9,723 research outputs found
Secret Communication over Broadcast Erasure Channels with State-feedback
We consider a 1-to- communication scenario, where a source transmits
private messages to receivers through a broadcast erasure channel, and the
receivers feed back strictly causally and publicly their channel states after
each transmission. We explore the achievable rate region when we require that
the message to each receiver remains secret - in the information theoretical
sense - from all the other receivers. We characterize the capacity of secure
communication in all the cases where the capacity of the 1-to- communication
scenario without the requirement of security is known. As a special case, we
characterize the secret-message capacity of a single receiver point-to-point
erasure channel with public state-feedback in the presence of a passive
eavesdropper.
We find that in all cases where we have an exact characterization, we can
achieve the capacity by using linear complexity two-phase schemes: in the first
phase we create appropriate secret keys, and in the second phase we use them to
encrypt each message. We find that the amount of key we need is smaller than
the size of the message, and equal to the amount of encrypted message the
potential eavesdroppers jointly collect. Moreover, we prove that a dishonest
receiver that provides deceptive feedback cannot diminish the rate experienced
by the honest receivers.
We also develop a converse proof which reflects the two-phase structure of
our achievability scheme. As a side result, our technique leads to a new outer
bound proof for the non-secure communication problem
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
Provably-secure symmetric private information retrieval with quantum cryptography
Private information retrieval (PIR) is a database query protocol that
provides user privacy, in that the user can learn a particular entry of the
database of his interest but his query would be hidden from the data centre.
Symmetric private information retrieval (SPIR) takes PIR further by
additionally offering database privacy, where the user cannot learn any
additional entries of the database. Unconditionally secure SPIR solutions with
multiple databases are known classically, but are unrealistic because they
require long shared secret keys between the parties for secure communication
and shared randomness in the protocol. Here, we propose using quantum key
distribution (QKD) instead for a practical implementation, which can realise
both the secure communication and shared randomness requirements. We prove that
QKD maintains the security of the SPIR protocol and that it is also secure
against any external eavesdropper. We also show how such a classical-quantum
system could be implemented practically, using the example of a two-database
SPIR protocol with keys generated by measurement device-independent QKD.
Through key rate calculations, we show that such an implementation is feasible
at the metropolitan level with current QKD technology.Comment: 19 page
Experimentally realizable quantum comparison of coherent states and its applications
When comparing quantum states to each other, it is possible to obtain an
unambiguous answer, indicating that the states are definitely different,
already after a single measurement. In this paper we investigate comparison of
coherent states, which is the simplest example of quantum state comparison for
continuous variables. The method we present has a high success probability, and
is experimentally feasible to realize as the only required components are beam
splitters and photon detectors. An easily realizable method for quantum state
comparison could be important for real applications. As examples of such
applications we present a "lock and key" scheme and a simple scheme for quantum
public key distribution.Comment: 14 pages, 5 figures, version one submitted to PRA. Version two is the
final accepted versio
A Tight High-Order Entropic Quantum Uncertainty Relation With Applications
We derive a new entropic quantum uncertainty relation involving min-entropy.
The relation is tight and can be applied in various quantum-cryptographic
settings.
Protocols for quantum 1-out-of-2 Oblivious Transfer and quantum Bit
Commitment are presented and the uncertainty relation is used to prove the
security of these protocols in the bounded quantum-storage model according to
new strong security definitions.
As another application, we consider the realistic setting of Quantum Key
Distribution (QKD) against quantum-memory-bounded eavesdroppers. The
uncertainty relation allows to prove the security of QKD protocols in this
setting while tolerating considerably higher error rates compared to the
standard model with unbounded adversaries. For instance, for the six-state
protocol with one-way communication, a bit-flip error rate of up to 17% can be
tolerated (compared to 13% in the standard model).
Our uncertainty relation also yields a lower bound on the min-entropy key
uncertainty against known-plaintext attacks when quantum ciphers are composed.
Previously, the key uncertainty of these ciphers was only known with respect to
Shannon entropy.Comment: 21 pages; editorial changes, additional applicatio
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