The Disclosure Power of Shared Objects

Shared objects are the means by which processes gather and exchange information about the state of a distributed system. Objects that disclose more information about the system—and thus provide a more centralized view—are therefore more desirable. In this paper, we propose the schedule reconstruction (SR) problem as a new metric for the disclosure power of shared memory objects. In schedule reconstruction, processes take steps which are interleaved to form a schedule; each process needs to be able to reconstruct the schedule up to its last step. We show that objects can be ranked in a hierarchy according to their ability to solve SR. In this hierarchy, stronger objects can implement weaker objects via a SR-based universal construction. We identify a connection between SR and consensus and prove that SR is at least as hard as consensus. Perhaps surprisingly, we show that objects that are powerful in solving consensus—such as compare-and-swap—are not always powerful in their ability to solve SR.

Peva François Blanchard, Rachid Guerraoui, Julien Michel Stainer, Mihail Igor Zablotchi

Shared objects are the means by which processes gather and exchange information about the state of a distributed system. Objects that disclose more information about the system are therefore more desirable. In this paper, we propose the schedule reconstruction (SR) problem as a new metric for the disclosure power of shared memory objects. In schedule reconstruction, processes take steps which are interleaved to form a schedule; each process needs to be able to reconstruct the schedule up to its last step. We show that objects can be ranked in a hierarchy according to their ability to solve SR. In this hierarchy, stronger objects can implement weaker objects via a SR-based universal construction. We identify a connection between SR and consensus and prove that SR is at least as hard as consensus. Perhaps surprisingly, we show that objects that are powerful in solving consensus—such as compare-and-swap—are not always powerful in their ability to solve SR.

2017

Exploitation de connaissances sémantiques externes dans les représentations vectorielles en recherche documentaire

Florian Seydoux

The work presented in this thesis deals with several problems met in information retrieval (IR), task which one can summarise as identifying, in a collection of "documents", a subset of documents carrying a sought information, i.e.. relevant for a request expressed by a user. In the case of textual documents, to which we limited ourselves within the framework of this thesis, a significant part of the difficulty lies in ambiguity inherent to human languages. The interaction with the user is also approached in our work, by studying a tool enabling a natural language access to a database. Finally, some techniques which permit the visualisation of large collections of documents are also presented. In this document we first of all describe the principal models of IR by highlighting the relations which exist with some manual technics of IR and document retrieval, developed during the past centuries. We present the principle of document indexing, allowing us to represent documents in a multidimensional space, and the use of this representation by a vectorial model. After having reviewed the principal improvements made these last years with vectorial research systems, including the preprocessings of collections, the indexing mechanism and measurements of similarities between documents, we detail some recent usecases of additional semantic resources (semantic dictionaries, thesaurus, networks, ontologies) reported in scientific literature for the indexing task. We then present more in detail the semantic indexing principle of textual documents by using a thesaurus, consisting in integrating in the document's representation space at least part of the informational contents of hierarchical semantic resources. We propose a general framework allowing us to describe and position various possible techniques to carry out the semantic indexing by adapting, if possible, the specificity of the descriptions resulting from the semantic resources to the data to be represented. We use this framework to describe three families of criteria usable for semantic indexing, each one having its own characteristics. For each of these families, we give the specific algorithms allowing the computation of the criteria. The first two families allow us to consider several criteria already known in feature selection. Moreover we show that, unfortunately, many of these criteria are in fact not very effective for the considered task. The third family allows us to introduce a completely new criterion, the Minimum Redundancy Cut criterion (MRC), built on the basis of the information theory and allowing us to obtain index terms having a probability of occurrence in the collection of documents as well balanced as possible. Finally, we treat the case of semantic index independent of the data (statically choosen), allowing a parameterisation of the level of generality of the index terms. Some of the criteria suggested for semantic indexing has been empirically evaluated. To judge their relevance, we used a well known vectorial system (the Smart IR system) and measured the performances of IR obtained with various reference collections. Those collections was indexed on the basis of the studied criterion, by taking into account the strongly structuring semantic relation of hyper/hyponymy ("is-a" relation), given by two different semantic resources. By comparing results obtained with the performances of a traditional indexing (using the lemmas of the words as representation space), we can show on one hand the relevance of the semantic indexings (in RD) and on the other hand the quality of the proposed criterion (MRC). Concerning man-machine interaction, we present a general outline allowing to build in a relatively fast and systematic way systems with mixed initiative, giving the human user a large (and natural) latitude in the control of the dialogue. This outline is usable in typical database research-task applications (where the database is hidden to the user, but the latter knows exactly which information they wish to find) as well as advice-task applications, for which the users does not necessarily have a precise idea of their needs, and uses the system not only for specifing their wishes, but also a set of propositions as a final result. We particularly stress the techniques allowing us to obtain a robust system, able to deal with speech recognizer failures. Concerning the visualisation of large textual data collections, we present an application of the correspondences analysis (allowing to highlight similarities and oppositions for various groups of entity, built on the basis of additional features present in the DB) to the case of patents data. In addition, we propose a method (based on the bootstrap replication principle) allowing us to determine a confidence interval for relative positionings of various groups, thus permit to immediately judge the reliability of the visually apparent similarities or oppositions.

EPFL2006

A Mobile World of Security

Christine Neuberg

Security in mobile communications is a topic of increasing relevance in everyday life. We all use mobile devices for everyday communications, maybe even for exchanging confidential information with the work place. This requires security systems that are reliable and stable even though mobility and number of users are increasing. An established solution in today's systems is to use symmetric cryptographic systems by relying on pre-established secrets. There are a few drawbacks. The first problem is that all security information is stored in one central storage place. This implies that users are required to trust this storage place. They need to trust that data is properly handled and well protected. The mechanism of storing pre-established long-term secrets at a central storage also makes it vulnerable to adversaries. As soon as someone gains access to it, he automatically has access to all secret information necessary to capture communications. A second problem is that security mechanisms implemented in today's mobile communication systems were designed for a moderate number of users. Since then, the number of users has grown significantly and continues to do so. It would thus be desirable to have a system that is flexible in terms of the number of users it can handle. The third and maybe the most crucial problem is the fact that today's security systems rely on computational cryptography. This means that the security mechanisms in cellular communication systems were designed with limited computational power of the adversary in mind. However, the computational power of computers is growing and systems might be breakable in the near future. A complementary notion of security is information-theoretic security. Systems built in this spirit are unbreakable even with unlimited computational power. These systems exploit the knowledge of statistical properties of the environment. This means mainly that security systems can be proved to be secure with respect to assumptions on the correlated sources and with respect to the nature of the knowledge the adversary can gain. The main idea of information-theoretic security is that two communication partners have access to correlated random sources. An adversary being potentially present in the system, has only degraded access to the source. The common knowledge can be used in order to generate sequences that are very similar among each other. Because the adversary has access to the random source, he is able to generate his own version of the sequence. It is fundamental that the sequences of the communicationpartners are stronger correlated among each other than with respect to the adversary. This advantage in knowledge can be interpreted as a secret among the communication partners to the adversary. In communicating over an authenticated but public channel, this secret can be extracted from the original sequence by both parties. In order for both keys to be equal, the original sequences need to coincide. This is not necessarily the case, but can be achieved by discussion over the public channel. In the first part of this thesis, we propose a security system that relies on information-theoretic security. We will demonstrate how this system can be implemented in cellular communication systems. The movement of the users is random and we want to use it as a random source. We assume that users are moving along a random itinerary while visiting a random sequence of cells. Users not only visit cells in a random order, they also stay for a random time in each cells. It turns out that it is the sequence of durations that contributes most to the randomness of the sequence. We call the sequence of durations timing. This sequence is known to users and to the infrastructure, but only parts of it are known to potential adversaries. By recording the sequence of cells along with the time of residence, both the user and the infrastructure will be able to acquire very similar sequences. After the collection process, a procedure for key agreement is applied. Due to the underlying process, the key will not be uniformly distributed. As this is a requirement for perfect secret keys, a final part of the protocol is privacy amplification, where the perfectly secret bits will be extracted. This system is lightweight as it is based on readily available information and only a small amount of post-processing is required. In the second part of this thesis, we analyze the performance of the system. For this, we demonstrate how the process of bit collection can be analytically modeled. Our analysis leads to two conclusions. It gives us intuition about how to choose parameters such that the scheme performs optimally. Our scheme can be applied to settings that fulfill certain conditions. Conclusions from analysis allow to specify such conditions. We will show the implication of this conclusion in sketching some alternative applications of our scheme.