Browsing by Author "Liedtke, Julian"

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Open Access
Concept Drift and Adaptation for Emotion Detection in Twitter
(2016) Liedtke, Julian
The classification task in dynamical environments is challenging. A reason for this is the change of their statistical properties over time. This characteristic is called concept drift and is one of the major topics in data mining. The objective of this thesis is to analyze, how accurate different systems are classifying in dynamical environments over a period of time. For this purpose, two different approaches are evaluated. One approach removes the features with the highest change in influence. The other is an ensemble based model which let experts vote between the outcomes. Although the models were not able to increase the accuracy after a long period of time, the results show that both models are able to achieve a higher accuracy than the baseline in particular cases. This outcome underlines that emotion detection in Twitter can be improved. New models or improvements to existing ones could be able to handle concept drift to achieve a higher accuracy.
Open Access
Nicht-interaktive Zero-Knowledge Beweise von Wissen mittels Fiat-Shamir Transformation
(2018) Liedtke, Julian
Sigma-Protokolle sind sehr effiziente Beweise von Wissen. Leider weisen sie nur die Special Honest Verifier Zero-Knowledge Eigenschaft auf, welche schwächer als die Zero-Knowledge Eigenschaft ist. Das liegt daran, dass bei Honest Verifier Zero-Knowledge nur für ehrliche Verifizierer ein Simulator existieren muss, während bei Zero-Knowledge auch für bösartige Verifizierer, das sind Verifizierer, die sich möglicherweise nicht an das Protokoll halten, eine erfolgreiche Simulationen verlangt werden. Eine Möglichkeit, Sigma-Protokolle in Zero-Knowledge Protokolle umzuwandeln, besteht in der Fiat-Shamir Transformation. Dabei entsteht nicht nur ein Zero-Knowledge Beweis von Wissen, sondern auch ein nicht-interaktives Beweissystem. Die Idee der Fiat-Shamir Transformation besteht darin, dass der Beweiser die Challenge mittels einer Hashfunktion aus dem gemeinsamen Eingabewort und dem Commitment berechnet. Trotz der aktiven Verwendung der Fiat-Shamir Transformation in der Praxis wurde erst 2012 in Arbeiten von Bernhard, Pereira und Warinschi sowie Faust, Kohlweiss, Marson, und Venturi der Versuch eines Beweises der Korrektheit erbracht. Der Beweis der ersten Arbeit wird in dieser Masterarbeit ausformuliert.
Open Access
Verifiable tally-hiding remote electronic voting
(2023) Liedtke, Julian; Küsters, Ralf (Prof. Dr.)
Electronic voting (e-voting) refers to casting and counting votes electronically, typically through computers or other digital interfaces. E-voting systems aim to make voting secure, efficient, convenient, and accessible. Modern e-voting systems are designed to keep the votes confidential and provide verifiability, i.e., everyone can check that the published election result corresponds to how voters intended to vote. Several verifiable e-voting systems have been proposed in the literature, with Helios being one of the most prominent ones. However, almost all verifiable e-voting systems reveal not just the voting result but also the tally, consisting of the exact number of votes per candidate or even all single votes. Publishing the tally causes several issues. For example, in elections with only a few voters (e.g., boardroom or jury votings), exposing the tally prevents ballots from being anonymous, thus deterring voters from voting for their actual preference. Furthermore, attackers can exploit the tally for so-called Italian attacks that allow for easily coercing voters. Often, the voting result merely consists of a single winner or a ranking of candidates, so disclosing only this information, not the tally, is sufficient. Revealing the tally unnecessarily embarrasses defeated candidates and causes them a severe loss of reputation. For these reasons, there are several real-world elections where authorities do not publish the tally but only the result - while the current systems for this do not ensure verifiability. We call the property of disclosing the tally tally-hiding. Tally-hiding offers entirely new opportunities for voting. However, a secure e-voting system that combines tally-hiding and verifiability does not exist in the literature. Therefore, this thesis presents the first provable secure e-voting systems that achieve both tally-hiding and verifiability. Our Ordinos framework achieves the strongest notion of tally-hiding: it only reveals the election result. Many real-world elections follow an alternative variant of tally-hiding: they reveal the tally to the voting authorities and only publish the election result to the public - so far without achieving verifiability. We, for the first time, formalize this concept and coin it public tally-hiding. We propose Kryvos, which is the first provable secure e-voting system that combines public tally-hiding and verifiability. Kryvos offers a new trade-off between privacy and efficiency that differs from all previous tally-hiding systems and allows for a radically new protocol design, resulting in a practical e-voting system. We implemented and benchmarked Ordinos and Kryvos, showing the practicability of our systems for real-world elections for significant numbers of candidates, complex voting methods, and result functions. Moreover, we extensively analyze the impact of tally-hiding on privacy compared to existing practices for various elections and show that applying tally-hiding improves privacy drastically.