228 research outputs found
Black swans or dragon kings? A simple test for deviations from the power law
We develop a simple test for deviations from power law tails, which is based
on the asymptotic properties of the empirical distribution function. We use
this test to answer the question whether great natural disasters, financial
crashes or electricity price spikes should be classified as dragon kings or
'only' as black swans
Block-Transitive Designs in Affine Spaces
This paper deals with block-transitive - designs in affine
spaces for large , with a focus on the important index case. We
prove that there are no non-trivial 5- designs admitting a
block-transitive group of automorphisms that is of affine type. Moreover, we
show that the corresponding non-existence result holds for 4- designs,
except possibly when the group is one-dimensional affine. Our approach involves
a consideration of the finite 2-homogeneous affine permutation groups.Comment: 10 pages; to appear in: "Designs, Codes and Cryptography
Perfect Secrecy Systems Immune to Spoofing Attacks
We present novel perfect secrecy systems that provide immunity to spoofing
attacks under equiprobable source probability distributions. On the theoretical
side, relying on an existence result for -designs by Teirlinck, our
construction method constructively generates systems that can reach an
arbitrary high level of security. On the practical side, we obtain, via cyclic
difference families, very efficient constructions of new optimal systems that
are onefold secure against spoofing. Moreover, we construct, by means of
-designs for large values of , the first near-optimal systems that are 5-
and 6-fold secure as well as further systems with a feasible number of keys
that are 7-fold secure against spoofing. We apply our results furthermore to a
recently extended authentication model, where the opponent has access to a
verification oracle. We obtain this way novel perfect secrecy systems with
immunity to spoofing in the verification oracle model.Comment: 10 pages (double-column); to appear in "International Journal of
Information Security
Attacks on quantum key distribution protocols that employ non-ITS authentication
We demonstrate how adversaries with unbounded computing resources can break
Quantum Key Distribution (QKD) protocols which employ a particular message
authentication code suggested previously. This authentication code, featuring
low key consumption, is not Information-Theoretically Secure (ITS) since for
each message the eavesdropper has intercepted she is able to send a different
message from a set of messages that she can calculate by finding collisions of
a cryptographic hash function. However, when this authentication code was
introduced it was shown to prevent straightforward Man-In-The-Middle (MITM)
attacks against QKD protocols.
In this paper, we prove that the set of messages that collide with any given
message under this authentication code contains with high probability a message
that has small Hamming distance to any other given message. Based on this fact
we present extended MITM attacks against different versions of BB84 QKD
protocols using the addressed authentication code; for three protocols we
describe every single action taken by the adversary. For all protocols the
adversary can obtain complete knowledge of the key, and for most protocols her
success probability in doing so approaches unity.
Since the attacks work against all authentication methods which allow to
calculate colliding messages, the underlying building blocks of the presented
attacks expose the potential pitfalls arising as a consequence of non-ITS
authentication in QKD-postprocessing. We propose countermeasures, increasing
the eavesdroppers demand for computational power, and also prove necessary and
sufficient conditions for upgrading the discussed authentication code to the
ITS level.Comment: 34 page
Lower bound for the quantum capacity of a discrete memoryless quantum channel
We generalize the random coding argument of stabilizer codes and derive a
lower bound on the quantum capacity of an arbitrary discrete memoryless quantum
channel. For the depolarizing channel, our lower bound coincides with that
obtained by Bennett et al. We also slightly improve the quantum
Gilbert-Varshamov bound for general stabilizer codes, and establish an analogue
of the quantum Gilbert-Varshamov bound for linear stabilizer codes. Our proof
is restricted to the binary quantum channels, but its extension of to l-adic
channels is straightforward.Comment: 16 pages, REVTeX4. To appear in J. Math. Phys. A critical error in
fidelity calculation was corrected by using Hamada's result
(quant-ph/0112103). In the third version, we simplified formula and
derivation of the lower bound by proving p(Gamma)+q(Gamma)=1. In the second
version, we added an analogue of the quantum Gilbert-Varshamov bound for
linear stabilizer code
The Threat of Capital Drain: A Rationale for Public Banks?
This paper yields a rationale for why subsidized public banks may be desirable from a regional perspective in a financially integrated economy. We present a model with credit rationing and heterogeneous regions in which public banks prevent a capital drain from poorer to richer regions by subsidizing local depositors, for example, through a public guarantee. Under some conditions, cooperative banks can perform the same function without any subsidization; however, they may be crowded out by public banks. We also discuss the impact of the political structure on the emergence of public banks in a political-economy setting and the role of interregional mobility
Directed Graph Representation of Half-Rate Additive Codes over GF(4)
We show that (n,2^n) additive codes over GF(4) can be represented as directed
graphs. This generalizes earlier results on self-dual additive codes over
GF(4), which correspond to undirected graphs. Graph representation reduces the
complexity of code classification, and enables us to classify additive (n,2^n)
codes over GF(4) of length up to 7. From this we also derive classifications of
isodual and formally self-dual codes. We introduce new constructions of
circulant and bordered circulant directed graph codes, and show that these
codes will always be isodual. A computer search of all such codes of length up
to 26 reveals that these constructions produce many codes of high minimum
distance. In particular, we find new near-extremal formally self-dual codes of
length 11 and 13, and isodual codes of length 24, 25, and 26 with better
minimum distance than the best known self-dual codes.Comment: Presented at International Workshop on Coding and Cryptography (WCC
2009), 10-15 May 2009, Ullensvang, Norway. (14 pages, 2 figures
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