# cryptography

Document Sample

```					We’ve broken our share of
algorithms, but we can
almost always find attacks
that bypass the algorithms
altogether...most of the time
we exploit the same old
mistakes that designers make
over and over again.            COUNTERPANE SYSTEMS

SECURITY PITFALLS
IN CRYPTOGRAPHY
by Bruce Schneier

M      agazine articles like to describe cryptography products in terms of algo-
rithms and key length. Algorithms make good sound bites: they can be
explained in a few words and they’re easy to compare with one another. “128-
bit keys mean good security.” “Triple-DES means good security.” “40-bit keys
mean weak security.” “2048-bit RSA is better than 1024-bit RSA.”
But reality isn’t that simple. Longer keys don’t always mean more security.
Compare the cryptographic algorithm to the lock on your front door. Most
door locks have four metal pins, each of which can be in one of ten positions.
A key sets the pins in a particular configuration. If the key aligns them all cor-
rectly, then the lock opens. So there are only 10,000 possible keys, and a bur-
glar willing to try all 10,000 is guaranteed to break into your house. But an
improved lock with ten pins, making 10 billion possible keys, probably won’t
make your house more secure. Burglars don’t try every possible key (a brute-
force attack); most aren’t even clever enough to pick the lock (a cryptographic
attack against the algorithm). They smash windows, kick in doors, disguise
themselves as policemen, or rob keyholders at gunpoint. One ring of art
thieves in California defeated home security systems by taking a chainsaw to
the house walls. Better locks don’t help against these attacks.
Strong cryptography is very powerful when it is done right, but it is not a
panacea. Focusing on the cryptographic algorithms while ignoring other
aspects of security is like defending your house not by building a fence around
it, but by putting an immense stake into the ground and hoping that the
adversary runs right into it. Smart attackers will just go around the algorithms.
Counterpane Systems has spent years designing, analyzing, and breaking cryp-
tographic systems. While we do research on published algorithms and proto-
cols, most of our work examines actual products. We’ve designed and analyzed
systems that protect privacy, ensure confidentiality, provide fairness, and facil-
itate commerce. We’ve worked with software, stand-alone hardware, and

everything in between. We’ve broken our share of algorithms, but we can
almost always find attacks that bypass the algorithms altogether. We don’t have
to try every possible key, or even find flaws in the algorithms. We exploit errors
in design, errors in implementation, and errors in installation. Sometimes we
invent a new trick to break a system, but most of the time we exploit the same
old mistakes that designers make over and over again.

ATTACKS AGAINST
CRYPTOGRAPHIC     A     cryptographic system can only be as strong as the encryption algorithms,
digital signature algorithms, one-way hash functions, and message
authentication codes it relies on. Break any of them, and you’ve broken the sys-
DESIGNS           tem. And just as it’s possible to build a weak structure using strong materials,
it’s possible to build a weak cryptographic system using strong algorithms and
protocols.
We often find systems that “void the warranty” of their cryptography by not
using it properly: failing to check the size of values, reusing random parame-
ters that should never be reused, and so on. Encryption algorithms don’t nec-
essarily provide data integrity. Key exchange protocols don’t necessarily ensure
that both parties receive the same key. In a recent research project, we found
that some—not all—systems using related cryptographic keys could be bro-
ken, even though each individual key was secure. Security is a lot more than
plugging in an algorithm and expecting the system to work. Even good engi-
neers, well-known companies, and lots of effort are no guarantee of robust
implementation; our work on the U.S. digital cellular encryption algorithm
illustrated that.
Random-number generators are another place where cryptographic systems
often break. Good random-number generators are hard to design, because
their security often depends on the particulars of the hardware and software.
Many products we examine use bad ones. The cryptography may be strong,
but if the random-number generator produces weak keys, the system is much
easier to break. Other products use secure random-number generators, but
they don’t use enough randomness to make the cryptography secure.
Recently Counterpane Systems has published new classes of attacks against
random-number generators, based on our work with commercial designs. One
of the most surprising things we’ve found is that specific random-number gen-
erators may be secure for one purpose but insecure for another; generalizing
security analyses is dangerous.
In another research result, we looked at interactions between individually
secure cryptographic protocols. Given a secure protocol, we show how to build
another secure protocol that will break the first if both are used with the same
keys on the same device.

ATTACKS AGAINST
IMPLEMENTATIONS
M       any systems fail because of mistakes in implementation. Some systems
don’t ensure that plaintext is destroyed after it’s encrypted. Other sys-
tems use temporary files to protect against data loss during a system crash, or
virtual memory to increase the available memory; these features can acciden-
tally leave plaintext lying around on the hard drive. In extreme cases, the oper-
ating system can leave the keys on the hard drive. One product we’ve seen used
a special window for password input. The password remained in the window’s

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memory even after it was closed. It didn’t matter how good that product’s cryp-
tography was; it was broken by the user interface.
Other systems fall to more subtle problems. Sometimes the same data is
encrypted with two different keys, one strong and one weak. Other systems
use master keys and then one-time session keys. We’ve broken systems using
partial information about the different keys. We’ve also seen systems that use
inadequate protection mechanisms for the master keys, mistakenly relying on
the security of the session keys. It’s vital to secure all possible ways to learn a
key, not just the most obvious ones.
Electronic commerce systems often make implementation trade-offs to
enhance usability. We’ve found subtle vulnerabilities here, when designers
don’t think through the security implications of their trade-offs. Doing
account reconciliation only once per day might be easier, but what kind of
damage can an attacker do in a few hours? Can audit mechanisms be flooded
to hide the identity of an attacker? Some systems record compromised keys on
“hotlists”; attacks against these hotlists can be very fruitful. Other systems can
be broken through replay attacks: reusing old messages, or parts of old mes-
sages, to fool various parties.
Systems that allow old keys to be recovered in an emergency provide another
area to attack. Good cryptographic systems are designed so that the keys exist
for as short a period of time as possible; key recovery often negates any securi-
ty benefit by forcing keys to exist long after they are useful. Furthermore, key
recovery databases become sources of vulnerability in themselves, and have to
be designed and implemented securely. In one product we evaluated, flaws in
the key recovery database allowed criminals to commit fraud and then frame
legitimate users.

ATTACKS AGAINST
PASSWORDS             M       any systems break because they rely on user-generated passwords. Left
to themselves, people don’t choose strong passwords. If they’re forced to
use strong passwords, they can’t remember them. If the password becomes a
key, it’s usually much easier—and faster—to guess the password than it is to
brute-force the key; we’ve seen elaborate security systems fail in this way. Some
user interfaces make the problem even worse: limiting the passwords to eight
characters, converting everything to lower case, etc. Even passphrases can be
weak: searching through 40-character phrases is often much easier than search-
ing through 64-bit random keys. We’ve also seen key-recovery systems that cir-
cumvent strong session keys by using weak passwords for key-recovery.

ATTACKS AGAINST
HARDWARE
S   ome systems, particularly commerce systems, rely on tamper-resistant hard-
ware for security: smart cards, electronic wallets, dongles, etc. These sys-
tems may assume public terminals never fall into the wrong hands, or that
those “wrong hands” lack the expertise and equipment to attack the hardware.
While hardware security is an important component in many secure systems,
we distrust systems whose security rests solely on assumptions about tamper
resistance. We’ve rarely seen tamper resistance techniques that work, and tools
for defeating tamper resistance are getting better all the time. When we design
systems that use tamper resistance, we always build in complementary securi-
ty mechanisms just in case the tamper resistance fails.

Counterpane Systems                                                                               page 3
The “timing attack” made a big press splash in 1995: RSA private keys could
be recovered by measuring the relative times cryptographic operations took.
The attack has been successfully implemented against smart cards and other
security tokens, and against electronic commerce servers across the Internet.
Counterpane Systems and others have generalized these methods to include
attacks on a system by measuring power consumption, radiation emissions,
and other “side channels,” and have implemented them against a variety of
public-key and symmetric algorithms in “secure” tokens. We’ve yet to find a
token that we can’t pull the secret keys out of by looking at side channels.
Related research has looked at fault analysis: deliberately introducing faults
into cryptographic processors in order to determine the secret keys. The effects
of this attack can be devastating.

ATTACKS AGAINST
TRUST MODELS      M      any of our more interesting attacks are against the underlying trust
model of the system: who or what in the system is trusted, in what way,
and to what extent. Simple systems, like hard-drive encryption programs or
telephone privacy products, have simple trust models. Complex systems, like
electronic commerce systems or multi-user e-mail security programs, have
complex (and subtle) trust models. An e-mail program might use uncrackable
cryptography for the messages, but unless the keys are certified by a trusted
source (and unless that certification can be verified), the system is still vulner-
able. Some commerce systems can be broken by a merchant and a customer
colluding, or by two different customers colluding. Other systems make
implicit assumptions about security infrastructures, but don’t bother to check
that those assumptions are actually true. If the trust model isn’t documented,
then an engineer can unknowingly change it in product development, and
compromise security.
Many software systems make poor trust assumptions about the computers
they run on; they assume the desktop is secure. These programs can often be
broken by Trojan horse software that sniffs passwords, reads plaintext, or oth-
erwise circumvents security measures. Systems working across computer net-
works have to worry about security flaws resulting from the network protocols.
Computers that are attached to the Internet can also be vulnerable. Again, the
cryptography may be irrelevant if it can be circumvented through network
insecurity. And no software is secure against reverse-engineering.
Often, a system will be designed with one trust model in mind, and imple-
mented with another. Decisions made in the design process might be com-
pletely ignored when it comes time to sell it to customers. A system that is
secure when the operators are trusted and the computers are completely under
the control of the company using the system may not be secure when the oper-
ators are temps hired at just over minimum wage and the computers are
untrusted. Good trust models work even if some of the trust assumptions turn
out to be wrong.

ATTACKS ON THE
USERS             E    ven when a system is secure if used properly, its users can subvert its secu-
rity by accident—especially if the system isn’t designed very well. The clas-
sic example of this is the user who gives his password to his co-workers so they
can fix some problem when he’s out of the office. Users may not report miss-
ing smart cards for a few days, in case they are just misplaced. They may not

page 4                                                                        Counterpane Systems
carefully check the name on a digital certificate. They may reuse their secure
passwords on other, insecure systems. They may not change their software’s
default weak security settings. Good system design can’t fix all these social
problems, but it can help avoid many of them.

ATTACKS AGAINST
FAILURE RECOVER
S  trong systems are designed to keep small security breaks from becoming big
ones. Recovering the key to one file should not allow the attacker to read
every file on the hard drive. A hacker who reverse-engineers a smart card
should only learn the secrets in that smart card, not information that will help
him break other smart cards in the system. In a multi-user system, knowing
one person’s secrets shouldn’t compromise everyone else’s.
Many systems have a “default to insecure mode.” If the security feature does-
n’t work, most people just turn it off and finish their business. If the on-line
credit card verification system is down, merchants will default to the less-
secure paper system. Similarly, it is sometimes possible to mount a “version
rollback attack” against a system after it has been revised to fix a security prob-
lem: the need for backwards compatibility allows an attacker to force the pro-
tocol into an older, insecure, version.
Other systems have no ability to recover from disaster. If the security breaks,
there’s no way to fix it. For electronic commerce systems, which could have
millions of users, this can be particularly damaging. Such systems should plan
to respond to attacks, and to upgrade security without having to shut the sys-
tem down. The phrase “and then the company is screwed” is never something
you want to put in your business plan. Good system design considers what will
happen when an attack occurs, and works out ways to contain the damage and
recover from the attack.

ATTACKS AGAINST THE
CRYPTOGRAPHY          S  ometimes, products even get the cryptography wrong. Some rely on pro-
prietary encryption algorithms. Invariably, these are very weak. Counter-
pane Systems has had considerable success breaking published encryption
algorithms; our track record against proprietary ones is even better. Keeping
the algorithm secret isn’t much of an impediment to analysis, anyway—it only
takes a couple of days to reverse-engineer the cryptographic algorithm from
executable code. One system we analyzed, the S/MIME 2 electronic-mail stan-
dard, took a relatively strong design and implemented it with a weak crypto-
graphic algorithm. The system for DVD encryption took a weak algorithm
We’ve seen many other cryptographic mistakes: implementations that repeat
“unique” random values, digital signature algorithms that don’t properly veri-
fy parameters, hash functions altered to defeat the very properties they’re being
used for. We’ve seen cryptographic protocols used in ways that were not
intended by the protocols’ designers, and protocols “optimized” in seemingly
trivial ways that completely break their security.

ATTACK PREVENTION
VS. ATTACK
M      ost cryptographic systems rely on prevention as their sole means of
defense: the cryptography keeps people from cheating, lying, abusing, or
whatever. Defense should never be that narrow. A strong system also tries to
DETECTION             detect abuse and to contain the effects of any attack. One of our fundamental

Counterpane Systems                                                                               page 5
design principles is that sooner or later, every system will be successfully
attacked, probably in a completely unexpected way and with unexpected con-
sequences. It is important to be able to detect such an attack, and then to con-
tain the attack to ensure it does minimal damage.
More importantly, once the attack is detected, the system needs to recover:
generate and promulgate a new key pair, update the protocol and invalidate
the old one, remove an untrusted node from the system, etc. Unfortunately,
many systems don’t collect enough data to provide an audit trail, or fail to pro-
tect the data against modification. Counterpane Systems has done consider-
able work in securing audit logs in electronic commerce systems, mostly in
response to system designs that could fail completely in the event of a success-
ful attack. These systems have to do more than detect an attack: they must also
be able to produce evidence that can convince a judge and jury of guilt.

BUILDING SECURE
CRYPTOGRAPHIC
S  ecurity designers occupy what Prussian general Carl von Clausewitz calls
“the position of the interior.” A good security product must defend against
every possible attack, even attacks that haven’t been invented yet. Attackers, on
SYSTEMS           the other hand, only need to find one security flaw in order to defeat the sys-
tem. And they can cheat. They can collude, conspire, and wait for technology
to give them additional tools. They can attack the system in ways the system
designer never thought of.
Building a secure cryptographic system is easy to do badly, and very difficult
to do well. Unfortunately, most people can’t tell the difference. In other areas
of computer science, functionality serves to differentiate the good from the
compression program will look worse in feature-comparison charts. Cryptog-
raphy is different. Just because an encryption program works doesn’t mean it
is secure. What happens with most products is that someone reads Applied
Cryptography, chooses an algorithm and protocol, tests it to make sure it works,
and thinks he’s done. He’s not. Functionality does not equal quality, and no
amount of beta testing will ever reveal a security flaw. Too many products are
merely “buzzword compliant”; they use secure cryptography, but they are not
secure.

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page 6                                                                       Counterpane Systems
COUNTERPANE SYSTEMS

C    ounterpane Systems is a cryptography and computer security consulting
firm. We are a virtual company based in Minneapolis, with three full-time
employees and six part-time contractors. Counterpane provides expert con-
sulting in the following areas:

This is the majority of Counterpane’s work: making and breaking commercial
DESIGN AND ANALYSIS   cryptographic systems and system designs. We can analyze all aspects of a secu-
rity system, from the threat model to the cryptographic algorithms, and from
the protocols to the implementation and procedures. Our detailed reports pro-
vide clients with information on security problems as well as suggested fixes.

Counterpane Systems has worked in areas such as:
Hard disk and file encryption
E-mail encryption and authentication
Software and information piracy prevention
Virtual private networks
Certificate Authority systems
Digital timestamping
Digital telecommunications security
Biometric security applications
JAVA security
Electronic commerce systems
Stored-value card security
Secure audit logs

Counterpane Systems also turns designs into commercial programs. We have
IMPLEMENTATION AND    implemented and tested many cryptographic systems, both from our own
TESTING               designs and from industry standards such as SET, S/MIME, and SSL. Coun-
terpane also performs security testing and verification of software implemen-
tations and products.

Using attack tree analysis, Counterpane Systems provides a comprehensive
THREAT MODELING       threat analysis of systems and products. This kind of analysis can determine a
system’s vulnerability and the avenues of attack most likely to succeed. We can
calculate the time, money, and resources necessary to attack a system, determine
the security effects of different business decisions, and list the security assump-
tions a system is based on. Attack trees can compare attacks and countermea-
sures, and isolate areas where security can most profitably be improved–or most
profitably be attacked.

Counterpane Systems assesses potential product ideas, and gives opinions on
PRODUCT RESEARCH      their viability in the marketplace. We also maintain a large database of com-
AND FORECASTING
petitive information, and can provide information on existing security-related
products. We publish occasional reports on different areas of commercial cryp-
tography–electronic commerce, Internet security, public-key infrastructure,
secure tokens–and make these reports available to clients.

Counterpane Systems                                                                               page 7
CLASSES AND TRAINING   Counterpane Systems provides a wide variety of training services, from hour-
long tutorials on the basics of computer security to week-long classes on the
mathematics of cryptography or the philosophy of secure system design. Other
classes include advanced protocol design and analysis, Internet security proto-
cols, public-key infrastructure, and electronic commerce security. Classes can
be tailored to suit individual needs.

Counterpane Systems has considerable experience writing patent disclosures
INTELLECTUAL           for cryptographic inventions. We provide opinions on patentability and prior
PROPERTY
art, and can help clients find new ways to implement systems which avoid
infringing on existing patents. We maintain a database of more than 1000
cryptography-related patents.

Counterpane Systems can help clients go through the process of receiving
EXPORT CONSULTING      Commodity Jurisdictions from the U.S. Department of State, and get their
products approved for export from the U.S. Department of Commerce.

Counterpane Systems continually pursues cryptographic research. By publish-
THEORETICAL AND        ing papers at international academic conferences, we maintain our state-of-
APPLIED                the-art knowledge and experience in cryptography.
CRYPTOGRAPHIC
RESEARCH

CLIENTS

C    ounterpane Systems has provided consulting services for clients on five
continents, including American Express, Canon, Citibank, Compaq,
Dallas Semiconductor, Disney, Hughes Data Systems, Intel, Intuit, MCI,
Merrill Lynch, Microsoft, Mitsubishi, National Semiconductor, Netscape,
NSA, Oracle, Security Dynamics, Silicon Graphics, Stac Electronics, Veridi-
com, Visa, and Xerox. Contracts range from short-term expert opinions and
design evaluations to multi-year design and development efforts.

page 8                                                                          Counterpane Systems
COUNTERPANE SYSTEMS PERSONNEL

B   RUCE SCHNEIER is president of Counterpane Systems. He is the
author of Applied Cryptography (John Wiley & Sons, 1994 & 1996), the
seminal work in its field. Now in its second edition, Applied Cryptography has
sold over 80,000 copies world-wide and has been translated into four lan-
guages. His papers have appeared at international conferences, and he has writ-
ten dozens of articles on cryptography for major magazines. He is a
contributing editor to Dr. Dobb’s Journal where he edited the “Algorithms
Alley” column, and has been a contributing editor to Computer and Com-
munications Security Reviews. He designed the popular Blowfish encryption
algorithm, still unbroken after years of cryptanalysis.
Schneier served on the Board of Directors of the International Association for
Cryptologic Research, is a member of the Advisory Board for the Electronic
Privacy Information Center, and is on the Board of Directors of the Voter’s Tel-
com Watch. Schneier has an M.S. in Computer Science from American Uni-
versity and a B.S. in Physics from the University of Rochester. He is a frequent
writer and lecturer on the topics of cryptography, computer security, and pri-
vacy.

J  OHN KELSEY is an experienced cryptographer, cryptanalyst, and pro-
grammer who has designed several algorithms and protocols. He pioneered
research on secure random number generators, differential related-key crypt-
analysis on block ciphers, and the chosen-protocol attack against cryptograph-
ic protocols. His research has been presented at several international
conferences, and he has broken many proposed commercial cryptographic
algorithm, protocol, and system designs. Kelsey has degrees in economics and
computer science from the University of Missouri—Columbia.

C     HRIS HALL is experienced in mathematical cryptography (including
elliptic curves), protocol design and analysis, and source-code security ver-
ification. He helped build various PGP products, including some crypto-
graphic protocols and software in PGPfone. He discovered a major weaknesses
in two different X Windows authentication schemes (the attacks and fixes
weren’t announced for six months so that major vendors could fix their soft-
ware). Hall has a B.S. in Computer Science and Mathematics at the Universi-

D    AVID WAGNER is a graduate student in cryptography at the Universi-
ty of California Berkeley. His cryptographic expertise includes both algo-
rithms and protocols. He has publicly cryptanalyzed the Netscape random
number generator, SSL 3.0, and the U.S. digital cellular encryption standard.

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Counterpane Systems                                                                               page 9
PUBLICATIONS
BOOKS
B. Schneier and D. Banisar, Electronic Privacy Sourcebook, John Wiley & Sons, 1997.
B. Schneier, Applied Cryptography, Second Edition, John Wiley & Sons, 1996.
B. Schneier, E-Mail Security, John Wiley & Sons, 1995.
B. Schneier, Protect Your Macintosh, Peachpit Press, 1994.
B. Schneier, Applied Cryptography, John Wiley & Sons, 1993.

PAPERS
J. Kelsey, B. Schneier, D. Wagner, and C. Hall, “Cryptan-        J. Kelsey and B. Schneier, “Conditional Purchase Orders,”
alytic Attacks on Pseudorandom Number Generators,”               4th ACM Conference on Computer and Communications
Fast Software Encryption, Fifth International Workshop Pro-      Security, ACM Press, April 1997, pp. 117-124.
ceedings (March 1998), Springer-Verlag, 1998, to appear.
J. Kelsey, B. Schneier, and D. Wagner, “Protocol
D. Coppersmith, D. Wagner, B. Schneier, and J. Kelsey,           Interactions and the Chosen Protocol Attack,” Security
“Cryptanalysis of TwoPrime,” Fast Software Encryption,           Protocols, International Workshop April 1996 Proceedings,
Fifth International Workshop Proceedings (March 1998),           Springer-Verlag, 1997, to appear.
Springer-Verlag, 1998, to appear.
B. Schneier and J. Kelsey, “Remote Auditing of Software
B. Schneier and J. Kelsey, “Cryptographic Support for            Outputs Using a Trusted Coprocessor,” Journal of Future
Secure Logs on Untrusted Machines,” The Seventh                  Generation Computer Systems, v.13, n.1, 1997, pp. 9-18.
USENIX Security Symposium Proceedings, USENIX Press,
January 1998, to appear.                                         B. Schneier, “Why Cryptography is Harder than it
Looks,” Information Security Bulletin, v. 2, n. 2, March
B. Schneier and C. Hall, “An Improved E-Mail Security            1997, pp. 31-36.
Protocol,” 13th Annual Computer Security Applications
Conference, ACM Press, December 1997, pp. 232-238.               B. Schneier and D. Whiting, “Fast Software Encryption:
Designing Encryption Algorithms for Optimal Software
C. Hall and B. Schneier, “Remote Electronic Gambling,”           Speed on the Intel Pentium Processor,” Fast Software
13th Annual Computer Security Applications Conference,           Encryption, Fourth International Workshop Proceedings
ACM Press, December 1997, pp. 227-230.                           (January 1997), LNCS #1267, Springer-Verlag, 1997,
pp. 242-259.
J. Kelsey, B. Schneier, and D. Wagner, “Related-Key
Cryptanalysis of 3-WAY, Biham-DES, CAST, DES-X,                  B. Schneier, “Cryptography, Security, and the Future,”
NewDES, RC2, and TEA,” ICICS ’97 Proceedings, LNCS               Communications of the ACM, v. 40, n. 1, January 1997, p.
#1334, Springer-Verlag, November 1997, pp. 233-246.              138.
J. Kelsey, B. Schneier, C. Hall, and D. Wagner, “Secure          J. Kelsey, B. Schneier, and C. Hall, “An Authenticated
Applications of Low-Entropy Keys,” 1997 Information              Camera,” 12th Annual Computer Security Applications
Security Workshop (ISW’97) Proceedings (September 1997),         Conference, ACM Press, December 1996, pp. 24-30.
Springer-Verlag, 1998, to appear.
B. Schneier and J. Kelsey, “A Peer-to-Peer Software
D. Wagner, B. Schneier, and J. Kelsey, “Cryptanalysis of         Metering System,” The Second USENIX Workshop on
the Cellular Message Encryption Algorithm,” Advances in          Electronic Commerce Proceedings, USENIX Press,
Cryptology–CRYPTO ’97 Proceedings, LNCS #1294,                   November 1996, pp. 279-286.
Springer-Verlag, August 1997, pp. 526-537.
D. Wagner and B. Schneier, “Analysis of the SSL 3.0
N. Ferguson and B. Schneier, “Cryptanalysis of Akelarre,”        Protocol,” The Second USENIX Workshop on Electronic
Fourth Annual Workshop on Selected Areas in Cryptography,        Commerce Proceedings, USENIX Press, November 1996,
August 1997, pp. 201-212.                                        pp. 29-40.
H. Abelson, R. Anderson, S.M. Bellovin, J. Benaloh, M.           B. Schneier, J. Kelsey, and J. Walker, “Distributed
Blaze, W. Diffie, J. Gilmore, P. G. Neumann, R.L. Rivest,        Proctoring,” ESORICS 96 Proceedings, LNCS #1146,
J.I. Schiller, and B. Schneier, “The Risks of Key Recovery,      Springer-Verlag, September 1996, pp. 172-182.
Key Escrow, and Trusted Third-Party Encryption,” World
Wide Web Journal, v.2, n.3, 1997, pp. 241-257.

page 10                                                                                             Counterpane Systems
J. Kelsey and B. Schneier, “Authenticating Outputs of         B. Schneier, “Blowfish–One Year Later,” Dr. Dobb’s
Computer Software Using a Cryptographic Coprocessor,”         Journal, September 1995.
Proceedings 1996 CARDIS, September 1996, pp. 11-24
M. Blaze and B. Schneier, “The MacGuffin Block Cipher
J. Kelsey, B. Schneier, and D. Wagner, “Key-Schedule          Algorithm,” Fast Software Encryption, Second International
Cryptanalysis of 3-WAY, IDEA, G-DES, RC4, SAFER,              Workshop Proceedings (December 1994), Springer-Verlag,
and Triple-DES,” Advances in Cryptology–CRYPTO ’96            LNCS #1008, 1995, pp. 97-110.
Proceedings, LNCS #1109, Springer-Verlag, August 1996,
pp. 237-251.                                                  B. Schneier, “The GOST Encryption Algorithm,” Dr.
Dobb’s Journal, v. 20, n. 1, January 95, pp. 123-124.
B. Schneier and J. Kelsey, “Automatic Event Stream
Notarization Using Digital Signatures,” Security Protocols,   B. Schneier, “A Primer on Authentication and Digital
International Workshop April 1996 Proceedings, LNCS           Signatures,” Computer Security Journal, v. 10, n. 2, 1994,
#1189, Springer-Verlag, 1997, pp. 155-169.                    pp. 38-40.

B. Schneier and J. Kelsey, “Unbalanced Feistel Networks       B. Schneier, “Designing Encryption Algorithms for Real
and Block Cipher Design,” Fast Software Encryption,           People,” Proceedings of the 1994 ACM SIGSAC New
Third International Workshop Proceedings (February 1996),     Security Paradigms Workshop, IEEE Computer Society
LNCS #1039, Springer-Verlag, 1996, pp. 121-144.               Press, August 1994, pp. 63-71.

M. Blaze, W. Diffie, R. Rivest, B. Schneier, T.               B. Schneier, “The Blowfish Encryption Algorithm,” Dr.
Shimomura, E. Thompson, and M. Weiner, “Minimal               Dobb’s Journal, v. 19, n. 4, April 1994, pp. 38-40.
Key Lengths for Symmetric Ciphers to Provide Adequate         B. Schneier, “Description of a New Variable-Length Key,
Commercial Security,” January 1996.                           64-Bit Block Cipher (Blowfish),” Fast Software Encryption,
M. Jones and B. Schneier, “Securing the World Wide            Cambridge Security Workshop Proceedings (December 1993),
Web: Smart Tokens and their Implementation,”                  LNCS #809, Springer-Verlag, 1994, pp. 191-204.
Proceedings of the Fourth International World Wide Web        B. Schneier, “One-Way Hash Functions,” Dr. Dobb’s
Conference, December 1995, pp. 397-409.                       Journal, v. 16, n. 9, September 1991, pp. 148-151.

Copies of most of these papers are available at http://www.counterpane.com/publish.html

Counterpane Systems                                                                                              page 11

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