70,862 research outputs found
Increasing the power of the verifier in Quantum Zero Knowledge
In quantum zero knowledge, the assumption was made that the verifier is only
using unitary operations. Under this assumption, many nice properties have been
shown about quantum zero knowledge, including the fact that Honest-Verifier
Quantum Statistical Zero Knowledge (HVQSZK) is equal to Cheating-Verifier
Quantum Statistical Zero Knowledge (QSZK) (see [Wat02,Wat06]).
In this paper, we study what happens when we allow an honest verifier to flip
some coins in addition to using unitary operations. Flipping a coin is a
non-unitary operation but doesn't seem at first to enhance the cheating
possibilities of the verifier since a classical honest verifier can flip coins.
In this setting, we show an unexpected result: any classical Interactive Proof
has an Honest-Verifier Quantum Statistical Zero Knowledge proof with coins.
Note that in the classical case, honest verifier SZK is no more powerful than
SZK and hence it is not believed to contain even NP. On the other hand, in the
case of cheating verifiers, we show that Quantum Statistical Zero Knowledge
where the verifier applies any non-unitary operation is equal to Quantum
Zero-Knowledge where the verifier uses only unitaries.
One can think of our results in two complementary ways. If we would like to
use the honest verifier model as a means to study the general model by taking
advantage of their equivalence, then it is imperative to use the unitary
definition without coins, since with the general one this equivalence is most
probably not true. On the other hand, if we would like to use quantum zero
knowledge protocols in a cryptographic scenario where the honest-but-curious
model is sufficient, then adding the unitary constraint severely decreases the
power of quantum zero knowledge protocols.Comment: 17 pages, 0 figures, to appear in FSTTCS'0
Cast-as-Intended Mechanism with Return Codes Based on PETs
We propose a method providing cast-as-intended verifiability for remote
electronic voting. The method is based on plaintext equivalence tests (PETs),
used to match the cast ballots against the pre-generated encrypted code tables.
Our solution provides an attractive balance of security and functional
properties. It is based on well-known cryptographic building blocks and relies
on standard cryptographic assumptions, which allows for relatively simple
security analysis. Our scheme is designed with a built-in fine-grained
distributed trust mechanism based on threshold decryption. It, finally, imposes
only very little additional computational burden on the voting platform, which
is especially important when voters use devices of restricted computational
power such as mobile phones. At the same time, the computational cost on the
server side is very reasonable and scales well with the increasing ballot size
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.Comment: 31 page
An Epistemic Approach to Coercion-Resistance for Electronic Voting Protocols
Coercion resistance is an important and one of the most intricate security
requirements of electronic voting protocols. Several definitions of coercion
resistance have been proposed in the literature, including definitions based on
symbolic models. However, existing definitions in such models are rather
restricted in their scope and quite complex.
In this paper, we therefore propose a new definition of coercion resistance
in a symbolic setting, based on an epistemic approach. Our definition is
relatively simple and intuitive. It allows for a fine-grained formulation of
coercion resistance and can be stated independently of a specific, symbolic
protocol and adversary model. As a proof of concept, we apply our definition to
three voting protocols. In particular, we carry out the first rigorous analysis
of the recently proposed Civitas system. We precisely identify those conditions
under which this system guarantees coercion resistance or fails to be coercion
resistant. We also analyze protocols proposed by Lee et al. and Okamoto.Comment: An extended version of a paper from IEEE Symposium on Security and
Privacy (S&P) 200
Dining Cryptographers with 0.924 Verifiable Collision Resolution
The dining cryptographers protocol implements a multiple access channel in
which senders and recipients are anonymous. A problem is that a malicious
participant can disrupt communication by deliberately creating collisions. We
propose a computationally secure dining cryptographers protocol with collision
resolution that achieves a maximum stable throughput of 0.924 messages per
round and which allows to easily detect disruptors.Comment: 11 pages, 3 figure
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