349 research outputs found
Ideal hierarchical secret sharing schemes
Hierarchical secret sharing is among the most natural generalizations of threshold secret sharing, and it has attracted a lot of attention from the invention of secret sharing until nowadays. Several constructions of ideal hierarchical secret sharing schemes have been proposed, but it was not known what access structures admit such a scheme. We solve this problem by providing a natural definition for the family of the hierarchical access structures and, more importantly, by presenting a complete characterization of the ideal hierarchical access structures, that is, the ones admitting an ideal secret sharing scheme. Our characterization deals with the properties of the hierarchically minimal sets of the access structure, which are the minimal qualified sets whose participants are in the lowest possible levels in the hierarchy. By using our characterization, it can be efficiently checked whether any given hierarchical access structure that is defined by its hierarchically minimal sets is ideal. We use the well known connection between ideal secret sharing and matroids and, in particular, the fact that every ideal access structure is a matroid port. In addition, we use recent results on ideal multipartite access structures and the connection between multipartite matroids and integer polymatroids. We prove that every ideal hierarchical access structure is the port of a representable matroid and, more specifically, we prove that every ideal structure in this family admits ideal linear secret sharing schemes over fields of all characteristics. In addition, methods to construct such ideal schemes can be derived from the results in this paper and the aforementioned ones on ideal multipartite secret sharing. Finally, we use our results to find a new proof for the characterization of the ideal weighted threshold access structures that is simpler than the existing one.Peer ReviewedPostprint (author's final draft
What is the topology of a Schwarzschild black hole?
We investigate the topology of Schwarzschild's black hole through the
immersion of this space-time in spaces of higher dimension. Through the
immersions of Kasner and Fronsdal we calculate the extension of the
Schwarzschild's black hole.Comment: 7 pages. arXiv admin note: substantial text overlap with
arXiv:1102.446
On the Design and Implementation of an Efficient DAA Scheme
International audienceDirect Anonymous Attestation (DAA) is an anonymous digital signature scheme that aims to provide both signer authentication and privacy. One of the properties that makes DAA an attractive choice in practice is the split signer role. In short, a principal signer (a Trusted Platform Module (TPM)) signs messages in collaboration with an assistant signer (the Host, a standard computing platform into which the TPM is embedded). This split aims to harness the high level of security offered by the TPM, and augment it using the high level of computational and storage ability offered by the Host. Our contribution in this paper is a modification to an existing pairing-based DAA scheme that significantly improves efficiency, and a comparison with the original RSA-based DAA scheme via a concrete implementation
Concurrent -vector fields and energy beta-change
The present paper deals with an \emph{intrinsic} investigation of the notion
of a concurrent -vector field on the pullback bundle of a Finsler manifold
. The effect of the existence of a concurrent -vector field on some
important special Finsler spaces is studied. An intrinsic investigation of a
particular -change, namely the energy -change
($\widetilde{L}^{2}(x,y)=L^{2}(x,y)+ B^{2}(x,y) with \
B:=g(\bar{\zeta},\bar{\eta})\bar{\zeta} \pi\Gamma\widetilde{\Gamma}\beta$-change of the fundamental linear connection in Finsler geometry: the
Cartan connection, the Berwald connection, the Chern connection and the
Hashiguchi connection. Moreover, the change of their curvature tensors is
concluded.
It should be pointed out that the present work is formulated in a prospective
modern coordinate-free form.Comment: 27 pages, LaTex file, Some typographical errors corrected, Some
formulas simpifie
A conformal boundary for space-times based on light-like geodesics: the 3-dimensional case
A new causal boundary, which we will term the l-boundary, inspired by the geometry of the space of light rays and invariant by conformal diffeomorphisms for space-times of any dimension m ≥ 3, proposed by one of the authors [R. J. Low, The Space of Null Geodesics (and a New Causal Boundary), Lecture Notes in Physics 692 (Springer, 2006), pp. 35-50] is analyzed in detail for space-times of dimension 3. Under some natural assumptions, it is shown that the completed space-time becomes a smooth manifold with boundary and its relation with Geroch-Kronheimer-Penrose causal boundary is discussed.Anumber of examples illustrating the properties of this newcausal boundary as well as a discussion on the obtained results will be provided
Horizon formation and far-from-equilibrium isotropization in supersymmetric Yang-Mills plasma
Using gauge/gravity duality, we study the creation and evolution of
anisotropic, homogeneous strongly coupled supersymmetric
Yang-Mills plasma. In the dual gravitational description, this corresponds to
horizon formation in a geometry driven to be anisotropic by a time-dependent
change in boundary conditions.Comment: 4 pages, typos corrected, published versio
Extended Absolute Parallelism Geometry
In this paper, we study Absolute Parallelism (AP-) geometry on the tangent
bundle of a manifold . Accordingly, all geometric objects defined in
this geometry are not only functions of the positional argument , but also
depend on the directional argument . Moreover, many new geometric objects,
which have no counterpart in the classical AP-geometry, emerge in this
different framework. We refer to such a geometry as an Extended Absolute
Parallelism (EAP-) geometry. The building blocks of the EAP-geometry are a
nonlinear connection assumed given a priori and linearly independent
vector fields (of special form) defined globally on defining the
parallelization. Four different -connections are used to explore the
properties of this geometry. Simple and compact formulae for the curvature
tensors and the W-tensors of the four defined -connections are obtained,
expressed in terms of the torsion and the contortion tensors of the EAP-space.
Further conditions are imposed on the canonical -connection assuming that it
is of Cartan type (resp. Berwald type). Important consequences of these
assumptions are investigated. Finally, a special form of the canonical
-connection is studied under which the classical AP-geometry is recovered
naturally from the EAP-geometry. Physical aspects of some of the geometric
objects investigated are pointed out and possible physical implications of the
EAP-space are discussed, including an outline of a generalized field theory on
the tangent bundle of Comment: 27 pages, LaTeX-file, The last version of this paper was replaced by
mistake (by arXiv: 0905.0209[gr-qc]
Area metric gravity and accelerating cosmology
Area metric manifolds emerge as effective classical backgrounds in quantum
string theory and quantum gauge theory, and present a true generalization of
metric geometry. Here, we consider area metric manifolds in their own right,
and develop in detail the foundations of area metric differential geometry.
Based on the construction of an area metric curvature scalar, which reduces in
the metric-induced case to the Ricci scalar, we re-interpret the
Einstein-Hilbert action as dynamics for an area metric spacetime. In contrast
to modifications of general relativity based on metric geometry, no continuous
deformation scale needs to be introduced; the extension to area geometry is
purely structural and thus rigid. We present an intriguing prediction of area
metric gravity: without dark energy or fine-tuning, the late universe exhibits
a small acceleration.Comment: 52 pages, 1 figure, companion paper to hep-th/061213
On the optimization of bipartite secret sharing schemes
Optimizing the ratio between the maximum length of the shares and the length of the secret value in secret sharing schemes for general access structures is an extremely difficult and long-standing open problem. In this paper, we study it for bipartite access structures, in which the set of participants
is divided in two parts, and all participants in each part play an equivalent role. We focus on the search of lower bounds by using a special class of polymatroids that is introduced here, the bipartite ones. We present a method based on linear programming to compute, for every given bipartite access structure, the best lower bound that can be obtained by this combinatorial method. In addition, we obtain some general lower bounds that improve the previously known ones, and we construct optimal secret sharing schemes for a family of bipartite access structures.Postprint (author’s final draft
Migrant women, place and identity in contemporary women's writing
While recent scholarship on migration has reflected growing attention to gender, and to the intersectionality of race, gender and sexuality, there has been little focus on women's emotional and bodily responses to migration in the context of larger structures of sexism, racism, and the legacies of colonialism. In this paper I examine some literary portrayals of how migrant women's relationships with specific places of origin and settlement, both steeped in structural relationships of unequal power and experienced on an immediate, psychological and bodily plane, are fundamental to migrant women's changing sense of belonging and identity. Jamaica Kincaid in her novel Lucy, Tsitsi Dangarembga in her novel Nervous Conditions, and Dionne Brand in the opening poems of her volume No Language is Neutral evoke some of the complex ways in which migration can affect women's lives and identities, thus both complementing and critiquing some contemporary theorisations of migration and migrant identities
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