Conventional wisdom suggests that in order to build a secure system, security must be an integral component in the system design. However, cost considerations drive most system designers to channel their efforts on the system's performance, scalability and usability. With little or no emphasis on security, such systems are vulnerable to a wide range of attacks that can potentially compromise confidentiality, integrity and availability of sensitive data. It is often cumbersome to redesign and implement massive systems with security as one of the primary design goals. This thesis advocates a proactive approach that cleanly retrofits security solutions into existing system architectures. The first step in this approach is to identify security threats, vulnerabilities and potential attacks on a system or an application. The second step is to develop security tools in the form of customizable and configurable plug-ins that address these security issues and minimally modify existing system code, while preserving its performance and scalability metrics. This thesis uses overlay network applications to shepherd through and address challenges involved in supporting security in large scale distributed systems. In particular, the focus is on two popular applications: publish/subscribe networks and VoIP networks. Our work on VoIP networks has for the first time identified and formalized caller identification attacks on VoIP networks. We have identified two attacks: a triangulation based timing attack on the VoIP network's route set up protocol and a flow analysis attack on the VoIP network's voice session protocol. These attacks allow an external observer (adversary) to uniquely (nearly) identify the true caller (and receiver) with high probability. Our work on the publish/subscribe networks has resulted in the development of an unified framework for handling event confidentiality, integrity, access control and DoS attacks, while incurring small overhead on the system. We have proposed a key isomorphism paradigm to preserve the confidentiality of events on publish/subscribe networks while permitting scalable content-based matching and routing. Our work on overlay network security has resulted in a novel information hiding technique on overlay networks. Our solution represents the first attempt to transparently hide the location of data items on an overlay network.

This diploma work is focused on the Cloud Computing and its possible applications in the Czech Republic. The first part of the work is theoretical and describes accessible sources analysis related to its topic. Historical circumstances are given in the relation to the evolution of this technology. Also few definitions are quoted, followed by the Cloud Computing basic taxonomy and common models of use. The chapter named Cloud Computing Architecture covers the generally accepted model of this technology in details. In the part focused on theory are mentioned some of the services opearating on this technology. At the end the theoretical part brings possibility of the Cloud Computing usage from the customer's and supplier's perspective. The practical part of the diploma work is divided into sections. The first one brings results of the questionnare research, performed by the author in the Czech Republic, focused on the usage of the Cloud Computing and virtualization services. In the sekond section is pre-feasibility study. The study is focused on the providing SaaS Services in the area of long-term and safe digital data store. Lastly there is an author's view on the Cloud Computing technology future and possible evolution.

Die vorliegende Arbeit verknüpft die Theorie der Interaktion psychischer Systeme von Kuhl (2000, 2001) mit dem Modell beruflicher Arbeitsleistung von Tett und Burnett (2003). Unter Anwendung reizorientierter arbeitspsychologischer Stressmodelle werden Hypothesen über einen durch das subjektive Bedrohungs- und Belastungspotenzial der Arbeitssituation moderierten Zusammenhang zwischen dem berufsbezogenen und nach Maßgabe der Item Response Theorie skalierbaren Persönlichkeitsmerkmal Handlungs- vs. Lageorientierung und beruflicher Arbeitsleistung aufgestellt. In drei Befragungen an N = 415, N = 331 sowie N = 49 Berufstätigen wurden querschnittliche Daten zur Hypothesenprüfung erhoben. Berufsbezogene Handlungs- vs. Lageorientierung zeigt sich als valides Subkonstrukt der allgemeinen Handlungs- vs. Lageorientierung, welches gemäß Graded Response Modell von Samejima (1969, 1997) mit 14 Items skalierbar ist. Prospektive berufsbezogene Handlungs- vs. Lageorientierung erklärt in multiplen hierarchischen Regressionsanalysen, im Gegensatz zu allgemeiner Handlungs- vs. Lageorientierung, inkrementell zu Gewissenhaftigkeit, Extraversion und Neurotizismus Anteile kontextueller und aufgabenbezogener Arbeitsleistung. Hypothesenkonträr werden diese Zusammenhänge nur marginal vom subjektiven Belastungspotential der Arbeitssituation moderiert. Die Prädiktorfunktion prospektiver berufsbezogener Handlungs- vs. Lageorientierung für berufliche Arbeitsleistung bleibt auch unter pfadanalytischer Kontrolle eines vorhandenen Common Method Bias erhalten. Die dispositionelle Fähigkeit, durch berufliche Hindernisse gehemmten positiven Affekt vorbewusst gegenregulieren zu können, scheint demnach ein bedeutender Prädiktor beruflicher Arbeitsleistung zu sein, insbesondere bei Führungskräften. Für die berufliche Eignungsbeurteilung ist es damit von diagnostischem Mehrwert, Handlungs- vs. Lageorientierung kontextualisiert zu erheben. Der Einsatz probabilistisch- testtheoretisch konstruierter Skalen steigert dabei die Effizienz des Beurteilungsprozesses. / The current paper combines personality systems interaction theory (Kuhl, 2000, 2001) with the model of job performance by Tett and Burnett (2003). Using established stress models from work psychology it is hypothesized that there is a relation between occupational action state orientation, scalable by means of items response theory, and job performance, which is moderated by the subjective stress level of job characteristics. Three surveys among samples of N = 415, N = 331, and N = 49 professionals yielded cross sectional data for investigating the hypotheses. Occupational action state orientation proves a valid construct which is compatible with Samejima’s (1969, 1997) Graded Response Model using a 14-item scale. As a result of multiple hierarchical regression analyses, the hesitation dimension of specifically occupational, in contrast to general action state orientation is a predictor of both contextual and task performance, incremental to conscientiousness, extraversion, and neuroticism. Contrary to expectations this relation is only marginally moderated by stress-relevant job characteristics. Even when controlling for an occurring common method bias by means of path analysis the occupational hesitation dimension’s predictor role perseveres. Therefore, the dispositional ability in subconsciously regulating inhibited positive affect due to occupational obstacles, seems to be a crucial predictor of job performance, especially regarding leaders. Hence, professional aptitude assessment benefits from assessing action state orientation in a contextualized manner. Application of item response theory-based scales further enhances assessment process efficiency.