26 research outputs found
Tight bounds for classical and quantum coin flipping
Coin flipping is a cryptographic primitive for which strictly better
protocols exist if the players are not only allowed to exchange classical, but
also quantum messages. During the past few years, several results have appeared
which give a tight bound on the range of implementable unconditionally secure
coin flips, both in the classical as well as in the quantum setting and for
both weak as well as strong coin flipping. But the picture is still incomplete:
in the quantum setting, all results consider only protocols with perfect
correctness, and in the classical setting tight bounds for strong coin flipping
are still missing. We give a general definition of coin flipping which unifies
the notion of strong and weak coin flipping (it contains both of them as
special cases) and allows the honest players to abort with a certain
probability. We give tight bounds on the achievable range of parameters both in
the classical and in the quantum setting.Comment: 18 pages, 2 figures; v2: published versio
Simple, near-optimal quantum protocols for die-rolling
Die-rolling is the cryptographic task where two mistrustful, remote parties
wish to generate a random -sided die-roll over a communication channel.
Optimal quantum protocols for this task have been given by Aharon and Silman
(New Journal of Physics, 2010) but are based on optimal weak coin-flipping
protocols which are currently very complicated and not very well understood. In
this paper, we first present very simple classical protocols for die-rolling
which have decent (and sometimes optimal) security which is in stark contrast
to coin-flipping, bit-commitment, oblivious transfer, and many other two-party
cryptographic primitives. We also present quantum protocols based on
integer-commitment, a generalization of bit-commitment, where one wishes to
commit to an integer. We analyze these protocols using semidefinite programming
and finally give protocols which are very close to Kitaev's lower bound for any
. Lastly, we briefly discuss an application of this work to the
quantum state discrimination problem.Comment: v2. Updated titl
Multiparty Quantum Coin Flipping
We investigate coin-flipping protocols for multiple parties in a quantum
broadcast setting:
(1) We propose and motivate a definition for quantum broadcast. Our model of
quantum broadcast channel is new.
(2) We discovered that quantum broadcast is essentially a combination of
pairwise quantum channels and a classical broadcast channel. This is a somewhat
surprising conclusion, but helps us in both our lower and upper bounds.
(3) We provide tight upper and lower bounds on the optimal bias epsilon of a
coin which can be flipped by k parties of which exactly g parties are honest:
for any 1 <= g <= k, epsilon = 1/2 - Theta(g/k).
Thus, as long as a constant fraction of the players are honest, they can
prevent the coin from being fixed with at least a constant probability. This
result stands in sharp contrast with the classical setting, where no
non-trivial coin-flipping is possible when g <= k/2.Comment: v2: bounds now tight via new protocol; to appear at IEEE Conference
on Computational Complexity 200
Fidelity of Quantum Strategies with Applications to Cryptography
We introduce a definition of the fidelity function for multi-round quantum strategies, which we call the strategy fidelity, that is a generalization of the fidelity function for quantum states. We provide many interesting properties of the strategy fidelity including a Fuchs-van de Graaf relationship with the strategy norm. We illustrate an operational interpretation of the strategy fidelity in the spirit of Uhlmann\u27s Theorem and discuss its application to the security analysis of quantum protocols for interactive cryptographic tasks such as bit-commitment and oblivious string transfer. Our analysis is very general in the sense that the actions of the protocol need not be fully specified, which is in stark contrast to most other security proofs. Lastly, we provide a semidefinite programming formulation of the strategy fidelity
A large family of quantum weak coin-flipping protocols
Each classical public-coin protocol for coin flipping is naturally associated
with a quantum protocol for weak coin flipping. The quantum protocol is
obtained by replacing classical randomness with quantum entanglement and by
adding a cheat detection test in the last round that verifies the integrity of
this entanglement. The set of such protocols defines a family which contains
the protocol with bias 0.192 previously found by the author, as well as
protocols with bias as low as 1/6 described herein. The family is analyzed by
identifying a set of optimal protocols for every number of messages. In the
end, tight lower bounds for the bias are obtained which prove that 1/6 is
optimal for all protocols within the family.Comment: 17 pages, REVTeX 4 (minor corrections in v2
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Unconditionally secure relativistic multi-party biased coin flipping and die rolling.
We introduce relativistic multi-party biased die-rolling protocols, generalizing coin flipping to M ≥ 2 parties and to N ≥ 2 outcomes for any chosen outcome biases and show them unconditionally secure. Our results prove that the most general random secure multi-party computation, where all parties receive the output and there is no secret input by any party, can be implemented with unconditional security. Our protocols extend Kent's (Kent A. 1999 Phys. Rev. Lett. 83, 5382) two-party unbiased coin-flipping protocol, do not require any quantum communication, are practical to implement with current technology and to our knowledge are the first multi-party relativistic cryptographic protocols