Design of Self-Healing Key Distribution Schemes

A self-healing key distribution scheme enables dynamic groups of users of an unreliable network to establish group keys for secure communication. In such a scheme, a group manager, at the beginning of each session, in order to provide a key to each member of the group, sends packets over a broadcast channel. Every user, belonging to the group, computes the group key by using the packets and some private information. The group manager can start multiple sessions during a certain time-interval, by adding/removing users to/from the initial group. The main property of the scheme is that, if during a certain session some broadcasted packet gets lost, then users are still capable of recovering the group key for that session simply by using the packets they have received during a previous session and the packets they will receive at the beginning of a subsequent one, without requesting additional transmission from the group manager. Indeed, the only requirement that must be satisfied, in order for the user to recover the lost keys, is membership in the group both before and after the sessions in which the broadcast messages containing the keys are sent. This novel and appealing approach to key distribution is quite suitable in certain military applications and in several Internet-related settings, where high security requirements need to be satisfied. In this paper we continue the study of self-healing key distribution schemes, introduced by Staddon et al. [37]. We analyze some existing constructions: we show an attack that can be applied to one of these constructions, in order to recover session keys, and two problems in another construction. Then, we present a new mechanism for implementing the self-healing approach, and we present an efficient construction which is optimal in terms of user memory storage. Finally, we extend the self-healing approach to key distribution, and we present a scheme which enables a user to recover from a single broadcast message all keys associated with sessions in which he is member of the communication group

Hoja oficial del lunes: Año XXXVI Número 1822 - 1972 Junio 19

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Adaptations to the neoliberal city:a comparative investigation of faith-based organisations in Rotterdam, the Netherlands and Philadelphia, USA

This thesis examines the role of faith-based organisations in Rotterdam, the Netherlands, and Philadelphia, USA. It was prompted by debates about the persistence of religion in seemingly secular societies and contexts and related questions about the role and relationship of faith-based organisations to the neighbourhood and non-faith community. Some scholars have suggested that increasingly the rhetorical moves of neoliberalism create specific spaces for faith-based organisations encouraging a kind of marketization in non-economic spheres. This study suggests that while the neoliberalizing urban carved out a niche for faith-based organisations, declining participation in conventional religious spaces and rites led these organisations to search for new meanings and roles for themselves. The forces that allowed these faith-based organisations a particular and meaningful role in the city were not religion versus secularity, but rather an unexpected interplay of neo-liberalism and secularism that can be seen the ways they attempt to create local and national ties through enhancing resident connections to neighbourhood and community The process of constant comparison has been central to this study and in doing so, it has enhanced our understanding of faith-based organisations by offering a new comparative perspective that recognises the significance of the interplay between neoliberalism and secularism. In adopting a comparative perspective that specifically juxtaposes a putatively secular nation (the Netherlands) against a putatively religious one (the U.S.A), it has been possible to discern common threads that have encouraged and facilitated these organisations in very different national contexts.