series of fixes, which we have since evaluated
and found wanting. They don't fix things nearly as well as Microsoft would
like people to believe.
And then there is a company like TriStrata, which claimed to have a
proprietary security solution without telling anyone how it works (because
it's patent pending). You have to trust them. They claimed to have a new
algorithm and new set of protocols that are much better than any that exist
today. And even if they make their system public, the fact that they've
patented it and retain proprietary control means that many cryptographers
won't bother analyzing their claims.
Leveraging the Collective Strength
You can choose any of these three systems to secure your virtual private
network. Although it's possible for any of them to be flawed, you want to
minimize your risk. If you go with IPSec, you have a much greater
assurance that the algorithms and protocols are strong. Of course, the
product could still be flawed -- there could be an implementation bug or a
bug in any of the odd little corners of the code not covered in the IPSec
standards -- but at least you know that the algorithms and protocols have
withstood a level of analysis and review that the Microsoft and TriStrata
options have not.
Choosing the TriStrata system is like going to a doctor who has no medical
degree and whose novel treatments (which he refuses to explain) have no
support by the AMA. Sure, it's possible (although highly unlikely) that
he's discovered a totally new branch of medicine, but do you want to be the
guinea pig?
The point here is that the best security methods leverage the collective
analytical ability of the cryptographic community. No single company
(outside the military) has the financial resources necessary to evaluate a
new cryptographic algorithm or shake the design flaws out of a complex
protocol. The same holds true in cryptographic libraries. If you write
your own, you will probably make mistakes. If you use one that's public
and has been around for a while, some of the mistakes will have been found
and corrected.
It's hard enough making strong cryptography work in a new system; it's just
plain lunacy to use new cryptography when viable, long-studied alternatives
exist. Yet most security companies, and even otherwise smart and sensible
people, exhibit acute neophilia and are easily blinded by shiny new pieces
of cryptography.
Following the Crowd
At Counterpane Systems, we analyze dozens of products a year. We review
all sorts of cryptography, from new algorithms to new implementations. We
break the vast majority of proprietary systems and, with no exception, the
best products are the ones that use existing cryptography as much as possible.
Not only are the conservative choices generally smarter, but they mean we
can actually analyze the system. We can review a simple cryptography
product in a couple of days if it reuses existing algorithms and protocols,
in a week or two if it uses newish protocols and existing algorithms. If
it uses new algorithms, a week is barely enough time to get started.
This doesn't mean that everything new is lousy. What it does mean is that
everything new is suspect. New cryptography belongs in academic papers,
and then in demonstration systems. If it is truly better, then eventually
cryptographers will come to trust it. And only then does it make sense to
use it in real products. This process can take five to ten years for an
algorithm, less for protocols or source-code libraries. Look at the length
of time it is taking elliptic curve systems to be accepted, and even now
they are only accepted when more trusted alternatives can't meet
performance requirements.
In cryptography, there is security in following the crowd. A homegrown
algorithm can't possibly be subjected to the hundreds of thousands of hours
of cryptanalysis that DES and RSA have seen. A company, or even an
industry association, can't begin to mobilize the resources that have been
brought to bear against the Kerberos authentication protocol, for example.
No one can duplicate the confidence that PGP offers, after years of people
going over the code, line by line, looking for implementation flaws. By
following the crowd, you can leverage the cryptanalytic expertise of the
worldwide community, not just a few weeks of some analyst's time.
And beware the doctor who says, "I invented and patented this totally new
treatment that consists of tortilla-chip powder. It has never been tried
before, but I just know it is much better and I'm going to give it to you."
There's a good reason we call new cryptography "snake oil."
Acknowledgments
Thanks to Matt Blaze for the analogy that opened this column. This
originally appeared in the March 1999 issue of IEEE Computer.
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News
A new Pentagon study acknowledges that U.S. Defense Computers are highly
vulnerable to attack. Big surprise.
http://abcnews.go.com/sections/tech/dailyNews/hackerspentagon990322.html
The Security and Freedom through Encryption (SAFE) Act (H.R. 850) passed
the House Judiciary Committee in late March. It was a victory, since an
amendment -- proposed by Rep. McCollum (R-Fl) -- that would mandate key
escrow as a condition for export was blocked by Rep. Goodlatte (R-VA) on
jurisdictional grounds. Rep. Lofgren (D-CA), a co-author of the bill,
compared the amendment to the Administration's failed Clipper Chip. Things
will get tougher for the bill in other committees; it now moves to the
International Relations Committee, where an intense debate on the foreign
availability of encryption products is expected.
http://abcnews.go.com/sections/tech/CNET/cnet_cryptobill990324.html
http://www.wired.com/news/news/politics/story/18708.html
Background materials on the SAFE bill:
http://www.cdt.org/crypto/legis_106/SAFE/
Proposed McCollum Amendment:
http://www.cdt.org/legislation/106th/encryption/McCollum.html
CDT's testimony in support of SAFE:
http://www.cdt.org/crypto/alantestimony.shtml
The Wassenaar Arrangement is an attempt to enforce on other countries the
same kinds of limits on strong cryptography that the U.S. has. This only
makes sense if the U.S. is the only source of strong cryptography. But it
isn't -- overseas security software is now just as good as work done by
U.S. programmers. And some of the signatories are taking advantage of the
wiggle room to liberalize their policies.
http://www.wired.com/news/news/politics/story/19018.html
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Counterpane Systems -- Featured Research
"The Solitaire Encryption Algorithm"
Bruce Schneier, appendix to CRYPTONOMICON, by Neal Stephenson, Avon Books,
1999.
Computers have revolutionized the field of cryptography. It is
relatively easy to design a computer algorithm that is secure against
adversaries with unimaginable computing power. Less attention has been
paid to pencil and paper algorithms, suitable for people who don't have
access to a computer but still want to exchange secret messages. Solitaire
is an OFB stream cipher that encrypts and decrypts using an ordinary deck
of playing cards. Even so, it is secure against computer cryptanalysis.
http://www.counterpane.com/solitaire.html
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Threats Against Smart Cards
Smart cards are viewed by some as the "magic bullets" of computer security,
multipurpose tools that can be used for access control, e-commerce,
authentication, privacy protection and a variety of other applications.
While the flexibility of smart cards makes them an attractive option for
numerous business uses, it also multiplies the number of threats to their
overall security. To date, there has been little analysis of these
wide-ranging security risks.
Because of the large number of parties involved in any smart card-based
system, there are many classes of attacks to which smart cards are
susceptible. Most of these attacks are not possible in conventional,
self-contained computer systems, since they would take place within a
traditional computer's security boundary. But in the smart card world, the
following attacks all pose a legitimate threat.
Attacks by the Terminal Against the Cardholder or Data Owner
These are the easiest attacks to understand. When a cardholder puts his
card into a terminal, he trusts the terminal to relay any input and output
from the card accurately. Prevention mechanisms in most smart card systems
center around the fact that the terminal only has access to a card for a
short period of time. The real prevention mechanisms, though, have nothing
to do with the smart card/terminal exchange; they are the back-end
processing systems that monitor the cards and terminals and flag suspicious
behavior.
Attacks by the Cardholder Against the Terminal
More subtle are attacks by the cardholder against the terminal. These
involve fake or modified cards running rogue software with the intent of
subverting the protocol between the card and the terminal. Good protocol
design mitigates the risk of these kinds of attacks. The threat is further
reduced when the card contains hard-to-forge physical characteristics
(e.g., the hologram on a Visa card) that can be manually checked by the
terminal owner.
Attacks by the Cardholder Against the Data Owner
In many smart card-based commerce systems, data stored on a card must be
protected from the cardholder. In some cases, the cardholder is not
allowed to know that data. If the card is a stored-value card, and the
user can change the value, he can effectively mint money. There have been
many successful attacks against the data inside a card, such as fault
analysis, reverse-engineering, and side-channel attacks such as power and
timing analysis.
Attacks by the Cardholder Against the Issuer
There are many financial attacks that appear to be targeting the issuer,
but in fact are targeting the integrity and authenticity of data or
programs stored on the card. If card issuers choose to put bits that
authorize use of the system in a card, they should not be surprised when
those bits are attacked. These systems rest on the questionable assumption
that the security perimeter of a smart card is sufficient for their purposes.
Attacks by the Cardholder Against the Software Manufacturer
Generally, in systems where the card is issued to an assumed hostile user,
the assumption is that the card will not have new software loaded onto it.
The underlying assumption may be that the split between card owner and
software owner is unassailable. However, attackers have shown a remarkable
ability to get the appropriate hardware sent to them, often gratis, to aid
in launching an attack.
Attacks by the Terminal Owner Against the Issuer
In some systems, the terminal owner and card issuer are different parties.
This split introduces several new attack possibilities. The terminal
controls all communication between the card and card issuer, and can always
falsify records or fail to complete one or more steps of a transaction in
an attempt to facilitate fraud or create customer service difficulties for
the issuer.
Attacks by the Issuer Against the Cardholder
In general, most systems presuppose that the card issuer has the best
interests of the cardholder at heart. But this is not necessarily the
case. These attacks are typically privacy invasions of one kind or
another. Smart card systems that serve as a substitute for cash must be
designed very carefully to maintain the essential properties of cash money:
anonymity and unlinkability.
Attacks by the Manufacturer Against the Data Owner
Certain designs by manufacturers may have substantial and detrimental
effects on the data owners in a system. By providing an operating system
that allows (or even encourages) multiple users to run programs on the same
card, a number of new security issues are opened up, such as subversion of
the operating system, intentionally poor random number generators or one
application on a smart card subverting another application running on the
same card.
Securing smart-card systems means recognizing these attacks and designing
them into a system. In the best systems, it doesn't manner if (for
example) the user can hack the card. It's very Zen: work with the security
model, not against it.
Adam Shostack is director of technology at Netect Inc.
Full paper: http://www.counterpane.com/smart-card-threats.html
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Attacking Certificates with Computer Viruses
How do you know an e-mail is authentic? You verify the digital signature,
of course. This means that you verify that the message was correctly
signed, using the sender's public key. How do you know that the sender's
(call her Alice) public key is valid? You check the signature on *that*
public key.
What you're checking is called a certificate. Someone else, call him Bob,
signs Alice's public key and confirms that it is valid. So you verify
Bob's signature on Alice's certificate, so you can verify Alice's signature
on her e-mail.
Okay, how do you know that Bob's signature is valid? Maybe Carol signs her
key (creating another certificate). That doesn't actually solve the
problem; it just moves it up another layer. Or maybe Bob also signed your
key, so you know to trust him. Or maybe Carol signed someone else's key
who signed your key. In the end, you have to trust someone.
This notion of a certificate chain is one of the biggest problems with
public-key cryptography, and one that isn't talked about very much. PGP
uses the notion of "trusted introducers"; Bob signs Alice's key because Bob
knows Alice and is her friend. You signed Bob's key for the same reason.
So when Alice sends you an e-mail you can note that her public key is
signed by Bob, and you trust Bob to introduce you to people. (Much like
Bob bringing Alice along to your party.)
Other Internet protocols -- S/MIME, SSL, etc. -- take a more hierarchical
approach. You probably got your public key signed by a company like
Verisign. A Web site's SSL public key might have been signed by Netscape.
Microsoft signs public keys used to sign pieces of ActiveX code you might
download from the net.
These so-called "root-level certificates" come hard-wired into your
browser. So when you try to establish an SSL connection with some Web
site, that Web site sends you its public-key certificate. You check to see
if that certificate is signed (using the public key in your browser); if it
is, you're happy. The you-have-to-trust-someone public keys are the ones
that come with your software. You trust them implicitly, with no outside
verification.
So if you're a paranoid computer-security professional, the obvious
question to ask is: can a rogue piece of software replace the root-level
certificates in my browser and trick me into trusting someone? Of course
it can.
It's even weirder than that. Researchers Adi Shamir and Nico van Someren
looked at writing programs that automatically search for public-key
certificates and replace them with phony ones. It turns out that the
randomness characteristics of a public key make them stick out like sore
thumbs, so they're easy to find.
This attack isn't without problems. If a virus replaces the root Netscape
certificate with a phony one, it can trick you into believing a fake
certificate is valid. But that replacement certificate can't verify any
real certificates, so you'll also believe that every real certificate is
invalid. (Hopefully, you'll notice this.) But it works well with
Microsoft's Authenticode. Microsoft had the foresight to include two
root-level Authenticode certificates, presumably for if one ever gets
compromised. But the software is designed to authenticate code if even one
checks out. So a virus can replace the Authenticode spare certificate.
Now rogue software signed with this rogue certificate verifies as valid,
and real software signed by valid Microsoft-approved companies still checks
out as valid.
This virus doesn't exist yet, but it could be written.
An okay story on the topic:
http://www.techweb.com/wire/story/TWB19990315S0001
The actual research paper:
http://www.ncipher.com/products/files/papers/anguilla/keyhide2.pdf
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Counterpane Systems News
John Wiley & Sons has published a book about the Twofish encryption
algorithm. The book contains all the information in the initial Twofish
submission and the first three Twofish tech reports, expanded and
corrected. It lists for $50, but I am offering it at a 20% discount.
http://www.counterpane.com/twofish-book.html
CardTech/SecureTech '99. Bruce Schneier will host and speak at the
Cryptography Technology panel at CardTech/SecureTech, on Wednesday
afternoon, May 14, in Chicago.
http://www.ctst.com
NetSec '99. Bruce Schneier will give the keynote speech at NetSec '99, a
computer security conference (June 14-16, in St. Louis). Even though
Bruce's talk will be at 8:00 in the morning on Tuesday, it will be
interesting. Schneier will also be speaking about securing legacy
applications at 2:00 that afternoon.
http://www.gocsi.com/conf.htm
The Black Hat Briefings '99 is a Computer Security Conference scheduled for
July 7 and 8 in Las Vegas, Nevada. DefCon is a hackers convention held the
weekend after. Bruce Schneier will be speaking at both.
http://www.blackhat.com/
http://www.defcon.org/
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Comments from Readers
From: Paul Shields <shields@passport.ca>
Subject: Home-made Cryptographic Algorithms
But it is not the "try to develop new algorithms" that is the danger so
much as the belief that it is safe to deploy those algorithms; and since
the designers are not always the decision makers, while the designer of an
algorithm can see its academic learning-by-doing value, the decision maker
can perhaps only see property to sell. Sadly, those decision makers are
business people who are not often well-acquainted with mathematical proofs,
and instead focus on risk and profit.
If no one ever tries to break it, the algorithm may not be secure; but if
the fact is that even after limited deployment no competent person ever
again tries to break it, then to the business mind that risk is covered.
I have to admit that in my youth I wrote a crypto application that, while
horribly insecure even to my untrained eyes, was unfortunately put into
service by just such a decision maker. The story goes that it was actually
used to protect highly sensitive information and that it passed a committed
attempt to break it; but this did not help me sleep at night.
To convince them I think such people need really compelling stories of such
systems that have fallen, at enormous cost to those who trusted them.
From: George Stults <gstults@vixel.com>
Subject: Peer Review vs. Secrecy
I was thinking about a point that you have made many times, namely that if
you don't use a published, peer reviewed, and analysed cryptographic
method, you are taking a big chance; it could be a good method, but the
odds are against it.
It occurs to me that there is another calculation you could make here.
Namely that a well known and widely adopted method such as DES is worth a
great deal more of effort to break. Enough so that special purpose
hardware becomes justifiable to break it, as in the recently publicized
machine which broke (40 bit key?) DES.
And the opposite would seem to be true of obscure methods.
Any agency that must deal with a large volume of cryptographic material
would surely prioritize its efforts. An unknown method requiring extra
time and effort to attack would seem less likely to get the required
attention. In effect, security by obscurity. The writers of the UK
security document (referenced in a previous issue) seemed to say as much.
From: Dave Emery <die@die.com>
Subject: Insecure Mobile Communications
Some of us whose hobby is poking around the RF spectrum to explore what is
out there have been trying to make this point about a number of widely used
rf links for many many years. Nice to see the issue hit something a bit
more mainstream.
Most of the MDTs used by police departments are not encrypted in any
meaningful sense, all the complexity of their signal formats is there to
provide error correction and detection and efficient use of the radio
spectrum. VHF and UHF rf links to moving vehicles are prone to both high
error rates and bursts of errors (fades) and both MDT and pager protocols
were designed to use very heavy (rate 1/2 or more) forward error correction
and interleaving to help cope with this. This and the synchronous
transmission mode used to reduce overall bandwidth make MDT signals complex
and non-standard by comparison with simple ASCII async start stop stuff.
And some of the providers were silly enough to sell customers on the
"security" of their systems by trying to convince them that the error
correcting, interleaved, randomized (scrambled, for better signal
statistics, not security), signals were so complex that nobody would be
able to figure them out.
But if you think that police MDTs running in the open are somewhat
shocking, you should also realize that virtually all pager traffic,
including email sent via pager systems, is also completely unencrypted and
transmitted using protocols designed for robustness in high error
environments rather than security. And software for monitoring pagers with
a scanner has also been widely circulated around the net for several years.
Included is the capability to target particular pagers or monitor
everything on the channel and grep for interesting traffic.
And the Mobitex protocol used by ARDIS and RAM mobile for wireless email is
another example of something that is complex for error correction and
robustness but has essentially no security. And software for monitoring
this circulates around the net as well. ARDIS does use XORing with a 32
bit constant of the day to provide some fig leaf of security, but obviously
determining the constant is trivial...
And this only scratches the surface of what can be found out in the ether
and intercepted with relatively simple gear and some software ingenuity...
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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) 1999 by Bruce Schneier