248 research outputs found
On Oblivious Amplification of Coin-Tossing Protocols
We consider the problem of amplifying two-party coin-tossing protocols: given a protocol where it is possible to bias the common output by at most ?, we aim to obtain a new protocol where the output can be biased by at most ?* < ?. We rule out the existence of a natural type of amplifiers called oblivious amplifiers for every ?* < ?. Such amplifiers ignore the way that the underlying ?-bias protocol works and can only invoke an oracle that provides ?-bias bits.
We provide two proofs of this impossibility. The first is by a reduction to the impossibility of deterministic randomness extraction from Santha-Vazirani sources. The second is a direct proof that is more general and also rules outs certain types of asymmetric amplification. In addition, it gives yet another proof for the Santha-Vazirani impossibility
Brief History of Quantum Cryptography: A Personal Perspective
Quantum cryptography is the only approach to privacy ever proposed that
allows two parties (who do not share a long secret key ahead of time) to
communicate with provably perfect secrecy under the nose of an eavesdropper
endowed with unlimited computational power and whose technology is limited by
nothing but the fundamental laws of nature. This essay provides a personal
historical perspective on the field. For the sake of liveliness, the style is
purposely that of a spontaneous after-dinner speech.Comment: 14 pages, no figure
Why Quantum Bit Commitment And Ideal Quantum Coin Tossing Are Impossible
There had been well known claims of unconditionally secure quantum protocols
for bit commitment. However, we, and independently Mayers, showed that all
proposed quantum bit commitment schemes are, in principle, insecure because the
sender, Alice, can almost always cheat successfully by using an
Einstein-Podolsky-Rosen (EPR) type of attack and delaying her measurements. One
might wonder if secure quantum bit commitment protocols exist at all. We answer
this question by showing that the same type of attack by Alice will, in
principle, break any bit commitment scheme. The cheating strategy generally
requires a quantum computer. We emphasize the generality of this ``no-go
theorem'': Unconditionally secure bit commitment schemes based on quantum
mechanics---fully quantum, classical or quantum but with measurements---are all
ruled out by this result. Since bit commitment is a useful primitive for
building up more sophisticated protocols such as zero-knowledge proofs, our
results cast very serious doubt on the security of quantum cryptography in the
so-called ``post-cold-war'' applications. We also show that ideal quantum coin
tossing is impossible because of the EPR attack. This no-go theorem for ideal
quantum coin tossing may help to shed some lights on the possibility of
non-ideal protocols.Comment: We emphasize the generality of this "no-go theorem". All bit
commitment schemes---fully quantum, classical and quantum but with
measurements---are shown to be necessarily insecure. Accepted for publication
in a special issue of Physica D. About 18 pages in elsart.sty. This is an
extended version of an earlier manuscript (quant-ph/9605026) which has
appeared in the proceedings of PHYSCOMP'9
Quantum cryptography: key distribution and beyond
Uniquely among the sciences, quantum cryptography has driven both
foundational research as well as practical real-life applications. We review
the progress of quantum cryptography in the last decade, covering quantum key
distribution and other applications.Comment: It's a review on quantum cryptography and it is not restricted to QK
Quantum Cryptography Beyond Quantum Key Distribution
Quantum cryptography is the art and science of exploiting quantum mechanical
effects in order to perform cryptographic tasks. While the most well-known
example of this discipline is quantum key distribution (QKD), there exist many
other applications such as quantum money, randomness generation, secure two-
and multi-party computation and delegated quantum computation. Quantum
cryptography also studies the limitations and challenges resulting from quantum
adversaries---including the impossibility of quantum bit commitment, the
difficulty of quantum rewinding and the definition of quantum security models
for classical primitives. In this review article, aimed primarily at
cryptographers unfamiliar with the quantum world, we survey the area of
theoretical quantum cryptography, with an emphasis on the constructions and
limitations beyond the realm of QKD.Comment: 45 pages, over 245 reference
Impossibility of Growing Quantum Bit Commitments
Quantum key distribution (QKD) is often, more correctly, called key growing.
Given a short key as a seed, QKD enables two parties, connected by an insecure
quantum channel, to generate a secret key of arbitrary length. Conversely, no
key agreement is possible without access to an initial key. Here, we consider
another fundamental cryptographic task, commitments. While, similar to key
agreement, commitments cannot be realized from scratch, we ask whether they may
be grown. That is, given the ability to commit to a fixed number of bits, is
there a way to augment this to commitments to strings of arbitrary length?
Using recently developed information-theoretic techniques, we answer this
question to the negative.Comment: 10 pages, minor change
Quantum Cryptography: Key Distribution and Beyond
Uniquely among the sciences, quantum cryptography has driven both foundational research as well as practical real-life applications. We review the progress of quantum cryptography in the last decade, covering quantum key distribution and other applications.Quanta 2017; 6: 1–47
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