Browsing by Author "Bauer, Christina"

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    Formal analysis of self-issued OpenID providers
    (2022) Bauer, Christina
    The Self-Issued OpenID Provider specification is an extension of OpenID Connect that allows users to authenticate at Relying Parties using an Identity Provider under the local control of the user. For this, the Self-Issued OpenID Provider specification introduces a notion of an identity that is self-asserted by the user.To supplement the self-asserted claims of the user with verifiable claims by a trusted party, a natural extension for the Self-Issued OpenID Provider specification is the use of Verifiable Credentials. The Self-Issued OpenID Provider specification defines two protocol flows: the same-device flow and the cross-device flow where the latter is known to be vulnerable to authentication request replay. In this thesis, we analyze the Self-Issued OpenID Provider specification using the Web Infrastructure Model. We model the Self-Issued OpenID Provider same-device protocol flow and uncover an attack on the protocol during formal analysis.After mitigating this attack in the model, we show that the security properties authentication, session integrity, and holder binding for Verifiable Credentials hold under certain assumptions. We also developed a variant of the cross-device flow that mitigates the request replay attack on the cross-device flow at the cost of introducing a web service associated with the Self-Issued OpenID Provider. For our variant of the cross-device protocol flow, we show that it is secure with respect to the authentication security property under suitable assumptions.
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    Optical properties of aperiodic metallic photonic crystal structures
    (2013) Bauer, Christina; Giessen, Harald (Prof. Dr.)
    In this thesis the linear optical properties of aperiodic metallic photonic crystals are studied. All structures consist of a metal grating on top of a waveguide material. The incident light can excite plasmonic modes in the metal as well as photonic modes in the waveguide underneath. These resonances are coupled to each other. In the first part of the thesis, the samples with a one-dimensional metal grating are studied. The structural arrangement of the metal wires on top of the dielectric waveguide layer is disordered, quasicrystalline, or fractal. For the disorder samples, the experimentally obtained coupling constant shows reduced values for larger disorder amounts. Additionally, the calculated coupling constants are compared to the experimentally obtained Urbach energies. It is found that the relation between these two parameters is dependent on the disorder model as well as on the average grating period. The optical properties of the samples with the quasicrystalline and fractal metal wire arrangement are analyzed with respect to their long, short, and average wire distances. The next part of the thesis deals with two-dimensional metal gratings. The metal disks are arranged in a quasicrystalline fashion with the disks being elliptically shaped and rotated with respect to the sample x axis. It is found that the optical properties of such structures are dependent on the eccentricity of the metal disks as well as on the rotation angle between the short main axis and the sample x axis. Afterwards, a theoretical model is developed in order to describe the optical properties of such structures. With the theoretical approach it is possible to calculate the normal incidence spectra as well as the oblique light incidence spectra. This model is used to predict the absorption enhancement of plasmonic solar cells.
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    Untersuchung der Durchführung der Offline Phase von MPC Protokollen
    (2019) Bauer, Christina
    Multiparty Computation Protokolle erlauben es einer Menge von Parteien gemeinsam eine Funktion zu berechnen. Diese Protokolle müssen die Korrektheit des Ergebnisses sowie die Geheimhaltung der Eingaben (Privacy) sicherstellen. Oft teilen sich solche Protokolle in eine Vorberechnungsphase (Offline Phase), welche von den Eingaben der Parteien unabhängig ist, und eine Phase für die tatsächliche Berechnung (Online Phase). In dieser Arbeit wird die Offline Phase für Multiparty Computation Protokolle basierend auf Paillier-Verschlüsselung betrachtet. Es wird ein Protokoll für die Erzeugung eines Schlüsselpaars des Paillier-Kryptosystems mit einem unter den Parteien aufgeteilten geheimen Schlüssel untersucht, ohne dass eine Partei, welcher alle anderen vertrauen, benötigt wird. Zudem wird betrachtet, wie Ciphertexte von zufälligen Werten in der Offline Phase vorberechnet werden können, ohne dass Parteien Kenntnis über den zugrundeliegenden Wert erlangen. Dazu werden Protokolle für die Generierung eines Zufallswerts, Zufallsbits sowie eines zufälligen Werts in einem eingegrenzten Bereich vorgestellt.