André Greubel

2020

1.
Greubel, A., Kounev, S.: Towards an Evaluation Framework for Tor Load Balancing. Work in Progress Paper at the 36th Annual Computer Security Applications Conference (ACSAC 2020). (2020).
[ Abstract ] [ BibTeX ] [ Download ]
 
Tor is a widely used anonymization network. Traffic is routed over different relay nodes to conceal the communication partner. The Tor Load Balancing Mechanism (TLBM) is responsible for assigning traffic to relays. However, up to this point, there are no agreed-upon quality and security metrics for evaluating the quality of the TLBM. In practice, multiple paper provide their own evaluation using custom metrics incomparable to related work. This paper aims to start bridging this gap by arguing for the necessity of a unified framework to evaluate TLBMs and by giving an overview of aspects that should be included in such a framework.
@article{noauthororeditor, abstract = {Tor is a widely used anonymization network. Traffic is routed over different relay nodes to conceal the communication partner. The Tor Load Balancing Mechanism (TLBM) is responsible for assigning traffic to relays. However, up to this point, there are no agreed-upon quality and security metrics for evaluating the quality of the TLBM. In practice, multiple paper provide their own evaluation using custom metrics incomparable to related work. This paper aims to start bridging this gap by arguing for the necessity of a unified framework to evaluate TLBMs and by giving an overview of aspects that should be included in such a framework.}, author = {Greubel, Andre and Kounev, Samuel}, journal = {Work in Progress Paper at the 36th Annual Computer Security Applications Conference (ACSAC 2020)}, keywords = {acsac evaluation framework hauptautor measurements paper tlbm tor wip}, title = {Towards an Evaluation Framework for Tor Load Balancing}, year = 2020 }

2.
Greubel, A., Pohl, S., Kounev, S.: Quantifying measurement quality and load distribution in Tor. Proceedings of the 36th Annual Computer Security Applications Conference (ACSAC 2020) (2020).
[ Abstract ] [ BibTeX ] [ URL ] [ DOI ] [ Download ]
 
Tor is a widely used anonymization network. Traffic is routed over different relay nodes to conceal the communication partners. However, if a single relay handles too much traffic, de-anonymization attacks are possible. The Tor Load Balancing Mechanism (TLBM) is responsible for balanced and secure load distribution. It must verify that relays cannot attract more traffic than they should by lying about their self-reported bandwidth. This work shows that the current bandwidth measurement method used for bandwidth verification is not suitable to verify the bandwidth of many relays. Most importantly, multiple measurements of high-bandwidth relays are uncorrelated to each other. Furthermore, we analyze the current load distribution in Tor. We show that the current load distribution reduces the resources necessary for several large-scale de-anonymization attacks by more than 80%. Additionally, as Tor favors fast relays during path selection, verifiable relays only handle a small fraction of Tor’s traffic. More precisely, we show that only 7.21% of all paths consist of entry and exit relays verifiable by measurements. We discuss these results’ security implications and argue that future TLBM research should focus at least as much on secure load distribution as on high traffic performance.
@inproceedings{greubel2020quantifying, abstract = {Tor is a widely used anonymization network. Traffic is routed over different relay nodes to conceal the communication partners. However, if a single relay handles too much traffic, de-anonymization attacks are possible. The Tor Load Balancing Mechanism (TLBM) is responsible for balanced and secure load distribution. It must verify that relays cannot attract more traffic than they should by lying about their self-reported bandwidth. This work shows that the current bandwidth measurement method used for bandwidth verification is not suitable to verify the bandwidth of many relays. Most importantly, multiple measurements of high-bandwidth relays are uncorrelated to each other. Furthermore, we analyze the current load distribution in Tor. We show that the current load distribution reduces the resources necessary for several large-scale de-anonymization attacks by more than 80%. Additionally, as Tor favors fast relays during path selection, verifiable relays only handle a small fraction of Tor’s traffic. More precisely, we show that only 7.21% of all paths consist of entry and exit relays verifiable by measurements. We discuss these results’ security implications and argue that future TLBM research should focus at least as much on secure load distribution as on high traffic performance.}, author = {Greubel, Andre and Pohl, Steffen and Kounev, Samuel}, booktitle = {Proceedings of the 36th Annual Computer Security Applications Conference (ACSAC 2020)}, keywords = {TLBM balancing hauptautor load myown paper tor}, title = {Quantifying measurement quality and load distribution in Tor}, year = 2020 }

3.
Greubel, A., Rudolph, T., Hennecke, M.: VeraSQL: An educational client for relational databases. 18. Fachtagung Bildungstechnologien (DELFI). (2020).
[ Abstract ] [ BibTeX ] [ URL ] [ Download ]
 
In this paper, we present VeraSQL, an educational database client specifically designed to support the transition between spreadsheets and relational databases as taught in many German school curricula. Most importantly, VeraSQL can execute SQL queries on CSV-files. VeraSQL is written in Java, open source, and can be used and downloaded free at www.verasql.de.
@article{greubel2020verasql, abstract = {In this paper, we present VeraSQL, an educational database client specifically designed to support the transition between spreadsheets and relational databases as taught in many German school curricula. Most importantly, VeraSQL can execute SQL queries on CSV-files. VeraSQL is written in Java, open source, and can be used and downloaded free at www.verasql.de.}, author = {Greubel, Andre and Rudolph, Theresa and Hennecke, Martin}, journal = {18. Fachtagung Bildungstechnologien (DELFI)}, keywords = {databases hauptautor paper sql verasql}, title = {VeraSQL: An educational client for relational databases}, year = 2020 }

4.
Greubel, A., Siller, H.-S., Hennecke, M.: Teaching Simulation Literacy with Evacuations: Concept, Technology and Material for a Novel Approach. European Conference for Technology Enhanced Learning. (2020).
[ Abstract ] [ BibTeX ] [ URL ] [ DOI ] [ Download ]
 
As signicant policies are based on their expected outcome in computer simulations, literacy of such simulations is necessary for political participation. In this paper, we propose ways to increase such simulation literacy.We discuss simulation literacy from a theoretical perspective and argue for simulating evacuations as a simple and potent topic to increase simulation literacy. Additionally, we present EVA, a novel educational tool to simulate the evacuating of buildings (not only) for classrooms. Lastly, we show different teaching scenarios and exercises for the usage of EVA in an exemplary way. EVA and further teaching material is available online at [blinded for review].
@article{greubel2020eva, abstract = {As signicant policies are based on their expected outcome in computer simulations, literacy of such simulations is necessary for political participation. In this paper, we propose ways to increase such simulation literacy.We discuss simulation literacy from a theoretical perspective and argue for simulating evacuations as a simple and potent topic to increase simulation literacy. Additionally, we present EVA, a novel educational tool to simulate the evacuating of buildings (not only) for classrooms. Lastly, we show different teaching scenarios and exercises for the usage of EVA in an exemplary way. EVA and further teaching material is available online at [blinded for review].}, author = {Greubel, André and Siller, Hans-Stefan and Hennecke, Martin}, journal = {European Conference for Technology Enhanced Learning}, keywords = {evakuierung hauptautor literacy myown paper simulation}, title = {Teaching Simulation Literacy with Evacuations: Concept, Technology and Material for a Novel Approach}, year = 2020 }

2019

1.
Pöhner, N., Schmidt, T., Greubel, A., Hennecke, M., Ehmann, M.: BlocklySQL: A new block-based editor for SQL. Proceedings of the 14th Workshop in Primary and Secondary Computing Education. S. 4:1–4:2. ACM, New York, NY, USA (2019).
[ Abstract ] [ BibTeX ] [ URL ] [ DOI ] [ Download ]
 
This paper introduces BlocklySQL, a new block-based editor for SQL. BlocklySQL was developed on the basis of Google's JavaScript library Blockly, a tool for developing own block-based editors, which comes with several technological and pedagogical advantages. BlocklySQL is also integrated into the learning platform dbup2date, which provides daily updated databases for the use in CS classrooms.
@inproceedings{P_hner_2019, abstract = {This paper introduces BlocklySQL, a new block-based editor for SQL. BlocklySQL was developed on the basis of Google's JavaScript library Blockly, a tool for developing own block-based editors, which comes with several technological and pedagogical advantages. BlocklySQL is also integrated into the learning platform dbup2date, which provides daily updated databases for the use in CS classrooms.}, address = {New York, NY, USA}, author = {Pöhner, Nicolai and Schmidt, Timo and Greubel, André and Hennecke, Martin and Ehmann, Matthias}, booktitle = {Proceedings of the 14th Workshop in Primary and Secondary Computing Education}, keywords = {SQL blockbasedprogramming computerscienceeducation databases education myown paper poster sql}, pages = {4:1-4:2}, publisher = {ACM}, title = {BlocklySQL: A new block-based editor for SQL}, year = 2019 }

2.
Berges, M., Ehmann, M., Gall, R., Greubel, A., Günzel-Weinkamm, N., Haller, V., Hennecke, M., Heuer, U., Kronawitter, J., Lindner, A., Pöhner, N.: Erfahrungsbericht zur Qualifizierungsmaßnahme Informatik als Erweiterungsfach (Lehramt Realschule) in Bayern. GI-Fachtagung Informatik und Schule (INFOS 2019). (2019).
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Teil des Programms BAYERN DIGITAL II ist die berufsbegleitende Nachqualifizierung von rund 180 Lehrerinnen und Lehrern der Realschule. Über die Ermäßigung des Stundendeputats wird den Teilnehmenden ermöglicht, sich innerhalb von zwei Jahren für das Fach Informatik an bayerischen Realschulen nachzuqualifizieren. Ausbildungsziel ist das einem regulären Studium entsprechende Staatsexamen. An mehreren universitären Standorten wurden daher entsprechende Maßnahmen aufgelegt. Durch die besonderen Rahmenbedingungen kommen vor allem Lehr-und Lernformen des "blended learning" Konzepts zum Tragen. Zweiwöchentliche Präsenztage sorgen dabei für eine kontinuierliche Flankierung, wobei versucht wird, im Sinne des "flipped classroom" Prinzips die Präsenzzeit möglichst lernerzentriert und interaktiv zu gestalten. Untersuchungen zur Zufriedenheit und Motivation nach dem ersten Halbjahr zeigen den Erfolg der Maßnahme, aber auch weiteres Verbesserungspotential.
@article{berges2019nachqualifizierung, abstract = {Teil des Programms BAYERN DIGITAL II ist die berufsbegleitende Nachqualifizierung von rund 180 Lehrerinnen und Lehrern der Realschule. Über die Ermäßigung des Stundendeputats wird den Teilnehmenden ermöglicht, sich innerhalb von zwei Jahren für das Fach Informatik an bayerischen Realschulen nachzuqualifizieren. Ausbildungsziel ist das einem regulären Studium entsprechende Staatsexamen. An mehreren universitären Standorten wurden daher entsprechende Maßnahmen aufgelegt. Durch die besonderen Rahmenbedingungen kommen vor allem Lehr-und Lernformen des "blended learning" Konzepts zum Tragen. Zweiwöchentliche Präsenztage sorgen dabei für eine kontinuierliche Flankierung, wobei versucht wird, im Sinne des "flipped classroom" Prinzips die Präsenzzeit möglichst lernerzentriert und interaktiv zu gestalten. Untersuchungen zur Zufriedenheit und Motivation nach dem ersten Halbjahr zeigen den Erfolg der Maßnahme, aber auch weiteres Verbesserungspotential.}, author = {Berges, Marc and Ehmann, Matthias and Gall, Rainer and Greubel, Andre and Günzel-Weinkamm, Nicole and Haller, Verena and Hennecke, Martin and Heuer, Ute and Kronawitter, Julia and Lindner, Annabel and Pöhner, Nicolai}, journal = {GI-Fachtagung Informatik und Schule (INFOS 2019)}, keywords = {Bayern Erfahrungsbericht Erweiterungsfach Informatik Nachqualifizierung Paper Staatsexamen myown}, month = {sep}, title = {Erfahrungsbericht zur Qualifizierungsmaßnahme Informatik als Erweiterungsfach (Lehramt Realschule) in Bayern}, year = 2019 }

3.
Greubel, A., Hegemann, T., Ifland, M., Hennecke, M.: Relevanz der Codequalität in einem Praktikum mit automatisch getesteten Programmierabgaben. Vierter Workshop zur Automatische Bewertung von Programmieraufgaben (ABP 2019). 27–34 (2019).
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Systeme zur automatisierten Bewertung von Programmieraufgaben bewerten üblicherweise nicht die Codequalität. In dieser Arbeit wird an einem Praktikum mit 212 Studierenden empirisch untersucht, welche Relevanz diese der Codequalität beimessen und welche Relevanz verschiedene Qualitätsmetriken auf das Bestehen des Praktikums haben. Des Weiteren wird analysiert, ob diese Metriken von externen Faktoren wie dem Studiengang oder der Praktikumswiederholung beeinflusst werden.
@article{greubel2019relevanz, abstract = {Systeme zur automatisierten Bewertung von Programmieraufgaben bewerten üblicherweise nicht die Codequalität. In dieser Arbeit wird an einem Praktikum mit 212 Studierenden empirisch untersucht, welche Relevanz diese der Codequalität beimessen und welche Relevanz verschiedene Qualitätsmetriken auf das Bestehen des Praktikums haben. Des Weiteren wird analysiert, ob diese Metriken von externen Faktoren wie dem Studiengang oder der Praktikumswiederholung beeinflusst werden.}, author = {Greubel, Andre and Hegemann, Tim and Ifland, Marianus and Hennecke, Martin}, journal = {Vierter Workshop zur Automatische Bewertung von Programmieraufgaben (ABP 2019)}, keywords = {Bewertung Codequalität Didaktik Hauptautor Paper Programmierpraktikum myown}, month = {oct}, pages = {27-34}, title = {Relevanz der Codequalität in einem Praktikum mit automatisch getesteten Programmierabgaben}, year = 2019 }

2018

1.
Greubel, A., Dmitrienko, A., Kounev, S.: SmarTor - Smarter Tor with Smart Contracts. Annual Computer Security Applications Conference (ACSAC 2018). (2018).
[ Abstract ] [ BibTeX ] [ URL ] [ DOI ] [ Download ]
 
In the Tor anonymity network, the distribution of topology information relies on the correct behavior of five out of the nine trusted directory authority servers. This centralization is concerning since a powerful adversary might compromise these servers and conceal information about honest nodes, leading to the full de-anonymization of all Tor users. Our work aims at distributing the work of these trusted authorities, such increasing resilience against attacks on core infrastructure components of the Tor network. In particular, we leverage several emerging technologies, such as blockchains, smart contracts, and trusted execution environments to design and prototype a system called SmarTor. This system replaces the directory authorities with a smart contract and a distributed network of untrusted entities responsible for bandwidth measurements. We prototyped SmarTor using Ethereum smart contracts and Intel SGX secure hardware. In our evaluation, we show that SmarTor produces significantly more reliable and precise measurements compared to the current measurement system. Overall, our solution improves the decentralization of the Tor network, reduces trust assumptions and increases resilience against powerful adversaries like law enforcement and intelligence services.
@article{greubel2018smartor, abstract = {In the Tor anonymity network, the distribution of topology information relies on the correct behavior of five out of the nine trusted directory authority servers. This centralization is concerning since a powerful adversary might compromise these servers and conceal information about honest nodes, leading to the full de-anonymization of all Tor users. Our work aims at distributing the work of these trusted authorities, such increasing resilience against attacks on core infrastructure components of the Tor network. In particular, we leverage several emerging technologies, such as blockchains, smart contracts, and trusted execution environments to design and prototype a system called SmarTor. This system replaces the directory authorities with a smart contract and a distributed network of untrusted entities responsible for bandwidth measurements. We prototyped SmarTor using Ethereum smart contracts and Intel SGX secure hardware. In our evaluation, we show that SmarTor produces significantly more reliable and precise measurements compared to the current measurement system. Overall, our solution improves the decentralization of the Tor network, reduces trust assumptions and increases resilience against powerful adversaries like law enforcement and intelligence services.}, author = {Greubel, Andre and Dmitrienko, Alexandra and Kounev, Samuel}, journal = {Annual Computer Security Applications Conference (ACSAC 2018)}, keywords = {Ethereum Hauptautor Paper Tor blockchains contracts environments execution hardware hauptautor intel network privacy secure security sgx smart trusted}, month = {dec}, title = {SmarTor - Smarter Tor with Smart Contracts}, year = 2018 }