The Performance of Elliptic Curve Based Group Diffie-Hellman Protocols for Secure Group Communication over Ad Hoc Networks

The security of the two party Diffie-Hellman key exchange protocol is currently based on the discrete logarithm problem (DLP). However, it can also be built upon the elliptic curve discrete logarithm problem (ECDLP). Most proposed secure group communication schemes employ the DLP-based Diffie-Hellman protocol. This paper proposes the ECDLP-based Diffie-Hellman protocols for secure group communication and evaluates their performance on wireless ad hoc networks. The proposed schemes are compared at the same security level with DLP-based group protocols under different channel conditions. Our experiments and analysis show that the Tree-based Group Elliptic Curve Diffie-Hellman (TGECDH) protocol is the best in overall performance for secure group communication among the four schemes discussed in the paper. Low communication overhead, relatively low computation load and short packets are the main reasons for the good performance of the TGECDH protocol.

Vanishing of Ext and Tor over complete intersections

Let (R,m) be a local complete intersection, that is, a local ring whose m-adic completion is the quotient of a complete regular local ring by a regular sequence. Let M and N be finitely generated R-modules. This dissertation concerns the vanishing of Tor(M, N) and Ext(M, N). In this context, M satisfies Serre's condition (S_{n}) if and only if M is an nth syzygy. The complexity of M is the least nonnegative integer r such that the nth Betti number of M is bounded by a polynomial of degree r-1 for all sufficiently large n. We use this notion of Serre's condition and complexity to study the vanishing of Tor_{i}(M, N). In particular, building on results of C. Huneke, D. Jorgensen and R. Wiegand [32], and H. Dao [21], we obtain new results showing that good depth properties on the R-modules M, N and MtensorN force the vanishing of Tor_{i}(M, N) for all i>0. We give examples showing that our results are sharp. We also show that if R is a one-dimensional domain and M and MtensorHom(M,R) are torsion-free, then M is free if and only if M has complexity at most one. If R is a hypersurface and Ext^{i}(M, N) has finite length for all i>>0, then the Herbrand difference [18] is defined as length(Ext^{2n}(M, N))-(Ext^{2n-1}(M, N)) for some (equivalently, every) sufficiently large integer n. In joint work with Hailong Dao, we generalize and study the Herbrand difference. Using the Grothendieck group of finitely generated R-modules, we also examined the number of consecutive vanishing of Ext^{i}(M, N) needed to ensure that Ext^{i}(M, N) = 0 for all i>>0. Our results recover and improve on most of the known bounds in the literature, especially when R has dimension two.

When the Romance is Over: Follower Perspectives of Aversive Leadership

While leadership is indisputably one of the most pervasive topics in our society, the vast majority of existing research has focused on leadership as a positive force. Taking a follower- centric approach to the study of leadership, we integrate research on the Romance of Leadership and the dark side of leadership by examining followers’ perceptions of aversive leadership in the context of public high schools. Although Meindl, Ehrlich, and Dukerich (1985) demonstrated that the Romance of Leadership also includes the overattribution of negative outcomes to leaders, subsequent research has failed to explore the implications of this potentially darker side of romanticizing leaders. Specifically, we examine perceptions of principals’ aversive leadership and traditional affective, behavioral, and performance outcomes of followers in a sample of 342 dyads. Followers assessed their principals’ leadership behaviors and self-rated their levels of job satisfaction, self-efficacy, and resistance, while principals assessed their followers’ citizenship behaviors, complaining behaviors, and job performance. Results show that perceptions of aversive leadership are positively related to follower resistance and negatively related to followers’ job satisfaction. In addition, a usefulness analysis revealed that follower-rated variables were significantly related to perceptions of aversive leadership above and beyond leader-rated variables, suggesting that the relationship between negative outcomes and aversive leadership may be more constructed than real. In sum, the tendency to romanticize leadership may also lead to a proclivity to readily misattribute or overattribute blame to leadership as a convenient scapegoat for negative outcomes. Alors que le leadership est incontestablement l’un des thèmes les plus envahissants de notre société, la grande majorité des recherches existantes a porté sur le leadership en tant que force positive. En adoptant une approche centrée sur le suiveur dans l’étude du leadership, nous examinant la perception qu’ont les collaborateurs du leadership insupportable dans le contexte des lycées publics. Quoique Meindl, Ehrlich, et Dukerich (1985) aient montré que la Romance du Leadership inclut aussi la surattribution de résultats négatifs aux leaders, les recherches ultérieures ont méconnu les implications de cet aspect potentiellement plus sombre des leaders idylliques. Nous analysons en particulier sur un échantillon de 342 dyades la perception du leadership répulsif du proviseur et les résultats habituels des collaborateurs en rapport avec l’affectivité, le comportement et les performances. Les collaborateurs ont noté les comportements de leadership de leur proviseur et auto-évalué leur niveau de satisfaction au travail, d’efficience et de résistance, alors que les proviseurs appréciaient les conduites de citoyenneté et de revendication, ainsi que la performance professionnelle. Les résultats montrent que la perception du leadership répulsif est Positivement reliée à la résistance du suiveur et négativement à sa satisfaction professionnelle. En outre, une analyse des plus fructueuses a révélé que les variables évaluées par les collaborateurs étaient significativement en relation avec la perception du leadership répulsif, bien plus qu’avec les variables évaluées par les leaders, ce qui indique que la relation entre les résultats médiocres et le leadership négatif serait plus construite que réelle. Au total, le penchant à l’idéalisation du leadership peut aussi bien conduire à une propension à trop facilement condamner à tort et à travers le leadership qu’à la désignation d’un bouc émissaire tout trouvé pour expliquer de mauvais résultats.