Binding cryptography.
A fraud-detectible alternative to
key-escrow proposals

The
Computer Law & Security Report, January-February 1997, pp. 3-14

© Eric Verheul, Bert-Jaap
Koops, Henk van Tilborg, 1997

This page may not be redistributed. Permission is granted for printing one
copy for personal use. Please credit if quoting.

Eric Verheul
Bert-Jaap
Koops
Henk van
Tilborg

Introduction
Information security, and so cryptography,
is essential in today's information society. A robust worldwide information
security infrastructure must be set up, including a Key Management
Infrastructure. However, the unconditional use of encryption by criminals poses
a threat to law enforcement. Consequently, governments have two tasks. The first
is stimulating the establishment of a security infrastructure that protects
their citizens, but which does not facilitate criminals to shield their
activities from law-enforcement agencies. The second task is coping with the use
of encryption by criminals in general - outside of this infrastructure. In this
article, Eric Verheul, Bert-Jaap Koops and Henk van Tilborg address the first
task. The authors review several (technical) proposals and a few government
initiatives, focusing on key-escrow proposals. They present a series of criteria
that acceptable solutions should meet, and note that all proposals so far fail
to meet many of these criteria. The establishment of a worldwide security
infrastructure can not be achieved without strong cooperation of governments. In
fact, the authors argue, governments themselves should take up the challenge of
establishing a security infrastructure based on public-key encryption, which
does not hamper law enforcement. The authors offer a new solution to achieve
this, "binding data", which improves upon current proposals. It helps the
establishment of a strong security infrastructure in which (unilateral) fraud
(i.e., not complying with the agreed rules) for criminal or subversive purposes
is discouraged; such abuse is difficult and detectible by arbitrary parties. The
proposal allows a straightforward monitoring of compliance with law-enforcement
regulations, without users having to deposit keys beforehand. This security
infrastructure does not solve criminal encryption use outside of this framework
- it is not meant to. Criminals can use encryption anyhow; they should only be
kept from gaining advantage in using the government infrastructure for this. The
authors envision how a security infrastructure can be established that is
flexible enough to be incorporated in any national crypto policy, on both the
domestic and international use of cryptography.

1. The Information Society
The information society is
in the making. More and more areas of life are being digitized - from work and
education to entertainment and shopping. Information and communications
technologies (ICT) are quickly gaining ground, as they are enhancing in an ever
faster pace. The convergence of various media and the digitisation of text,
sound and image will lead to an information infrastructure in which the
telephone, television, computer and facsimile machine are integrated into a
user-friendly and everyday device. Moreover, the liberalization of the
telecommunications sector is creating a customer-oriented market, and users will
be able to pick and choose the services best catered to their needs.

Thus, the users of the information society face a wide variety of services,
which can significantly enhance the quality of their lives. This is also true
for governments: several aspects of their administration, such as electronic
publishing of administrative information and on-line tax collection, will
improve through ICT.

But governments also face a regulatory task with respect to the information
society. Both the National Information Infrastructure (NII) initiative of the US
administration and the EU Bangemann Report emphasize that while private
investment is needed to develop the information infrastructure, governments
should focus on creating a stimulating and balanced regulatory environment.

The task of providing such a stimulating and balanced regulatory environment
includes the solving of several problems that currently hamper an untroubled
development of the information society. For instance, interconnection, universal
service, protecting intellectual property rights and privacy are issues that
have to be addressed. Another major issue is information security.

2. Information Security and Cryptography
Currently,
some two thirds of the (Western) economy depend on information-related jobs. As
the information society relies so much on information, the security of
information transport and storage is of the utmost importance. Through the ever
increasing interconnection of computers into large and often open networks, the
enhancement of ICT and the growing computer-literacy of the younger generations,
the threats to which information is exposed are serious. Sophisticated attacks
on computer and telecommunications systems show that information security is a
dire necessity.

Information security consists of the confidentiality, integrity and
availability of information. Confidentialityis the requirement that
information is kept secret from people who are not authorized to access it.
Integrityis the requirement that information is unaltered; we take
integrity to include authenticity, the requirement that information indeed comes
from the purported source. Availability is the requirement that
information and information and communications systems are available at the
right times to their users. Another requirement often associated with
information security is non-repudiation, which means that the sender or
receiver of a message cannot deny having sent or received the message.

The threats to information vary widely. Many security incidents are caused
through carelessness - people spill coffee over a machine, spread a home-copied
virus or choose easily guessable passwords. Threats from the outside -
cr/hackers and viruses - have received much attention, but inside attacks (by
employees, managers or system operators) often cause much larger damage.

Because of the variety of threats, an adequate information security plan must
consist of various measures to lower the overall risk. Typical measures include
physical measures (such as isolating key information and vital information
systems in closely guarded rooms), technological measures (firewalls,
authentication mechanisms, cryptography), procedural measures (such as
responsibility division or the regular changing of passwords), control and
auditing.

Within an information security plan, cryptography plays an important part. It
safeguards the integrity and confidentiality of stored or transported data; it
can also be used for non-repudiation of the sender. Indeed, for many purposes,
cryptography is the only way to effectively shield information from unauthorized
access or altering. For instance, cryptography protects billions of dollars of
financial transactions that are processed daily over the global financial
networks; it also provides for electronic payment, both in sending credit card
numbers securely and with digital cash. Encrypted services, such as pay-TV or
video-on-demand constitute a growing market. Likewise, the soaring market for
mobile communications is enabled through cryptographic protection. E-mail can be
encrypted to safeguard the confidentiality of privacy-related or sensitive
company information; it also provides integrity, which is essential in
electronic business transactions, EDI, online tax declaration, and government
information publishing. Cryptography will enable new applications such as road
pricing and electronic voting. A few of these applications are somewhat exotic,
but many are everyday necessities.

In short, cryptography is essential in today's information society. The
establishment of a good infrastructure for information security that
incorporates cryptography is not only a private concern, but it is also a
government task: they must provide the necessary regulatory environment and
stimulate the establishment of a good information security infrastructure. This
need is recognized in several policy documents, such as the 1992 OECD
Guidelines for the security of information systems and the European
Commission's draft Green Book on the Security of Information Systems.

3. Public Key Infrastructures
For understanding the
intricacies of the issue at stake, a conceptual understanding of today's
cryptography is essential. We will therefore describe in this section, in a
non-technical way, recent developments in cryptography.

Until the 1980s, the security of digital communication between two (business)
partners Alice and Bob has usually been established by symmetric
cryptography. Metaphorically speaking, this uses an (adjustable) combination
lock: Alice thinks of a combination and adjusts the lock to it. Then Alice puts
a (confidential) document in a safe, seals it with the combination lock and
sends the safe to Bob by regular, unprotected mail. If the safe and the
combination lock are good enough, then access to the document can only be
achieved by access to the combination. Hence, Alice must have made known the
combination to Bob in a secure fashion, i.e., in such a way that the combination
is not revealed to others. Remark that Bob can use the same combination to
securely send documents to Alice.

This method does not only safeguard the confidentiality of the document, it
also assures Bob of the integrity of the document. Indeed, as the combination
opens the safe, the document must be from Alice. However, Bob can never convince
anybody else (for instance, a judge) of this, as he could have opened the safe,
put any document in the safe himself and closed it again. In other words, this
method does not provide for non-repudiation. Apart from the problem of securely
sending the combination, a disadvantage of this method is the poor scalability
of it. For each partner Alice wants to securely communicate with, she has to
create and securely send a combination. For instance, if ten partners all want
to communicate with each other, forty-five combinations are needed. These
disadvantages make the sole use of symmetric cryptography not a suitable
security solution for the information society.

In 1976, Diffie and Hellman introduced the concept of public-key
encryption (pke). In terms of the previous metaphor, this introduces
combination locks with a pair of combinations: a close-combination and
an open-combination. With the close-combination, one can close the combination
lock, and only with the open-combination can one open it again. A requirement of
these pke-combinations locks is that it must be easy to create a pair of
close/open combinations, but it must be impossible to derive the
open-combination from the close-one or vice versa. Since 1976, several
constructions for pke-combination locks have been published; the construction of
Rivest, Shamir and Adleman (RSA) is the most famous and widely used.

Suppose Alice wants to securely send a confidential document to Bob. First,
Bob generates a random pair of close/open-combinations and sends only the
close-combination to Alice. Now Alice takes a pke-combination lock, puts the
document in a safe, seals it with the close-combination and sends the safe to
Bob by regular mail. As only Bob has access to the corresponding
open-combination of the pke-combination lock, only he can open the safe and
obtain the document. It follows that, although Bob must keep his
open-combination secret, he can publish the close-combination widely. So, he can
use it to securely communicate with as many people he likes. This concept is
used on the World Wide Web for securely sending sensitive data: browsers contain
a public close-combination, the secret open-combination of which is incorporated
in (secure) servers.

Apart from confidentiality, pke-combination locks can be used to let Bob
determine the integrity of documents sent by Alice. To this end, Alice generates
a random pair of close/open combinations and only sends the
open-combination to Bob. Now, Alice takes a pke-combination lock, puts
the document in a safe, seals it with the combination lock and the
close-combination, and sends it to Bob by regular mail. On delivery, Bob can
open the safe with the open-combination. Assuming that Alice's close-combination
has not been compromised, she can never repudiate safes closed by her
close-combination. This specific sealing by Alice is called "digital signing";
the opening by Bob is called "verifying".

In both applications of public-key encryption, one of the combinations may be
(and usually is) publicly known, while the other one must remain secret: in pke
jargon they are called respectively the public keyand the private
key. Whether the close-combination or the open-combination is publicly
known depends on which of the two applications, confidentiality or integrity, is
needed. In both applications, only one public key is sufficient for
communication with all partners. It is this important improvement over
the use of symmetric cryptography that makes public-key encryption the
appropriate basis for security in the information society.

In practice, e.g., in the freeware program Pretty Good Privacy (PGP), both
applications are usually combined to obtain safeguards for both confidentiality
and integrity. In this fashion, Alice first signs a message with her private
key, and then encrypts the result with the public key of Bob. On delivery, Bob
decrypts the incoming message with his own private key and then verifies the
result with Alice's public key.

As public-key encryption on whole messages is too time-consuming, usually a
combination of symmetric and public-key cryptography (a hybrid system)
is used. For confidentiality purposes, the message is (symmetrically) enciphered
with a randomly generated session key, and this - short - session key
is (public-key) enciphered with the public key of the receiver. This
public-key-enciphered session key is sent along with the (symmetrically)
enciphered message. On delivery, the receiver can use his private key to find
the session key, and hence decrypt the message through this session key. By
signing a short digest (hash) instead of the whole message, integrity can also
be efficiently realised by a hybrid system.

Securing communication in the information society now seems quite
straightforward. Each citizen generates two public key pairs, one for
confidentiality and one for integrity purposes. Publishing both public keys can
be done very efficiently by using a directory server, a digital version
of a telephone book. In effect, PGP makes possible the construction of these
public keys. Usually they are sent to others by Internet E-mail, but directory
servers for PGP public keys exist as well. To illustrate a serious problem in
this setting, suppose that Alice receives a digitally signed message from
somebody calling himself Bob, a business partner of hers, asking her to send
some confidential company information. She looks up Bob in the directory server
and fetches his (verification) public key and verifies the signature. Now
suppose the verification succeeds, then how sure can she be of the integrity of
the message? Well, just as sure as she can be of the integrity of "Bob's" public
key, since somebody impersonating Bob may have put this public key on the
server. A similar conclusion holds for Alice's use of Bob's public key - also
fetched from the server - to protect confidential information she sends to Bob:
Alice can only be as sure that confidentiality is safeguarded as she can be of
the integrity of this public key.

So, in public-key encryption, the integrity of public keys is
crucial, just as the confidentiality of keys is with conventional, symmetric
cryptography. Does this mean that Alice should only trust public keys when she
has received them from Bob in person? Luckily not, as this would make the key
management of public-key encryption nearly as inconvenient as that of
conventional cryptography. The solution that PGP uses is a "web of trust", based
on the paradigm "a friend of a friend is a friend". Here, users can bind public
keys to their owner by creating certificates: the public key plus the
identity of the owner signed by a user. Users only certify public keys when they
can vouch for their integrity, e.g., when they were handed over in person.
Usually, PGP public keys are signed by several people. So, if the public keys of
Alice and Bob are signed by a mutual friend, they can trust each other's public
keys. Apart from the problem that a friend of a friend need not be a friend,
people will have different notions about vouching for integrity. And how is a
user going to securely communicate with somebody (e.g., a digital store) not
contained in his "web of trust"? Moreover, as there are no formal
responsibilities in the scheme, whom is a user going to sue if he has been
cheated? Another problem is public key revocation: how is a user going to
quickly revoke his public key if he has lost his private key? A horizontal
key-certification structure, then, is too problematic to serve as a basis for an
information security infrastructure.

A better solution is to bring in a party trusted by everyone working
in a well-established framework. Both Alice and Bob go to this party, a
certifying authority (CA), identify themselves, e.g., with a passport,
and hand over their public keys. The CA digitally certifies these public keys,
makes them publicly available in a directory server and hands over its own
(verification) public key, so that both Alice and Bob can verify other public
keys signed by this CA. Most of the standards for public-key certificates give
rise to a tree-like Public Key Infrastructure (PKI) depicted below. There is one
"root" which creates the overall guidelines for the entire PKI and signs the
public keys of all the CAs directly underneath it (CA1 and CA2). All CAs are
expected to certify the public keys of users and the CAs below them in
accordance with the overall guidelines.

To illustrate the resulting structure, suppose Alice wants to verify
the public key of Carol. If Alice has the public (verification) key of the root,
she can first verify the integrity of the public key of CA2 signed by the root,
with CA2's key she can verify the public key of CA6 signed by CA2. Finally, with
that key she can verify the public key of Carol signed by CA6.

A (certified) public key can be considered a digital passport in the
information society. This metaphor is particularly appropriate as authentication
of the owner of a public key prior to certification is usually based on a
conventional passport. It is generally believed that PKIs will provide an
important tool for security in the information society. Several organizations
(e.g., banks, multinationals, telephone companies) and governments (USA - NIST,
Canada) are developing separate PKIs for secure communications for themselves
and with their clients (customers and citizens). PKIs will indeed present an
information security structure in the information society, provided, first, the
PKIs are part of a legal framework with adequate supervision, and, second, there
are not too many separate PKIs: citizens in the information society should not
have to use a different private/public key pair for each branch of trade they do
business with. So, PKIs should be developed for large target groups.

The development of a legal framework for PKIs is obviously a task for
governments. Moreover, there are at least three reasons why the
supervision of PKIs should be in the hands of governments as well.
First of all, a PKI can be considered the digital equivalent (with respect to
authentication) or extension (with respect to confidentiality) of the "structure
of passports" which has always, and for good reasons, been in the hands of
governments. It seems only natural that in future, governments themselves will
distribute certified public keys for their citizens, on smart cards, as part of
a passport. This, for instance, is one of the aims of the Danish Department of
the Interior with the "Danish citizen card". So, governments will become the
operators of large PKIs. Second, governments are in a good position to make
worldwide agreements on the international recognition, standards and policies
for these PKIs. Indeed, governments are in a good position to develop a
worldwide PKI, which is very tempting for the private sector to hook on
to. Third, governments will be the prime and most central users of PKIs: large
exchanges of information between governments, businesses and citizens (e.g.,
publishing new legislation, online tax declaration, electronic voting) require
integrity and confidentiality. Therefore, although the actual construction of
PKIs could and should be done mainly by the private sector, governments (in
particular the departments of Commerce and the Interior) are in the best
position to lead and supervise this development. Actually, it is in this fashion
the government of the US state of Utah has recently started to set up a
Utah-wide PKI (for digital signatures).

4. Problems for Law Enforcement and National
Security
There is a problem, though. If everyone can use
cryptography to shield his personal communications and his stored data, so will
criminals and subversive persons - thus leaving law enforcement and intelligence
agencies powerless to trace them. Indeed, for a long time, cryptography has been
regarded a weapon that must be prevented from falling into the wrong hands.
Governments have a long tradition of export controls and, to a lesser extent,
domestic regulations of cryptography.

The concern for law enforcement and national security is a serious one. In
the past few decades, criminal and terrorist organizations have spread around
the world and rule large portions of illegal markets. Moreover, with the advent
of the information society, computer crime is on the rise. Although the
incidence of computer crime is hard to estimate, it is clear that hacking, data
diddling, fraud and electronic forgery are a serious threat in a society that is
based on information. The problem at stake is that criminal organizations,
computer criminals and, increasingly, terrorists are sophisticated, professional
and computer-literate; they often use state-of-the-art and expensive
technologies in planning and committing their crimes. Cryptography is one of the
tools they will gladly use to shield themselves from judicial and intelligence
scrutiny.

The problems for law enforcement and national security surface in wiretaps
and searches. Encrypted communications render wiretapping useless - and
law-enforcement agencies generally regard wiretapping an effective, often
essential, tool in gaining information on criminals and their networks. Also,
the encryption of information storage effectively prevents the police from
gaining evidence during a search or seizure.

In this article, we focus on the wiretapping problem - as far as it is
related to an information security infrastructure. Apart from addressing the
overall threat of cryptography to law enforcement and national security [see Koops], governments
face the task of stimulating the establishment of a good information security
infrastructure for "lawful" use. Thus, governments must address this issue,
while taking care that this infrastructure does not facilitate criminals in
escaping law-enforcement scrutiny. Many governments are looking at (voluntary or
mandatory) key escrow (i.e., key deposits) (see Section 6) as a viable solution.
Preventing abuse in such a government-supported security infrastructure does not
mean to solve the general problem of criminal encryption use outside of the
infrastructure. It merely wants to prevent criminals from gaining advantage in
using the infrastructure.

5. A Security Infrastructure: Criteria for
Solutions
Several solutions for security in the information society
have already been proposed. We consider key-escrow proposals essentially an
effort to address the problem of establishing a good security infrastructure:
the initiatives offer voluntary systems which provide users with good
cryptography. To prevent abuse of these systems, most initiatives incorporate a
back-door for law-enforcement access. Such systems are inadequate, though, to
cope with the overall problem of criminal use of cryptography, and so they are
really solutions to the problem of establishing an information security
structure which discourages abuse of the infrastructure itself.

Still, many of these proposals are incomplete or unrealistic. In this
section, we propose a set of criteria for acceptable solutions. We
focus on the security issues of information confidentiality and integrity.
Taking into account the opinions expressed by governments, the private sector
and citizens on the issue of security in the information society, we come up
with the following criteria for accomplishing solutions. We distinguish between
"essential" criteria, that have to be met to a large extent by any solution, and
"desirable" criteria, which describe desirable characteristics that solutions
should try to incorporate as much as possible, within the constraints of the
essential criteria.

Essential criteria
a. Compatible solutions for integrity
and confidentiality. Just as in current society, there should be compatible
methods in the information society to sign electronic correspondence and to put
it in an electronic envelope.

b. Worldwide usability. A security solution must be flexible enough to
allow cooperating countries to implement different cryptography policies on the
domestic and international use of encryption in a coherent way.

c. Enough strength to resist any realistic criminal threat, relative
to the sensitivity of the application and the risk involved.

d. Details of solutions should be public. Subjects in the information
society who want to communicate securely have to trust the solutions they use.
To accomplish this trust, it is essential that anyone is able to closely
examine, or to have experts closely examine, the details of the solutions.

e. Not frustrate law enforcement. The tools in the information society
that law-abiding subjects can use to secure their communication, can also be
used by subjects to support criminal or subversive activities. As this
subversive use of cryptography is contrary to the interest of society at large,
security solutions in the information society should try and prevent abuse. This
criterion does not prevent criminal or subversive subjects from finding other
security solutions for protecting their communication. It merely prevents their
using regular solutions for this.

f. Abuse is difficult and easily detectible. Not only should the
regular use of security solutions in the information society be of no aid to
criminal or subversive activities, it should also be difficult to circumvent the
measures taken to achieve this. More in particular, it should be difficult or
even impossible to achieve such advantage unilaterally [cf. LWY, p.199]. To
illustrate this point, the Clipper chip (see below) can be manipulated in a
unilateral way to make law-enforcement decryption impossible [Blaze]. It should be
difficult for two colluding criminals to effectively gain advantage from the
solution, i.e., having its encryption advantages (such as key management) but
not its law-enforcement "disadvantages". Especially with software-only
solutions, abuse by collusion is hard to prevent. So, as an extra safeguard, we
suggest that solutions should have some means to make abuse at least
detectible by a wide range of third parties (e.g., network and service
operators). This ability of detection should preferably not give these parties
access to confidential information. Finally, we remark that the measures taken
for preventing abuse should be such that the benefit/effort trade-off for
criminals in abusing the system is negative; abuse need not be impossible, it
should only be made difficult enough.

g. Incorporation into an organizational, legal framework. In order to
prosecute cheaters and to address liability issues, a security solution must be
accompanied by a trustworthy organizational framework that is legally
recognized.

h. Consideration for constitutional rights. Security solutions in the
information society must take into account constitutional and fundamental
rights, such as the freedom of speech and the privacy of correspondence and
communications. In essence, this consideration should be such that the current
balance between the privacy of users and the coercive measures of
law-enforcement agencies remains the same in the information society. As an
illustration: when law-enforcement agencies are given "master-keys" of the
person they are lawfully wiretapping, then also the person's past and future
privacy might be jeopardized. Therefore, it must remain possible for the courts
to enforce the time-limits of a warrant (time-boundedness).

Desirable criteria
i. Great flexibility in choice of trusted
third parties. An information security infrastructure will inevitably
incorporate trusted third parties, comparable with notaries. These are parties
that can be trusted with respect to the integrity of information obtained from
it (e.g., time stamps, certification of public keys) and with respect to
maintaining confidentiality of information handed over to it (e.g., keys). Users
should be able to flexibly choose such trusted third parties (TTPs). It should,
for instance, be easy to switch from one TTP to another. Ultimately, if in a
user's opinion there are no parties he can trust, he should be able to set up
his own TTP. As a consequence of other criteria, however, setting up a TTP
should meet certain requirements, to prevent abuse.

j. Be flexible enough to follow private-sector developments.
The (practical) development of the information society and its security is
mainly in the hands of the private sector. Hence, security solutions should be
flexible enough to use newly developed algorithms and standards securing newly
developed applications. Consequently, security solutions should as much as
possible be based on technology-independent concepts.

k. Be easily available. Given the importance of information security,
solutions should be easily available to everyone, and thus not be too expensive.
For security solutions of high quality and performance, it is best to put some
components in (tamper-proof) hardware. Other security solutions, especially
those used for basic security, should also be completely deliverable in (usually
cheaper) software.

l. Offer optional safeguards ("safety belts") for users. Security
solutions, especially those to maintain confidentiality, can also work
against users and companies. If, for instance, keys are lost,
deliberately destroyed or withheld (e.g., by disgruntled employees), this can
effectively imply loss of information. Security solutions should have optional,
flexible safeguards to prevent this.

6. Proposed Data Recovery Solutions
So far, no overall
solutions for an information security structure in the information society have
been proposed. Some governments are taking initiatives to develop data
recoverystructures: information infrastructures in which law-enforcement
agencies can get access to (decrypted) communications. The most straightforward
form of data recovery is (master) key escrow where participants deposit
(voluntarily or mandatorily) all their private-key information
(referred to us by "master keys") with a Key Escrow Agency (KEA), which
can facilitate law-enforcement agencies. This is a sure way to data recovery,
but it is definitely not the only way.

In addition to government initiatives, various cryptographic
proposals for data recovery have been proposed, in more or less detail, and for
more or less specific situations. In this section we will describe several of
these proposals, indicating which of the criteria they do or do not meet. We
will start with a partial solution, then describe variants of key-escrow using
Trusted Third Parties (TTPs) (cf. [Denn]), and end with
proposals that can be described as session-key recovery.

For precision's sake, we will distinguish between various sorts of TTPs: we
shall reserve the term TTPs for those parties who perform "everyday" functions
for (public-key) cryptography: certification, time-stamping, key distribution
and revocation, and the like. TTPs who hold (escrowed) master keys in deposit we
will call Key Escrow Agencies (KEAs), and TTPs who help law-enforcement agencies
in decrypting tapped messages shall be called Trusted Recovery Parties (TRPs).
TRPs can provide law-enforcement access by operating as a key-escrow agency, but
also as a session-key escrow agency by serving as a virtual addressee
(see below).

The US Digital Signature Standard (DSS)
In order
to promote the separation of integrity and confidentiality cryptography, the US
government has developed a Digital Signature Standard (DSS), which can be used
for the first purpose only. Thus, the algorithm fulfils a large part of the
information security needs in the information society, but it does not hamper
law enforcement. Several patent holders of other digital signature algorithms
have claimed infringement, which has negatively affected the worldwide
acceptance of DSS.

The DSS has some potential to become the basis for a worldwide Public Key
Infrastructure with respect to integrity, but it only presents a partial
solution for a security structure in the information society. This security
structure should be augmented by government with a (public-key) standard for
confidentiality as well. If governments fail to offer such a public-key security
architecture, they risk facing a situation in which the DSS infrastructure for
authentication will be (ab)used by people to sign their public keys of a
public-key encryption scheme (say, PGP) used for confidentiality. People will
then trust the public keys of others, and so they may unrestrictedly use robust
encryption which hampers law-enforcement, thereby taking advantage of the DSS
infrastructure. If, on the other hand, governments in the near future offer a
PKI which includes certification services for confidentiality encryption, they
can build in features to at least prevent abuse of the infrastructure to the
extent possible.

As such, this proposal scores badly on criterion a and, possibly, on criteria
e and f.

European Trusted Services
The European Commission is working on
a draft proposal concerning the establishment of a Europe-wide network of
Trusted Third Party Services (ETS). Such services will constitute a core-network
of information security through certification and other services for public-key
encryption. TTPs participating in the ETS will be private organizations; they
will probably need accreditation to operate within the ETS. The proposal
primarily addresses the establishment of a PKI; it tries to discourage abuse for
criminal purposes by demanding TTPs to cooperate with judicial warrants for
accessing keys. The willingness to cooperate would likely be a requirement for
receiving TTP accreditation.

The proposal is vague on its specific implementation - the private sector
will likely have to address several of the as yet unresolved issues. A recent
call for tenders by the European Commission invites preparatory work on
operational, technical and legal aspects, addressing infrastructure
specification, design and implementation in the form of pilots [Call].

As such, the proposal meets criteria a and i. Criteria j and k are likely to
be met, but the proposal leaves this in the open. As a solution for establishing
an information security infrastructure, the proposal scores well, but as to
discouraging abuse, it is less adequate. A major problem of the proposal is the
way data recovery is handled. The proposal seems to imply that private keys are
escrowed with KEAs. Apart from raising privacy issues (criterion h), the
proposal is too vague to be able to assess the ways in which it wants to
discourage abuse. Also, as the proposal does not (yet) entail a harmonisation of
national rules nor impose a specific solution, international cooperation within
the EU (KEAs yielding (session) keys to foreign judiciary) will be questionable,
let alone worldwide cooperation (criterion b).

Escrowed Encryption Standard (Clipper)
In order to provide
citizens with a reliable crypto system that does not hamper law enforcement, the
US government has developed a key-escrow initiative, generally referred to as
Clipper, after its first implementation in the Clipper chip. The Escrowed
Encryption Standard (EES) is a voluntary standard for secure communications. It
uses a chip with Skipjack, a classified symmetric algorithm. When two people
communicate through EES-enabled phones, a law-enforcement access field (LEAF) is
transmitted, which enables the tracing of the chip used, and therefore, access
of the chip's master key that is deposited in two parts with trusted parties.
The judiciary can access the key parts with a court warrant. Whereas initially
the deposit agencies were government bodies, the administration has agreed that
also independent organizations could serve as such (the so-called Clipper II
initiative). For data communications, EES has been implemented in the Capstone
chip.

Although the EES (presumably) scores well on criteria c (strength) and i
(flexibility of trusted parties), and possibly on h (maintaining the level of
constitutional protection, as long as use of the chip is voluntary), it is a
partial and incomplete solution. It uses a classified algorithm (contrary to d),
will not be internationally accepted (most foreign industry would never consent
to US government access to their information, thwarting criterion b) and is
inflexible (not meeting criterion j). Moreover, it is only hardware-available
(contrary to criterion k) and the EES does not explicitly address
time-boundedness (contrary to h).

Trusted Information Systems [TIS] has proposed a
software variant of Clipper using publicly known (public-key) encryption
techniques. In [LWY],
a technique is proposed that would make it possible for a KEA to release chips'
master keys with an expiry date, safeguarding time-boundedness (criterion h).

Partial Data Recovery
If a TRP becomes corrupt, it might
decipher all communications of any of its participants. Moreover, it can
routinely decipher the communications of all of its participants,
searching for "interesting" information. The EES uses two KEAs to lower the
probability that either of this occurs. Adi Shamir has proposed a countermeasure
to the second threat: users should not give TRPs access to a complete
(master/session) key but only to a part. For each communication TRPs want to
assist a LEA in decrypting, they have to find the rest by "trying", which will
take enough time (say, a minute) to defeat the second threat. This concept might
strengthen the properties of data recovery proposals in respecting
constitutional rights (criterion h).

US Public Key Infrastructure (Clipper III)
A recent, unofficial
draft of the US Interagency Working Group on Cryptography Policy [IWG] envisions a form of
key-escrow incorporated in a government-sponsored, voluntary PKI usable for both
confidentiality and integrity. The PKI itself would be supported by
private-sector key-management organizations - certifying authorities. These
would also hold in deposit private keys, and will operate within performance
standards set by law. Thus, they serve as a Key Escrow Agency (KEA).

An important benefit of this concept is that there is no need to set
restrictions on whichpublic-key or symmetric systems are used
(criterion j).

By the time-boundedness condition (criterion h), the KEA should not hand over
private keys of participants but rather effect decryption itself. A potential
problem with this scheme (cf. [Frank]), as with any
scheme that relies on law-enforcement decryption through accessing a private
key, is that it works only one-way: the addressee's private key is
needed for the decryption, not the sender's. This means that if someone
suspected of criminal activities is being wiretapped and sends a message to a
"good guy", law enforcement need cooperation of the KEA to decrypt the message
with the good guy's private key. For this, the KEA must be convinced the message
came from the suspect in the period covered by the warrant. Therefore, the
system would require mandatory time-stamping and signing of messages, or network
operators would have to provide KEAs with evidence of the time and origin of
messages. (This is not enough to overcome the problem of a policeman conspiring
with a criminal to recover a highly sensitive message; see below for a solution
to the "tempted policeman".)

Another problem is, of course, when the addressee lives abroad. The concept
is not easy to use if one of the communicating parties and its KEA are outside
the jurisdiction of the law-enforcement agency conducting a wiretap. For various
reasons, international cooperation of the KEA with the law-enforcement agency
might be difficult and time-consuming. For instance, the conditions for
interception of communications differ per country. Also, fears of economic
espionage will make countries wary of handing over keys to foreign governments;
an international treaty of countries agreeing to mutually hand over keys to
foreign law-enforcement agencies is unlikely to be agreed upon soon.

Another reason why the general PKI key-escrow concept is less acceptable to
users is that they have to trust the KEA unconditionally: if it is corrupted,
then so are the user's private keys. In short, this proposal scores badly on
criterion b and possibly on i. Assuming the KEAs will not hand over private
keys, the proposal does protect constitutional rights as much as under existing
wiretap laws (criterion h).

Fair Cryptography
To overcome
the problem of KEA corruption, one might - as in the Escrowed Encryption
Standard - let the user split his private key into two or more pieces which he
hands over to different key-escrow agencies of his choice: only with all shares
can the user's private key be reconstructed. Although more acceptable to users,
this concept is easily abusable by criminal of subversive subjects. Indeed,
receiving a single share of the key, a KEA has no way of telling it is indeed
part of the private key, or that all shares have been deposited with a
KEA: a user might have sent all KEAs some useless data. Silvio Micali has
proposed a solution to this problem, which he calls fair public-key cryptography
[Mica]. He proposes
a splitting method in which the pieces of the key have the additional property
that they can be individually verified by the KEA to be correct, without
reconstructing the private key. Although fulfilling criterion i and helping in
meeting criteria f and h, this proposal does not address international
cooperation (criterion b).

The Royal Holloway International TTP Concept
Jefferies,
Mitchell and Walker of Royal Holloway, London, propose a solution to the latter
problem [Holl].
Here, each citizen in the information society is associated with a national Key
Escrow Agency (KEA) (or, in the terminology of the paper, TTP).

All such KEAs have to trust and recognize each other, forming a worldwide KEA
structure. As part of this recognition, all KEAs bilaterally agree on some
mutual secret key, for instance, K(A,B) for the mutual key of KEAA in
the US and KEAB in Britain.

Now suppose citizens Alice in the USA and Bob in Britain (associated with
respectively KEAAand KEAB) want to have a confidential
communication. Then, Alice and Bob are each given - in a secure fashion -
private/public key pairs by their respective KEAs. The construction of the
private/public key pairs of Alice and Bob happens in a special way: they are
constructed solely from their identity and the mutual secret key K(A,B).
Therefore, the American KEAA can also construct the public/private
key pair of British Bob (and vice versa) without having to communicate
with its foreign counterpart. So, if there is a warrant for legal interception
of the communication between Alice and Bob, the intercepting party can retrieve
information related to the private keys of both Alice and Bob from the
associated key-escrow agency within its jurisdiction and thus decrypt
international communications. This property makes the concept potentially usable
worldwide.

A problem with the Royal Holloway concept is that users will need a separate
private/public key pair for each foreign KEA with whose users they are
communicating. Moreover, the Royal Holloway concept uses a rather rigid
public-key encryption scheme that makes possible the secure exchange of only one
shared key per public key: for a new shared key, either the sender or the
receiver has to obtain a new public key from his KEA. Therefore, this concept
scores rather badly on criteria a, j and possibly on i.

Trusted Information System's Commercial Key Escrow
(TIS-CKE)
TIS Commercial Key Escrow (CKE) is used in conjunction with
public-key encryption and focuses on the escrow of session keys instead of
(master) private keys [cf. TIS]. So, an interesting
feature is that users do not need to give secret information (notably, keys) to
key-escrow agencies beforehand. Law-enforcement access is based on the
information components in encrypted messages that enable addressees to determine
("decrypt") session keys, i.e., the session key encrypted with the public key of
the addressee. The idea is that each such information component is accompanied
by another one for a TRP, i.e., the session key encrypted with the public key of
the Trusted Recovery Party.

In effect, the TRP is treated as a virtual addressee, although the message is
not sent to it. When a law-enforcement agency is conducting a lawful intercept
and strikes upon an enciphered message, they take the information component of
the TRP to the TRP. If shown an appropriate warrant, the TRP decrypts the
information component and hands over the session key, so that the
law-enforcement agency has access to the message. The concept of "virtual
escrow" has been the basis of several products (AT&T Crypto Backup, RSA
Secure, Translucent Cryptography).

By sending along useless data instead of a session key encrypted with the
public key of the TRP, unilateral abuse is easily possible and will only be
detected in case of a lawful wiretap. This is prevented in TIS-CKE (or actually
in its successor RecoverKey International) by having the decryption
software of the addressee first validate whether the session key encrypted with
the public key of the TRP matches the third component; if it does not, the
software refuses to decrypt. However, abuse by collusion of sender and receiver
- through manipulation of this validation in the software - is still (easily)
possible and will only be detected in case of a lawful wiretap (contrary to
criterion f).

No solution for obtaining foreign access to session keys is given; therefore,
law-enforcement agencies could only decrypt outgoing messages - unless the
sender includes keys for both a domestic and a foreign TRP. However, the
proposal does not include the option of virtually addressing keys to more than
one TRP. Thus, the proposal scores less on criteria b and e. Because of the
fixed 64-bit key length, the proposal does not score well on criterion c. Also,
abuse by colluding is (easily) possible (contrary to criterion f). In the
available information, the property of time-boundedness is not explicitly
addressed. In principle, users have a wide choice in TRPs, although they have to
trust them completely, since key splitting is not supported. So, the proposal
scores reasonably on criterion i. Finally, the proposal scores well on criteria
d, j, k and l.

Translucent cryptography
With current key-escrow proposals, in
which "either government has access (if the keys are escrowed) or it does not
(if the keys are not escrowed)", decision making on the subject is difficult,
since there is no "middle ground", Bellare and Rivest argue [BelR]. To form such a
middle ground, the authors propose that a probability p be chosen by
parliament to represent the probability that a particular intercepted message
can be decrypted by law-enforcement agencies when conducting a lawful intercept.
So, p=0means absolute privacy for users, and p=1 is similar to
conventional data recovery solutions. The p for domestic communications
might change with each US administration; the p for foreign
communications might vary with the country. The authors argue that with such
translucent cryptography, an appropriate balance can be found between
the desire for law-enforcement agencies to recover encrypted data and the desire
for absolute privacy by (most) users.

Bellare and Rivest present several implementations of translucent
cryptography. All implementations are based on the same technique used in
TIS-CKE. To give this method translucent properties, the TRP has several public
keys that people can use: a fraction p of them are useful to the TRP.
The user has no way of knowing which of the keys can yield decryption, and
therefore can not deliberately pick invalid keys.

One can argue that this concept strengthens the properties of any TIS-CKE
type proposal with respect to constitutional rights (criterion h).

7. The Binding Alternative
In this section we will
combine the government need for establishing a PKI and the previous proposals
into a new information security proposal for the information society. This
proposal will be evaluated according to the criteria put forward.

The Technical Perspective
Metaphorically speaking, our
solution consists of equipping public-key encryption systems used for
confidentiality with a metal detector, as used at boarding gates on airports.
The specifications of this detector are rather general, and so many systems can
probably be equipped with it. It is inspired by TIS-CKE, as its avoidance of
master key-escrow appeals to us. In this system, users' encrypted messages
roughly consist of three components:

1. the (actual) message encrypted with a symmetric system, using a random
session key;

2. the session key encrypted (using pke) with the public key(s) of the
addressee(s);

3. the session key encrypted (using pke) with the public key of a TRP.

As explained above, a drawback of this concept is that unilateral abuse is
easily possible by sending nonsense in the third component. This can be
prevented by having the decryption software of the addressee first validate
whether the session key encrypted with the public key of the TRP matches the
third component; if it does not, the software could refuse to decrypt. However,
abuse by colluding of sender and receiver - through manipulation of this
validation in the software - is still easily possible. So, the solution is
almost entirely unenforceable. Therefore, we propose a binding
alternative, which adds a fourth component to the encrypted message:

4. binding data.

The idea is that any third party, e.g., a network or service provider, who
has access to components 2, 3 and 4 (but not to any additional (secret)
information) can - either online of offline:

a) determine that the session keys in components 2 and 3 coincide;

b) not determine any information on the actual session key.

In this way, fraud is easily detectible: a sender that attempts to virtually
address a session key to the TRP (component 3) that is different from the real
one he uses on the message (or just nonsense) will be discovered by anyone
checking the binding data. If such checking happens regularly, fraud can be
properly discouraged and, if desirable, fined.

Constructing binding data is feasible. An outline of the construction for
binding data for an important public-key encryption system (ElGamal) can be
found at [W3]. This
outline will be elaborated in a separate article [VT]. The construction is
based on the technique of zero-knowledge proofs. We expect that this
construction can be improved and that many other public-key encryption systems
can be equipped with binding data. We present this as a challenge to the
cryptographic research community. Use of the ElGamal scheme is particulary
interesting as on 29 April 1997, ElGamal will be unencumbered by patents in the
USA [Schn, p.479].
Moreover, the next version of PGP (3.0) will include ElGamal.

For non-technical readers, we hope a simile may give some understanding of
how the binding data work. Imagine an envelope (which serves as a safe as in the
analogy in section 3) is sealed with a combination lock which can be locked with
the public close-combination of the receiver. Alice uses this system to securely
send a session key to Bob. She puts a letter (the session key) in an envelope
and seals this with a combination lock which only Bob can open. Then, she puts a
letter (which should be the same session key) in an envelope and seals it with
the close-combination of a TRP. She sends both envelopes as a message to Bob.
Now, a monitor should be able to check whether the two letters are identical,
without seeing the letters himself. For this, the system requires special
envelopes, the back of which is a (coloured) filter selectively transparent to a
certain colour. The letter will be printed in many different colours, in such a
way that each letter is built up of hundreds of differently-coloured pixels; this is illustrated in the figure (different characters
represent different colours). The sender now computes the binding data, which is
the colour that the filter should be selectively transparent to; this
computation depends on many data, such as Alice's and Bob's identity and the
date and time. In effect, Alice can not predict beforehand the colour of the
filter at the back of the envelop, and so she can not manipulate the message.
Now, the monitor (e.g., a postman) can first compute and check the colour of the
filter and then check whether the letters in the envelopes match by simply
checking the pixel patterns appearing through the filter (the appearing "q" in
the figure). If these match, the letters must be identical - after all, the
colour was chosen arbitrarily. Moreover, the monitor does not get any
information on the letter's contents; although in the simile he might gain a
slight amount of information, in our technical proposal, complete
confidentiality is guaranteed.

The binding concept that we envision supports the virtual addressing of
session keys to several TRPs (or none for that matter), for instance, one to a
TRP in the country of the sender and one in the country of the addressee. The
solution therefore offers the same advantage for worldwide useability as the
Royal Holloway concept. We also remark that the concept supports the use of
controllable key splitting in the sense of Micali as well, even in two ways.
First, the private TRP key can be splitted in several parts and be deposited
with several TRPs. It appears that some systems (e.g., ElGamal) can very
conveniently support the splitting and reconstruction of private TRP keys by
users themselves (details will appear in [VT]). Second, a sender
can split the session key and address all the shares separately to the addressee
and virtually to various TRPs using the binding concept. Moreover, the number of
shares and the TRPs can - in principle - be chosen freely by each user. Finally
we remark that the time-boundedness condition (criterion h) can be fulfilled by
additionally demanding that encrypted information (or all components) be
timestamped and signed by the sender; a condition that can be publicly verified
by any third party as well.

An additional feature could prevent the threat of the "tempted policeman".
This tempted policeman might conspire with a criminal and have the criminal
resend (or "receive") an intercepted, highly confidential business message. The
TRP, thinking the message originated from the (wiretapped) criminal, would
assist the policeman in decrypting. In the binding scheme, this can be prevented
by additionally requiring senders to virtually address the session key to
themselves as well, thereby binding the message to both sender and addressee.
The TRP should check this component before assisting law-enforcement, and
monitors could check on compliance. Incidentally, this feature can also solve a
similar problem in TIS-CKE and in the US Key Management Infrastructure
initiative. In the latter, it also overcomes the problem of international
communications: the KEA has got the private key of the sender and can therefore
retrieve the session key. Thus, binding cryptography can also benefit other
proposals.

The Organizational Perspective
We will now explain how
we envision the framework in which the binding concept could present an
acceptable security tool in the information society.

Initiated by an intergovernmental organization (e.g., the OECD or the
European Union), agreements are made with as many governments as possible on the
development of an international PKI usable for both integrity and
confidentiality purposes. International cooperation is established to develop a
PKI. As pointed out in Section 3, governments have good reasons to develop an
international PKI anyhow.

Being the developers of this PKI, governments can determine the standards the
PKI will support and the ways the PKI may be used. With respect to
confidentiality, the infrastructure should support only the use of public-key
encryption systems that incorporate binding data; acceptance of new systems must
be agreed within the governmental cooperation. The PKI has the four following
players:

Users, i.e., governments, businesses, and citizens,

TTPs offering trusted services (e.g., time-stamping and certification of
public keys),

TRPs aiding law-enforcement agencies with decrypting legally intercepted
messages,

Monitors, monitoring communications encrypted via the PKI on compliance
with binding regulations.

In a dialogue with citizen and business organizations, every cooperating
country will regulate on its own, to the extent desired, the following points.

1. General legislation on PKIs: the use and responsibilities concerning
digital signatures, certifying authorities etc.

2. Whether virtually addressing keys to domestic or foreign TRPs for domestic
or international encrypted communications is obligatory for participation in the
PKI, and if so, to what extent the splitting variant of fair cryptography may be
used.

3. The conditions under which TTPs and TRPs may be set up, the conditions
under which the roles of TTPs and TRPs may be combined, and the conditions under
which TRPs have to cooperate with law-enforcement agencies.

4. Who plays the role of monitor, and what are the sanctions for abuse, e.g.,
blocking throughput, administrative fining and/or reporting to the judiciary.
This part can be performed by network operators such as PTTs, but also by
Internet Service Providers, who increasingly provide value-added services.
Complying with this part could be a legal requirement for becoming a network or
service provider. Moreover, the monitors could be monitored themselves by random
checks by governments. Recall that for the monitor role, only access to the
communications is necessary, not any additional (secret) information. The
monitoring could likely be automated to a large extent.

5. The prohibition of the use, manufacture or trade of software/ hardware
that use the PKI (in particular, the public-key certificates issued by
the PKI TTPs), but do not comply with the binding rules of the PKI. This
discourages, for example, a version of PGP that makes use of the PKI without
complying to the binding rules. A complying version of PGP, which can be easily
made, would be readily accepted.

6. The formulation of additional rules on cryptography that does not use the
PKI and its binding policy. There could be a ban on (other) types of encryption,
or on specific (illegal) use of it. We feel, however, that one should not ban
certain types or use of cryptography, as this could infringe upon constitutional
rights and would likely disproportionately harm the beneficent use of
cryptography.

A user who wants to use the PKI for securing his communication must comply
with the binding regulations of both his own country and the country he is
communicating with (provided, of course, that that country is a party to the
international agreement).

Evaluation of the Binding Alternative
The binding
alternative offers an compatible solution for both integrity and confidentiality
(criterion a), gives sufficient and scalable strength of security (criterion c),
and all details are public (criterion d). Also, the concept allows cooperating
countries to implement different cryptography policies on the domestic and
international use of encryption in a coherent way. After all, the international
treaty ensures that each country accepts the foreign policies for TRPs, and so
each country will require its citizens to comply with the virtually addressing
of session keys to the TRPs in the country they are communicating with; domestic
monitors can check this. Therefore, the concept is in principle usable worldwide
(criterion b), which is a significant improvement over TIS-CKE. Moreover, the
concept (if supported by legislation) gives a flexible choice of TRPs (criterion
i). The latter choice can, in principle, be changed instantaneously, so that
session keys for different types of confidential messages (financial, work or
personal) can be virtually addressed to different TRPs. Moreover, the concept
supports two ways of enforcing that two or more TRPs are needed to retrieve
information. By construction, the principle can follow any new development
concerning the use of session keys, and any new developments in public-key
encryption that can incorporate binding data (criterion j). The concept does not
require expensive or complicated implementations (criterion k). Indeed, given
the US administration's statement of 1 October 1996, crypto industry could
incorporate the data recovery potential of binding cryptography to profit from
the proposed export relaxation.

The concept does not frustrate the lawful interception of communications that
use the PKI. Abuse of the concept (i.e., not complying with the binding rules)
can be made difficult by checks in supporting software/hardware; it can also be
discouraged (e.g., by fining) and can be easily detected by third parties
(criteria e and f) without their accessing the information content. As
organizations can simply create their own TRPs (in fact, creating a public key
suffices), the principle also supports (voluntary) data-recovery for
organizations; moreover, organizations can easily check outgoing messages for
compliance with data recovery rules (criterion l). The concept is incorporated
in an organizational, legal framework (criterion g). Finally, we believe that
the concept takes sufficient account of constitutional and fundamental rights
(criterion h), since the monitoring of compliance leaves the confidentiality of
communications unharmed.

8. Conclusion
Governments should stimulate the
establishment of a security infrastructure that protects and is accepted by
their citizens, but which does not facilitate criminal or subversive activities.
Such an infrastructure, for safeguarding both information integrity and
confidentiality, will likely be based on public-key encryption. We think that
governments have good reasons to develop the accompanying public-key
infrastructure (PKI). We have presented an inventory of criteria such an
infrastructure should meet, and we have observed that current proposals for
key-escrow fail to take sufficient account of these criteria. We have proposed
an alternative solution for the establishment of a PKI that does not hamper
law-enforcement, using binding data, which at least to some extent
solves the problems of other proposals. It can be implemented using the ideas
and advantages of other proposals. Special properties of the binding alternative
include giving users in principle a flexible choice on who to trust with their
confidential communication; moreover, there need be no vulnerable parties
holding (master) keys in deposit. Also, abuse of the system is not only
difficult but also detectible by third parties (e.g., network or service
providers) without harming the privacy of law-abiding users.

In our opinion, the properties of the binding alternative are flexible enough
to allow cooperating countries to implement different cryptography policies on
the domestic and international use of encryption in a coherent way, which will
be acceptable to all citizens in the information society. In fact, we believe
the binding PKI can serve well for the EU's proposal on European Trusted
Services.

Bibliography

[BelR] M. Bellare, R.L. Rivest, Translucent Cryptography. An
Alternative to Key Escrow, and its Implementation via Fractional Oblivious
Transfer, see http://theory.lcs.mit.edu/~rivest

[Blaze] M. Blaze, 'Protocol Failure in the Escrowed Encryption Standard',
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[Denn] D.E. Denning's Descriptions of Key Escrow Systems, http://www.cosc.georgetown.edu/~denning/crypto/Appendix.html

[DSS] Proposed Federal Information Processing Standard for Digital
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[ETS] Proposal for a Council Decision in the field of security of
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[Frank] Y. Frankel, M. Yung, 'Escrow Encryption Systems Visited: Attacks,
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[Holl] N. Jefferies, C. Mitchell, M. Walker, 'A Proposed Architecture for
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98-104.

[IWG] US Interagency Working Group on Cryptographic Policy, Enabling
Privacy, Commerce, Security and Public Safety in the Global Information
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[Koops] B.J. Koops, 'A
survey of cryptography laws and regulations', Computer Law and Security
Report, November/December 1996, pp. 349-355.

[LWY] A.K. Lenstra, P. Winkler, Y. Yacobi, 'A key escrow system with
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Springer-Verlag, 1995, pp.197-207.

[Mica] S. Micali, 'Fair Public-key Cryptosystems', Advances in
Cryptology - CRYPTO '92 Proceedings, Springer-Verlag, 1993, pp. 113-138.

[Schn] B. Schneier, Applied Cryptography, Protocols, Algorithms and
Source Code in C, Second Edition, John Wiley and Sons, 1996.

[TIS] D.M. Balenson et al., (TIS Inc.), 'A New Approach to Software
Escrow Encryption', in: L.J. Hoffman (ed.), Building in Big Brother
(Springer-Verlag, NY, 1996), pp. 180-207. See also http://www.tis.com/.

[VT] E.R. Verheul, H.C.A. van Tilborg, Binding ElGamal: A
fraud-detectable alternative to key-escrow solutions, submitted to
Eurocrypt '97. A summary
is available online.

[W3] http://cwis.kub.nl/~frw/people/koops/binding.htm.

Eric Verheul, Ministry of the Interior, P.O. Box 20010, 2500
EA The Hague, Netherlands, e-mail: [email-masked]

Bert-Jaap Koops, Centre for Law, Administration and Informatisation, Tilburg
University, P.O. Box 90153, 5000 LE Tilburg, Netherlands, e-mail: [email-masked], WWW: http://cwis.kub.nl/~frw/people/koops/bertjaap.htm

Henk van Tilborg, Department of Mathematics and Computing Science, Eindhoven
University of Technology, P.O. Box 513, 5600 MB Eindhoven, Netherlands, e-mail:
[email-masked]

About the authors
Eric Verheul is working on
information security, both policy and practice; the views expressed in this
paper are his own and are not necessarily in accordance with those of his
employer. Bert-Jaap Koops is a Ph.D. student at Tilburg University and Eindhoven
University of Technology, researching judicial and private concerns with respect
to cryptography. Henk van Tilborg is Professor of Discrete Mathematics at
Eindhoven University of Technology. His main fields of research are coding
theory and cryptology.