Dr.-Ing. Kai Gellert

Short Vita

• 2024: Completed my habilitation and achieved the Lehrbefähigung (i.e., qualification to teach) in Computer Science, specializing in IT Security and Cryptography
• 2023: Appointed Akademischer Rat auf Lebenszeit (i.e., achieved tenure)
• 2021: Appointed Akademischer Rat auf Probe (probation period)
• 2020: Completed my PhD with distinction under supervision of Tibor Jager and with Colin Boyd as second examiner
• 2019-2021: Research assistant at University of Wuppertal, Germany
• 2017-2019: Reseach assistant at Paderborn University, Germany
• 2015-2017: Completed my master's degree in IT Security and Information Engineering at Ruhr University Bochum
• 2012-2015: Completed my bachelor's degree in IT Security and Information Engineering at Ruhr University Bochum

My research interests lie at the intersection of theoretical and real-world cryptography, where I enjoy analyzing and improving the security of real-world communication protocols. This includes, e.g., the analysis of complex protocols such as the WhatsApp end-to-end encrypted backup protocol, developing efficient protocols with tailored security properties, or investigating if certain security properties are even possible to achieve in practice (and how to potentially evade such impossibility results).

Honors and Awards

• 2024: Two of my students won a Barmenia-Förderpreis for outstanding bachelor theses on a topic with a strong mathematical focus
• GDD-Wissenschaftspreis 2020 for an outstanding PhD thesis in the areas of data protection and data security
• Deutschlandstipendium 2016, awarded to students with outstanding achievements and social commitment
• Deutschlandstipendium 2015, awarded to students with outstanding achievements and social commitment

Media Coverage

• Wissenschaftliche Codeknacker; University of Wuppertal, 2022
• GDD-Wissenschaftspreis für Wuppertaler Kryptographie-Forscher; University of Wuppertal, November 2020
• It's time you were T0RTT a lesson: Here's how you could build a better Tor, say boffins; The Register, December 2019
• Effizientere Sicherheit gegen Datenkraken und Massenüberwachung; University of Wuppertal, December 2019

Independent Teaching Experience

During my time at the University of Wuppertal, I have independently taught the following courses:

• Grundlagen der IT-Sicherheit (WiSe 2022/23, WiSe 2023/24, WiSe 2024/25)
• Moderne Kryptographie (SoSe 2022, SoSe 2023, SoSe 2024)
• Effiziente Implementierung kryptographischer Verfahren (WiSe 2020/21, WiSe 2021/22, WiSe 2022/23, WiSe 2024/25)
• Communication Security for Modern Applications (SoSe 2021)

In addition, I have independently supervised and examined > 15 bachelor's and master's theses. Two of my students have won the Barmenia-Förderpreis for an outstanding bachelor thesis.

Academic Service

I have served as an external reviewer for the following conferences and journals:

• 2024: EUROCRYPT, CRYPTO
• 2023: ASIACRYPT, CCS, Designs, Codes and Cryptography, ISC
• 2022: EUROCRYPT, CRYPTO, ESORICS, IEEE Transactions on Dependable and Secure Computing (TDSC), IEEE Transactions on Mobile Computing (TMC)
• 2021: IEEE S&P, ESORICS, ITC
• 2020: Journal of Cryptology, ASIACRYPT, ASIACCS, ISC
• 2019: CCS, PKC, ASIACCS, ACISP
• 2018: CCS, ACNS, CT-RSA
• 2017: PKC, ISC

Peer-Reviewed Publications

2024

• Manuel Barbosa, Kai Gellert, Julia Hesse, Stanislaw Jarecki
  Bare PAKE: Universally Composable Key Exchange from just Passwords
  44th Annual International Cryptology Conference - CRYPTO 2024

2023

• Gareth T. Davies, Sebastian Faller, Kai Gellert, Tobias Handirk, Julia Hesse, Máté Horvath, Tibor Jager
  Security Analysis of the WhatsApp End-to-End Encrypted Backup Protocol
  43th Annual International Cryptology Conference - CRYPTO 2023
• Kai Gellert, Kristian Gjøsteen, Håkon Jacobsen, Tibor Jager
  On Optimal Tightness for Key Exchange with Full Forward Secrecy via Key Confirmation
  43th Annual International Cryptology Conference - CRYPTO 2023

2022

• Kai Gellert, Tibor Jager, Lin Lyu, Tom Neuschulten
  On Fingerprinting Attacks and Length-Hiding Encryption
  RSA Conference, Cryptographers’ Track - CT-RSA 2022

2021

• Denis Diemert, Kai Gellert, Tibor Jager, Lin Lyu
  Digital Signatures with Memory-Tight Security in the Multi-Challenge Setting
  27th Annual International Conference on the Theory and Applications of Cryptology and Information Security - ASIACRYPT 2021
• Colin Boyd, Gareth T. Davies, Bor de Kock, Kai Gellert, Tibor Jager, Lise Millerjord
  Symmetric Key Exchange with Full Forward Security and Robust Synchronization
  27th Annual International Conference on the Theory and Applications of Cryptology and Information Security - ASIACRYPT 2021
• Kai Gellert, Tobias Handirk
  A Formal Security Analysis of Session Resumption Across Hostnames
  26th European Symposium on Research in Computer Security - ESORICS 2021
• Denis Diemert, Kai Gellert, Tibor Jager, Lin Lyu
  More Efficient Digital Signatures with Tight Multi-User Security
  24th International Conference on Practice and Theory of Public-Key Cryptography - PKC 2021
• Nimrod Aviram, Kai Gellert, Tibor Jager
  Session Resumption Protocols and Efficient Forward Security for TLS 1.3 0-RTT
  Journal of Cryptology - Special Issue on TLS 1.3, 2021
• David Derler, Kai Gellert, Tibor Jager, Daniel Slamanig, Christoph Striecks
  Bloom Filter Encryption and Applications to Efficient Forward-Secret 0-RTT Key Exchange
  Journal of Cryptology, 2021

2020

• Fynn Dallmeier, Jan P. Drees, Kai Gellert, Tobias Handirk, Tibor Jager, Jonas Klauke, Simon Nachtigall, Timo Renzelmann, Rudi Wolf
  Forward-Secure 0-RTT Goes Live: Implementation and Performance Analysis in QUIC
  19th International Conference on Cryptology and Network Security 2020
• Colin Boyd, Kai Gellert
  A Modern View on Forward Security
  The Computer Journal, 2020
• Sebastian Lauer, Kai Gellert, Robert Merget, Tobias Handirk, Jörg Schwenk
  T0RTT: Non-Interactive Immediate Forward-Secret Single-Pass Circuit Construction
  Proceedings on Privacy Enhancing Technologies 2020

2019

• Nimrod Aviram, Kai Gellert, Tibor Jager
  Session Resumption Protocols and Efficient Forward Security for TLS 1.3 0-RTT
  38th Annual International Conference on the Theory and Applications of Cryptographic Techniques - EUROCRYPT 2019

Doctoral Dissertation

• Kai Gellert
  Construction and Security Analysis of 0-RTT Protocols
  PhD Thesis, 2020