241 research outputs found
Efficient pebbling for list traversal synopses
We show how to support efficient back traversal in a unidirectional list,
using small memory and with essentially no slowdown in forward steps. Using
memory for a list of size , the 'th back-step from the
farthest point reached so far takes time in the worst case, while
the overhead per forward step is at most for arbitrary small
constant . An arbitrary sequence of forward and back steps is
allowed. A full trade-off between memory usage and time per back-step is
presented: vs. and vice versa. Our algorithms are based on a
novel pebbling technique which moves pebbles on a virtual binary, or -ary,
tree that can only be traversed in a pre-order fashion. The compact data
structures used by the pebbling algorithms, called list traversal synopses,
extend to general directed graphs, and have other interesting applications,
including memory efficient hash-chain implementation. Perhaps the most
surprising application is in showing that for any program, arbitrary rollback
steps can be efficiently supported with small overhead in memory, and marginal
overhead in its ordinary execution. More concretely: Let be a program that
runs for at most steps, using memory of size . Then, at the cost of
recording the input used by the program, and increasing the memory by a factor
of to , the program can be extended to support an
arbitrary sequence of forward execution and rollback steps: the 'th rollback
step takes time in the worst case, while forward steps take O(1)
time in the worst case, and amortized time per step.Comment: 27 page
A Secure Cooperative Sensing Protocol for Cognitive Radio Networks
Cognitive radio networks sense spectrum occupancy
and manage themselves to operate in unused bands without disturbing licensed users. Spectrum sensing is more accurate if jointly performed by several reliable nodes. Even though cooperative sensing is an active area of research, the secure
authentication of local sensing reports remains unsolved, thus empowering false results. This paper presents a distributed protocol based on digital signatures and hash functions, and an
analysis of its security features. The system allows determining a final sensing decision from multiple sources in a quick and secure way.Las redes de radio cognitiva detectora de espectro se las arreglan para operar en las nuevas bandas sin molestar a los usuarios con licencia. La detección de espectro es más precisa
si el conjunto está realizado por varios nodos fiables. Aunque la detección cooperativa es un área activa de investigación, la autenticación segura de informes locales de detección no ha sido resuelta, por lo tanto se pueden dar resultados falsos. Este trabajo presenta un protocolo distribuido basado en firmas digitales y en funciones hash, y un análisis de sus características de seguridad. El sistema permite determinar una decisión final de detección de múltiples fuentes de una manera rápida y segura.Les xarxes de ràdio cognitiva detectora d'espectre se les arreglen per operar en les noves bandes sense destorbar els usuaris amb llicència. La detecció d'espectre és més precisa
si el conjunt està realitzat per diversos nodes fiables. Encara que la detecció cooperativa és una àrea activa d'investigació, l'autenticació segura d'informes locals de detecció no ha estat resolta, per tant es poden donar resultats falsos. Aquest treball presenta un protocol distribuït basat en signatures digitals i en funcions hash, i una anàlisi de les seves característiques de seguretat. El sistema permet determinar una decisió final de detecció de múltiples fonts d'una manera ràpida i segura
One-Time Signatures Revisited: Have They Become Practical?
One-time signatures have been known for more than two decades, and
have been studied mainly due to their theoretical value. Recent
works motivated us to examine the practical use of one-time
signatures in high-performance applications. In this paper we
describe FMTseq - a signature scheme that merges recent
improvements in hash tree traversal into Merkle\u27s one-time signature
scheme. Implementation results show that the scheme provides a
signature speed of up to 35 times faster than a 2048-bit RSA
signature scheme, for about one million signatures, and a signature
size of only a few kilobytes. We provide an analysis of practical
parameter selection for the scheme, and improvements that can be
applied in more specific scenarios
Swiftmend: Data Synchronization in Open mHealth Applications with Restricted Connectivity
Open mHealth applications often include mobile devices and cloud services with replicated data between components. These replicas need periodical synchronization to remain consistent. However, there are no guarantee of connectivity to networks which do not bill users on the quantity of data usage. This thesis propose Swiftmend, a system with synchronization that minimize the quantity of I/O used on the network.
Swiftmend includes two reconciliation algorithms; Rejuvenation and Regrowth. The latter utilizes the efficiency of the Merkle tree data structure to reduce the I/O. Merkle trees can sum up the consistency of replicas into compact fingerprints. While the first reconciliation algorithm, Rejuvenation simply inspects the entire replica to identify consistency. Regrowth is shown to produce less quantity of I/O than Rejuvenation when synchronizing replicas. This is due to the compact fingerprints
Visually Mining the Datacube using a Pixel-Oriented Technique
International audienceThis paper introduces a new technique easing the navigation and interactive exploration of huge multidimensional datasets. Following the pixel-oriented paradigm, the key ingredients enabling the interactive navigation of extreme volumes of data rely on a set of functions bijectively mapping data elements to screen pixels. The use of the mapping from data elements to pixels constrain the computational complexity for the rendering process to be linear with respect to the number of rendered pixels on the screen as opposed to the dataset size. Our method furthermore allows the implementation of usual information visualization techniques such as zoom and pan, anamorphosis and texturing. As a proof-of-concept, we show how our technique can be adapted to interactively explore the Datacube, turning our approach into an efficient system for visual datamining. We report experiments conducted on a Datacube containing 50 millions of items. To our knowledge, our technique outperforms all existing ones and push the scalability limit close to the billion of elements. Supporting all basic navigation techniques, and being moreover flexible makes it easily reusable for a large number of applications
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