619 research outputs found
Eliminating Recursion from Monadic Datalog Programs on Trees
We study the problem of eliminating recursion from monadic datalog programs
on trees with an infinite set of labels. We show that the boundedness problem,
i.e., determining whether a datalog program is equivalent to some nonrecursive
one is undecidable but the decidability is regained if the descendant relation
is disallowed. Under similar restrictions we obtain decidability of the problem
of equivalence to a given nonrecursive program. We investigate the connection
between these two problems in more detail
On The Power of Tree Projections: Structural Tractability of Enumerating CSP Solutions
The problem of deciding whether CSP instances admit solutions has been deeply
studied in the literature, and several structural tractability results have
been derived so far. However, constraint satisfaction comes in practice as a
computation problem where the focus is either on finding one solution, or on
enumerating all solutions, possibly projected to some given set of output
variables. The paper investigates the structural tractability of the problem of
enumerating (possibly projected) solutions, where tractability means here
computable with polynomial delay (WPD), since in general exponentially many
solutions may be computed. A general framework based on the notion of tree
projection of hypergraphs is considered, which generalizes all known
decomposition methods. Tractability results have been obtained both for classes
of structures where output variables are part of their specification, and for
classes of structures where computability WPD must be ensured for any possible
set of output variables. These results are shown to be tight, by exhibiting
dichotomies for classes of structures having bounded arity and where the tree
decomposition method is considered
Self-Enforcing Access Control for Encrypted RDF
The amount of raw data exchanged via web protocols is
steadily increasing. Although the Linked Data infrastructure could
potentially be used to selectively share RDF data with different individuals
or organisations, the primary focus remains on the unrestricted
sharing of public data. In order to extend the Linked Data paradigm to
cater for closed data, there is a need to augment the existing infrastructure
with robust security mechanisms. At the most basic level both access
control and encryption mechanisms are required. In this paper, we propose
a flexible and dynamic mechanism for securely storing and efficiently
querying RDF datasets. By employing an encryption strategy based on
Functional Encryption (FE) in which controlled data access does not
require a trusted mediator, but is instead enforced by the cryptographic
approach itself, we allow for fine-grained access control over encrypted
RDF data while at the same time reducing the administrative overhead
associated with access control management
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