## All papers in 1998 (26 results)

Comparing Entropies in Statistical Zero-Knowledge with Applications to the Structure of SZK

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We consider the following (promise) problem, denoted ED (for Entropy
Difference): The input is a pairs of circuits, and YES instances
(resp., NO instances) are such pairs in which the first (resp.,
second) circuit generates a distribution with noticeably higher
entropy.
On one hand we show that any language having a (honest-verifier)
statistical zero-knowledge proof is Karp-reducible to ED. On the other
hand, we present a public-coin (honest-verifier) statistical
zero-knowledge proof for ED. Thus, we obtain an alternative proof of
Okamoto's result by which HVSZK (i.e., Honest-Verifier Statistical
Zero-Knowledge) equals public-coin HVSZK. The new proof is much simpler
than the original one. The above also yields a trivial proof that HVSZK
is closed under complementation (since ED easily reduces to its
complement). Among the new results obtained is an equivalence of a weak
notion of statistical zero-knowledge to the standard one.

Secure Distributed Storage and Retrieval

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In his well-known Information Dispersal Algorithm paper, Rabin showed
a way to distribute information in n pieces among n servers in such a
way that recovery of the information is possible in the presence of up
to t inactive servers. An enhanced mechanism to enable construction
in the presence of malicious faults, which can intentionally modify
their pieces of the information, was later presented by Krawczyk.
Yet, these methods assume that the malicious faults occur only at
reconstruction time. <P>
In this paper we address the more general problem of secure storage
and retrieval of information (SSRI), and guarantee that also the
process of storing the information is correct even when some of the
servers fail. Our protocols achieve this while maintaining the
(asymptotical) space optimality of the above methods. <P>
We also consider SSRI with the added requirement of confidentiality,
by which no party except for the rightful owner of the information is
able to learn anything about it. This is achieved through novel
applications of cryptographic techniques, such as the distributed
generation of receipts, distributed key management via threshold
cryptography, and ``blinding.'' <P>
An interesting byproduct of our scheme is the construction of a secret
sharing scheme with shorter shares size in the amortized sense. An
immediate practical application of our work is a system for the secure
deposit of sensitive data. We also extend SSRI to a ``proactive''
setting, where an adversary may corrupt all the servers during the
lifetime of the system, but only a fraction during any given time
interval.

The Disparity between Work and Entropy in Cryptology

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A brief theory of work is developed. In it, the work-factor and
guesswork of a random variable are linked to intuitive notions of time
complexity in a brute-force attack. Bounds are given for a specific
work-factor called the minimum majority. Tight bounds are given for
the guesswork in terms of variation distance. Differences between
work-factor, guesswork and the entropy of a random variable are
pointed out, calling into question a common misconception about
entropy indicating work.

Security amplification by composition: The case of doubly-iterated, ideal ciphers

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We investigate, in the Shannon model, the security of constructions
corresponding to double and (two-key) triple DES. That is, we
consider F<sub>k1</sub>(F<sub>k2</sub>(.)) and
F<sub>k1</sub>(F<sub>k2</sub><sup>-1</sup>(F<sub>k1</sub>(.))) with
the component functions being ideal ciphers. This models the
resistance of these constructions to ``generic'' attacks like meet
in the middle attacks.
We obtain the first proof that composition actually
increases the security in some meaningful sense. We compute a bound
on the probability of breaking the double cipher as a function of
the number of computations of the base cipher made, and the number
of examples of the composed cipher seen, and show that the success
probability is the square of that for a single key cipher. The
same bound holds for the two-key triple cipher. The first bound
is tight and shows that meet in the middle is the best possible
generic attack against the double cipher.

Insecurity of Quantum Computations

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It had been widely claimed that quantum mechanics can protect private
information during public decision in for example the so-called
two-party secure computation. If this were the case, quantum
smart-cards could prevent fake teller machines from learning the PIN
(Personal Identification Number) from the customers' input. Although
such optimism has been challenged by the recent surprising discovery
of the insecurity of the so-called quantum bit commitment, the
security of quantum two-party computation itself remains unaddressed.
Here I answer this question directly by showing that all *one-sided*
two-party computations (which allow only one of the two parties to
learn the result) are necessarily insecure. As corollaries to my
results, quantum one-way oblivious password identification and the
so-called quantum one-out-of-two oblivious transfer are impossible. I
also construct a class of functions that cannot be computed securely
in any <i>two-sided</i> two-party computation. Nevertheless, quantum
cryptography remains useful in key distribution and can still provide
partial security in ``quantum money'' proposed by Wiesner.

Relations among Notions of Security for Public-Key Encryption Schemes

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We compare the relative strengths of popular notions of security for
public key encryption schemes. We consider the goals of
indistinguishability and non-malleability, each under chosen plaintext
attack and two kinds of chosen ciphertext attack. For each of the
resulting pairs of definitions we prove either an implication (every
scheme meeting one notion must meet the other) or a separation (there
is a scheme meeting one notion but not the other, assuming the first
notion can be met at all). We similarly treat plaintext awareness, a
notion of security in the random oracle model. An additional
contribution of this paper is a new definition of non-malleability
which we believe is simpler than the previous one.

Almost All Discrete Log Bits Are Simultaneously Secure

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Let G be a finite cyclic group with generator \alpha and with an
encoding so that multiplication is computable in polynomial time. We
study the security of bits of the discrete log x when given
exp<sub>\alpha</sub>(x), assuming that the exponentiation function
exp<sub>\alpha</sub>(x) = \alpha<sup>x</sup> is one-way. We reduce the
general problem to the case that G has odd order q. If G has odd order
q the security of the least-significant bits of x and of the most
significant bits of the rational number x/q \in [0,1) follows from the
work of Peralta [P85] and Long and Wigderson [LW88]. We generalize
these bits and study the security of consecutive <i> shift bits</i>
lsb(2<sup>-i</sup>x mod q) for i=k+1,...,k+j. When we restrict
exp<sub>\alpha</sub> to arguments x such that some sequence of j
consecutive shift bits of x is constant (i.e., not depending on x) we
call it a 2<sup>-j</sup>-<i>fraction</i> of exp<sub>\alpha</sub>.
For groups of odd group order q we show that every two
2<sup>-j</sup>-fractions of exp<sub>\alpha</sub> are equally one-way
by a polynomial time transformation: Either they are all one-way or
none of them. Our <i> key theorem</i> shows that arbitrary j
consecutive shift bits of x are simultaneously secure when given
exp<sub>\alpha</sub>(x) iff the 2<sup>-j</sup>-fractions of
exp<sub>\alpha</sub> are one-way. In particular this applies to the j
least-significant bits of x and to the j most-significant bits of x/q
\in [0,1). For one-way exp<sub>\alpha</sub> the individual bits of x
are secure when given exp<sub>\alpha</sub>(x) by the method of Hastad,
Naslund [HN98]. For groups of even order 2<sup>s</sup>q we show that
the j least-significant bits of \lfloor x/2<sup>s</sup>\rfloor, as
well as the j most-significant bits of x/q \in [0,1), are
simultaneously secure iff the 2<sup>-j</sup>-fractions of
exp<sub>\alpha'</sub> are one-way for \alpha' := \alpha<sup>2<sup>s</sup>
</sup>.
We use and extend the models of generic algorithms of Nechaev (1994)
and Shoup (1997). We determine the generic complexity of inverting
fractions of exp<sub>\alpha</sub> for the case that \alpha has prime
order q. As a consequence, arbitrary segments of (1-\varepsilon)\lg q
consecutive shift bits of random x are for constant \varepsilon >0
simultaneously secure against generic attacks. Every generic algorithm
using t generic steps (group operations) for distinguishing bit
strings of j consecutive shift bits of x from random bit strings has
at most advantage O((\lg q)j\sqrt{t} (2<sup>j</sup>/q)<sup>1/4</sup>).

Many-to-one Trapdoor Functions and their Relation to Public-key Cryptosystems

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The heart of the task of building public key cryptosystems is viewed
as that of ``making trapdoors;'' in fact, public key cryptosystems and
trapdoor functions are often discussed as synonymous. How accurate is
this view? In this paper we endeavor to get a better understanding of
the nature of ``trapdoorness'' and its relation to public key
cryptosystems, by broadening the scope of the investigation: we look
at general trapdoor functions; that is, functions that are not
necessarily injective (ie., one-to-one). Our first result is somewhat
surprising: we show that non-injective trapdoor functions (with
super-polynomial pre-image size) can be constructed {from} any one-way
function (and hence it is unlikely that they suffice for public key
encryption). On the other hand, we show that trapdoor functions with
polynomial pre-image size are sufficient for public key encryption.
Together, these two results indicate that the pre-image size is a
fundamental parameter of trapdoor functions. We then turn our
attention to the converse, asking what kinds of trapdoor functions can
be constructed from public key cryptosystems. We take a first step by
showing that in the random-oracle model one can construct injective
trapdoor functions from any public key cryptosystem.

Security and Composition of Multi-party Cryptographic Protocols

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We present general definitions of security for multiparty cryptographic
protocols and show that, using these definitions, security is preserved
under a natural composition method.
The definitions follow the general paradigm of known definitions;
yet some substantial modifications and simplifications are introduced. In
particular, `black-box simulation' is no longer required. The composition
method is essentially the natural `subroutine substitution' method suggested
by Micali and Rogaway.
We first present the general definitional approach. Next we consider several
settings for multiparty protocols. These include the cases of non-adaptive
and adaptive adversaries, as well as the information-theoretic and the
computational models.

Making An Empty Promise With A Quantum Computer (Or, A Brief Review on the Impossibility of Quantum Bit Commitment)

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Alice has made a decision in her mind.
While she does not want to reveal it to
Bob at this moment, she would like to convince Bob that she is committed to
this particular decision and that she cannot change it at a later time. Is
there a way for Alice to get Bob's trust? Until recently, researchers had
believed that the above task can be performed with the help of quantum
mechanics. And the security of the quantum scheme lies on the uncertainty
principle. Nevertheless, such optimism was recently shattered by Mayers and by
us, who found that Alice can always change her mind if she has a quantum
computer. Here, we survey this dramatic development and its implications on
the security of other quantum cryptographic schemes.

Last updated: 1999-01-01

Quantum Computers Render Quantum Key Distribution Unconditionally Secure Over Arbitrarily Long Distances

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Abstract: Quantum cryptography has long been claimed to be useful for
achieving many tasks that are impossible from the perspective of
conventional cryptography. Arguably, the most important problem
in quantum cryptography has been a rigorous proof of the security of
quantum key distribution, the most well-known application.
This notoriously hard problem has eluded researchers for years and has
become even more important after the recent surprising demonstration
of the insecurity of many other quantum cryptographic schemes including
quantum bit commitment. Here, we solve this long standing problem by
showing that, given quantum computers, quantum key distribution over an
arbitrarily long distance of a realistic noisy channel can be made
unconditionally secure. The novel technique we use is reduction from a
quantum scheme to a classical scheme. The security in realistic noisy
environments is then proven by using the recent theory of fault-tolerant
quantum computation.

More on Proofs of Knowledge

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The notion of proofs of knowledge is central to cryptographic
protocols, and many definitions for it have been proposed. In this work
we explore a different facet of this notion, not addressed by prior
definitions. Specifically, prior definitions concentrate on capturing
the properties of the verifier, and do not pay much attention to the
properties of the prover.
Our new definition is strictly stronger than previous ones, and captures
new and desirable properties. In particular, it guarantees prover
feasibility, that is, it guarantees that the time spent by the prover
in a proof of knowledge is comparable to that it spends in an "extraction"
of this knowledge. Our definition also enables one to consider meaningfully
the case of a single, specific prover.

Randomness versus Fault-Tolerance

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We investigate the relations between two major requirements of multiparty
protocols: {\em fault tolerance} (or {\em resilience}) and {\em randomness}.
Fault-tolerance is measured in terms of the maximum number of colluding faulty
parties, t, that a protocol can withstand and still maintain the privacy of the inputs and the correctness of the outputs (of the honest parties). Randomness
is measured in terms of the total number of random bits needed by the parties
in order to execute the protocol.
Previously, the upper bound on the amount of randomness required by general
constructions for securely computing any non-trivial function f was polynomial
both in $n$, the total number of parties, and the circuit-size C(f). This was
the state of knowledge even for the special case t=1 (i.e., when there is at
most one faulty party). In this paper, we show that for any linear-size
circuit, and for any number t < n/3 of faulty parties, O(poly(t) * log n)
randomness is sufficient. More generally, we show that for any function f
with circuit-size C(f), we need only O(poly(t) * log n + poly(t) * C(f)/n)
randomness in order to withstand any coalition of size at most t.
Furthermore, in our protocol only t+1 parties flip coins and the rest of
the parties are deterministic. Our results generalize to the case of adaptive
adversaries as well.

A Random Server Model for Private Information Retrieval (or How to Achieve Information Theoretic PIR Avoiding Data Replication)

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Private information retrieval (PIR) schemes enable users to obtain
information from databases while keeping their queries secret from the
database managers. We propose a new model for PIR, utilizing
auxiliary random servers to provide privacy services for database
access. In this model, prior to any on-line communication where users
request queries, the database engages in an initial preprocessing
setup stage with the random servers. Using this model we achieve the
first PIR information theoretic solution in which the database does
not need to give away its data to be replicated, and with minimal
on-line computation cost for the database. This solves privacy and
efficiency problems inherent to all previous solutions.
In particular, all previous information theoretic PIR schemes required
multiple replications of the database into separate entities which are
not allowed to communicate with each other; and in all previous
schemes (including ones which do not achieve information theoretic
security), the amount of computation performed by the database on-line
for every query is at least linear in the size of the database.
In contrast, in our solutions the database does not give away its
contents to any other entity; and after the initial setup stage, which
costs at most O(n log n) in computation, the database needs to
perform only O(1) amount of computation to answer questions of users
on-line. All the extra on-line computation is done by the auxiliary
random servers.

Maintaining Authenticated Communication in the Presence of Break-ins

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We study the problem of maintaining authenticated communication over untrusted
communication channels, in a scenario where the communicating parties may be
occasionally and repeatedly broken into for transient periods of time. Once
a party is broken into, its cryptographic keys are exposed and perhaps
modified. Yet, we want parties whose security is thus compromised to regain
their ability to communicate in an authenticated way aided by other parties.
In this work we present a mathematical model for this highly adversarial
setting, exhibiting salient properties and parameters, and then describe
a practically-appealing protocol for the task of maintaining authenticated
communication in this model.
A key element in our solution is devising {\em proactive distributed signature
(PDS) schemes} in our model. Although PDS schemes are known in the literature,
they are all designed for a model where authenticated communication and
broadcast primitives are available. We therefore show how these schemes can be
modified to work in our model, where no such primitives are available a-priori.
In the process of devising the above schemes, we also present a new definition
of PDS schemes (and of distributed signature schemes in general). This
definition may be of independent interest.

The Random Oracle Methodology, Revisited

We take a critical look at the relationship between the security of
cryptographic schemes in the Random Oracle Model, and the security of the
schemes that result from implementing the random oracle by so called
"cryptographic hash functions".
The main result of this paper is a negative one: There exist signature and
encryption schemes that are secure in the Random Oracle Model, but for which
any implementation of the random oracle results in insecure schemes.
In the process of devising the above schemes, we consider possible definitions
for the notion of a "good implementation" of a random oracle, pointing out
limitations and challenges.

Chameleon Hashing and Signatures

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We introduce CHAMELEON SIGNATURES that provide with an undeniable
commitment of the signer to the contents of the signed document (as regular
digital signatures do) but, at the same time, do not allow the recipient
of the signature to disclose the contents of the signed information to any
third party without the signer's consent. These signatures are closely
related to Chaum's "undeniable signatures", but chameleon signatures allow
for simpler and more efficient realizations than the latter.
In particular, they are essentially non-interactive and do not involve the
design and complexity of zero-knowledge proofs on which traditional undeniable
signatures are based. Instead, chameleon signatures are generated
under the standard method of hash-then-sign. Yet, the hash functions
which are used are CHAMELEON HASH FUNCTIONS. These hash functions are
characterized by the non-standard property of being collision-resistant
for the signer but collision tractable for the recipient.
We present simple and efficient constructions of chameleon hashing and
chameleon signatures. The former can be constructed based on standard
cryptographic assumptions (such as the hardness of factoring or discrete
logarithms) and have efficient realizations based on these assumptions.
For the signature part we can use any digital signature (such as RSA or DSS)
and prove the unforgeability property of the resultant chameleon signatures
solely based on the unforgeability of the underlying digital signature
in use.

A Modular Approach to the Design and Analysis of Authentication and Key Exchange Protocols

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We present a general framework for constructing and analyzing authentication
protocols in realistic models of communication networks. This framework
provides a sound formalization for the authentication problem and suggests
simple and attractive design principles for general authentication and key
exchange protocols. The key element in our approach is a modular treatment of
the authentication problem in cryptographic protocols; this applies to the
definition of security, to the design of the protocols, and to their analysis.
In particular, following this modular approach, we show how to systematically
transform solutions that work in a model of idealized authenticated
communications into solutions that are secure in the realistic setting of
communication channels controlled by an active adversary.
Using these principles we construct and prove the security of simple and
practical authentication and key-exchange protocols. In particular, we provide
a security analysis of some well-known key exchange protocols (e.g.
authenticated Diffie-Hellman key exchange), and of some of the techniques
underlying the design of several authentication protocols that are currently
being deployed on a large scale for the Internet Protocol and other
applications.

An Efficient Non-Interactive Statistical Zero-Knowledge Proof System for Quasi-Safe Prime Products

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We present efficient zero-knowledge proof systems for quasi-safe
prime products and other related languages. Quasi-safe primes are a
relaxation of safe primes, a class of prime numbers useful in many
cryptographic applications.
Our proof systems achieve higher security and better
efficiency than all previously known ones.
In particular, all our proof systems are perfect or statistical
zero-knowledge, meaning that even a computationally
unbounded adversary cannot extract any information
from the proofs.
Moreover, our proof systems are extremely efficient because they do
not use general reductions to NP-complete problems, can be easily
parallelized preserving zero-knowledge, and are non-interactive for
computationally unbounded provers. The prover can also be efficiently
implemented given some trapdoor information and using very little
interaction.
We demonstrate the applicability of quasi-safe primes
by showing how they can be effectively used in the
context of RSA based undeniable signatures to enforce
the use of ``good'' public keys, i.e.,
keys such that if a signer can convince a recipient
of the validity of a signature, then he won't be able to
subsequently deny the same signature in case of a dispute.

Fast Batch Verification for Modular Exponentiation and Digital Signatures

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Many tasks in cryptography (e.g., digital signature
verification) call for verification of a basic operation like modular
exponentiation in some group: given (g,x,y) check that g<sup>x</sup>=y.
This is typically done by re-computing g<sup>x</sup> and checking we get y.
We would like to do it differently, and faster.
The approach we use is batching. Focusing first on the basic modular
exponentiation operation, we provide some probabilistic batch verifiers, or
tests, that verify a sequence of modular exponentiations significantly faster
than the naive re-computation method. This yields speedups for several
verification tasks that involve modular exponentiations.
Focusing specifically on digital signatures, we then suggest a weaker notion of
(batch) verification which we call ``screening.'' It seems useful for many
usages of signatures, and has the advantage that it can be done very fast;
in particular, we show how to screen a sequence of RSA signatures at the
cost of one RSA verification plus hashing.

A Practical Public Key Cryptosystem Provably Secure against Adaptive Chosen Ciphertext Attack

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A new public key cryptosystem is presented that is provably secure
against adaptive chosen ciphertext attack.
The scheme is quite practical, and the proof of security relies
only on standard intractability assumptions.

On the possibility of basing Cryptography on the assumption that $P \neq NP$

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Recent works by Ajtai and by Ajtai and Dwork
bring to light the old (general) question of whether it is
at all possible to base the
security of cryptosystems on the assumption that $\P\neq\NP$.
We discuss this question and in particular review and extend
a two-decade old result of Brassard regarding this question.
Our conclusion is that the question remains open.

Universal Service Providers for Database Private Information Retrieval

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We consider the question of private information retrieval in the
so-called ``commodity-based'' model. This model was recently proposed
by Beaver for practically-oriented service-provider internet
applications. In this paper, we show the following, somewhat
surprising, results regarding this model for the problem of
private-information retrieval: (1) the service-provider model allows
to dramatically reduce the overall communication involving the user,
using off-line pre-processing messages from ``service-providers'' to
databases, where the service-providers do not need to know the
database contents, nor the future user's requests; (2) our
service-provider solutions are resilient against more than a majority
(in fact, all-but-one) coalitions of service-providers; and (3) these
results hold for {\em both} the computational and the
information-theoretic setting.
More specifically, we exhibit a service-provider algorithm which can
``sell'' (i.e., generate and send) ``commodities'' to users and
databases, that subsequently allow users to retrieve database contents
in a way which hides from the database which particular item the user
retrieves. The service-providers need not know anything about the
contents of the databases nor the nature of the user's requests in
order to generate commodities. Our commodity-based solution
significantly improves communication complexity of the users (i.e.,
counting both the size of commodities bought by the user from the
service-providers and the subsequent communication with the databases)
compared to all previously known on-line private information retrieval
protocols (i.e., without the help of the service-providers).
Moreover, we show how commodities from different service-providers can
be {\em combined} in such a way that even if ``all-but-one'' of the
service-providers collude with the database, the user's privacy
remains intact. Finally, we show how to re-use commodities in case of
multiple requests (i.e., in the amortized sense), how to ``check''
commodity-correctness, and how some of the solutions can be extended
to the related problem of {\em Private Information Storage}.

Private Information Retrieval by Keywords

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Private information retrieval (PIR) schemes enable a user to
access one or more servers that hold copies of
a database and {\em privately} retrieve parts of the $n$
bits of data stored in the database. This means that the queries
give each individual
database no partial information (in the information theoretic or computational
sense) on the identity of the item retrieved by the user.
All known PIR schemes assume that the user knows the {\em physical address}
of the sought item. This is usually not the case when accessing a public
database that is not managed by the user. Such databases are typically
presented with keywords, which are then internally translated (at the
database end) to physical addresses, using an appropriate search
structure (for example, a hash table or a binary tree). In this note we
describe a simple, modular way to privately access data by keywords.
It combines {\em any} conventional search structure with {\em any}
underlying PIR scheme (including single server schemes). The transformation
requires no modification in the way that the search structure is maintained.
Therefore the same database will support both private and regular (non
private) searches.

The Graph Clustering Problem has a Perfect Zero-Knowledge Proof

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The input to the Graph Clustering Problem
consists of a sequence of integers $m_1,...,m_t$
and a sequence of $\sum_{i=1}^{t}m_i$ graphs.
The question is whether the equivalence classes,
under the graph isomorphism relation,
of the input graphs have sizes which match the input sequence of integers.
In this note we show that this problem has a (perfect) zero-knowledge
interactive proof system.
This result improves over <a href="http:../1996/96-14.html">record 96-14</a>,
where a parametrized (by the sequence of integers)
version of the problem was studied.

On Protocol Divertibility

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In this paper, we establish the notion of divertibility as a
protocol property
as opposed to the existing notion as a language property (see Okamoto,
Ohta). We give a definition of protocol divertibility that applies to
arbitrary 2-party protocols and is compatible with Okamoto and Ohta's
definition
in the case of interactive zero-knowledge proofs. Other important examples
falling under the new definition are blind signature protocols. A sufficient
criterion for divertibility is presented and found to be satisfied by many
examples of protocols in the literature. The generality of the definition is
further demonstrated by examples from protocol classes that have not been
considered for divertibility before. We show diverted El-Gamal encryption and
diverted Diffie-Hellman key exchange.