49 research outputs found
Device-independent randomness extraction for arbitrarily weak min-entropy source
Expansion and amplification of weak randomness plays a crucial role in many
security protocols. Using quantum devices, such procedure is possible even
without trusting the devices used, by utilizing correlations between outcomes
of parts of the devices. We show here how to extract random bits with an
arbitrarily low bias from a single arbitrarily weak min-entropy source in a
device independent setting. To do this we use Mermin devices that exhibit
super-classical correlations. Number of devices used scales polynomially in the
length of the random sequence . Our protocol is robust, it can tolerate
devices that malfunction with a probability dropping polynomially in at the
cost of a minor increase of the number of devices used.Comment: 5 pages + 3 pages supplementary materia
Weak randomness completely trounces the security of QKD
In usual security proofs of quantum protocols the adversary (Eve) is expected
to have full control over any quantum communication between any communicating
parties (Alice and Bob). Eve is also expected to have full access to an
authenticated classical channel between Alice and Bob. Unconditional security
against any attack by Eve can be proved even in the realistic setting of device
and channel imperfection. In this Letter we show that the security of QKD
protocols is ruined if one allows Eve to possess a very limited access to the
random sources used by Alice. Such knowledge should always be expected in
realistic experimental conditions via different side channels
Non-malleable encryption of quantum information
We introduce the notion of "non-malleability" of a quantum state encryption
scheme (in dimension d): in addition to the requirement that an adversary
cannot learn information about the state, here we demand that no controlled
modification of the encrypted state can be effected.
We show that such a scheme is equivalent to a "unitary 2-design" [Dankert et
al.], as opposed to normal encryption which is a unitary 1-design. Our other
main results include a new proof of the lower bound of (d^2-1)^2+1 on the
number of unitaries in a 2-design [Gross et al.], which lends itself to a
generalization to approximate 2-design.
Furthermore, while in prime power dimension there is a unitary 2-design with
=< d^5 elements, we show that there are always approximate 2-designs with
O(epsilon^{-2} d^4 log d) elements.Comment: 8 pages. Title changed in v2, a couple of mistakes corrected, the
results are essentially unchanged; accepted for publication at J Math Phy
Purification and correlated measurements of bipartite mixed states
We prove that all purifications of a non-factorable state (i.e., the state
which cannot be expressed in a form ) are
entangled. We also show that for any bipartite state there exists a pair of
measurements which are correlated on this state if and only if the state is
non-factorable.Comment: 4 revtex pages, to appear in Phys. Rev.
Optimality of private quantum channels
We addressed the question of optimality of private quantum channels. We have
shown that the Shannon entropy of the classical key necessary to securely
transfer the quantum information is lower bounded by the entropy exchange of
the private quantum channel and von Neumann entropy of the ciphertext
state . Based on these bounds we have shown that decomposition
of private quantum channels into orthogonal unitaries (if exists) is optimizing
the entropy. For non-ancillary single qubit PQC we have derived the optimal
entropy for arbitrary set of plaintexts. In particular, we have shown that
except when the (closure of the) set of plaintexts contains all states, one bit
key is sufficient. We characterized and analyzed all the possible single qubit
private quantum channels for arbitrary set of plaintexts. For the set of
plaintexts consisting of all qubit states we have characterized all possible
approximate private quantum channels and we have derived the relation between
the security parameter and the corresponding minimal entropy.Comment: no commen
Quantum key distribution and cryptography: a survey
I will try to partially answer, based on a review on recent work, the following question:
Can QKD and more generally quantum information be useful to cover some practical security requirements in current (and future) IT infrastructures ?
I will in particular cover the following topics
- practical performances of QKD
- QKD network deployment - SECOQC project
- Capabilities of QKD as a cryptographic primitive - comparative advantage with other solution, in order to cover practical security requirements
- Quantum information and Side-channels
- QKD security assurance
- Thoughts about "real" Post-Quantum Cryptograph