# Christoph Egger

Christoph Egger
Lehrstuhl für angewandte Kryptographie
Fürther Str. 246c / Eingang 5
90429 Nurnberg

Email: Christoph.Egger@fau.de
Jabber: christoph@egger.im

PGP

9FED 5C6C E206 B70A 5857  70CA 9655 22B9 D49A E731
3C1F 32FB E637 85F2 4461  4AD2 53C2 B1F9 83C5 BAA3


I am a senior PhD Student in Cryptography. As a master student I worked on Modal Logic and Category Theory and as a Bachelor Student on Anonymous Communication and Software Product Lines. I am also a founding member of the FAUST CTF team and its own competition, FAUST-CTF. Finally I am a Free Software person. I have been a Debian Developer for more than 10 years and have contributed to a variety of software projects including the Linux kernel and the Git version control system.

# Research Interests

My current research focus is on cryptographic proofs: Composability (both in the classical Universal Composability interpretation as well as composition and modularization of proofs) and extending security from cryptographic primitives towards their concrete use (for example concerning Ring Signatures and Steganography).

In the future, I would like to work on computational notions of entropy, the limits of black box techniques (and related fields like oracle separations) as well as fine-grained and bounded-space cryptography.

In the past I have worked, among other things, on System Software Engineering and Software Product Lines, Coalgebraic Modal Logic and Anonymity Networks.

# Free Software

I am a Debian Developer since December 2009, my first contributions go back to 2016. From roughly 2010 to 2016 I have been a core member of the kFreeBSD team supporting this rather unusual combination of BSD and GNU components. Additionally, many small contributions to different Free Software projects are the result of my Debian work. I have initiated the internationalization effort of Unknown Horizons originally implementing its multi-language support. As a research assistant in the VAMOS research project I contributed more than 50 changes to the Linux Kernel. I also added public key pinning support to the Git version control system.

# Comunity Service

• 2020, 2021: External Reviewer for Proceedings on Privacy Enhancing Technologies
• 2018: External Reviewer for IEEE Transactions on Information Forensics & Security
• 2021: External Reviewer for ACM Transactions on Privacy and Security
• Subreviewer on multiple occasions including TCC'21, Crypto('19, '20. '21), CCS'19 and S&P'21

# Teaching

In summer 2021 Viktoria Ronge and I designed and taught a one-week summer school course for high-school students. Focus of the course was on cryptographic methodology and zero-knowledge proof systems. Also with Viktoria Ronge in Fall 2019 I organized a (graduate level) seminar on privacy notions.

In addition I have been (co-)responsible for the exercise sessions in multiple courses including "Secure Multi-Party Computation", "Password Based Cryptography" and blockchain-related lectures.

# Publications

Key-schedule Security for the TLS 1.3 Standard
In: IACR Cryptol. ePrint Arch. 2021(467)
@article{DBLP:journals/iacr/BrzuskaDEFKK21,
author    = {Chris Brzuska and
Antoine Delignat{-}Lavaud and
Christoph Egger and
C{\'{e}}dric Fournet and
Markulf Kohlweiss},
title     = {Key-schedule Security for the {TLS} 1.3 Standard},
journal   = {{IACR} Cryptol. ePrint Arch.},
pages     = {467},
year      = {2021},
url       = {https://eprint.iacr.org/2021/467},
timestamp = {Fri, 23 Apr 2021 12:06:25 +0200},
bibsource = {dblp computer science bibliography, https://dblp.org}
}


A Security Framework for Distributed Ledgers
Mike Graf, Daniel Rausch, Viktoria Ronge, Christoph Egger, Ralf Küsters, Dominique Schröder
In: CCS '21: 2021 ACM SIGSAC Conference on Computer and Communications Security, Virtual Event, Republic of Korea, November 15-19, 2021
[PDF | DOI | BibTeX]
@article{DBLP:journals/iacr/EggerGKRRS21,
author    = {Christoph Egger and
Mike Graf and
Ralf K{\"{u}}sters and
Daniel Rausch and
Viktoria Ronge and
Dominique Schr{\"{o}}der},
title     = {A Security Framework for Distributed Ledgers},
journal   = {{IACR} Cryptol. ePrint Arch.},
pages     = {145},
year      = {2021},
url       = {https://eprint.iacr.org/2021/145},
timestamp = {Tue, 02 Mar 2021 23:07:15 +0100},
biburl    = {https://dblp.org/rec/journals/iacr/EggerGKRRS21.bib},
bibsource = {dblp computer science bibliography, https://dblp.org}
}


Foundations of Ring Sampling
Viktoria Ronge, Christoph Egger, Russell W. F. Lai, Dominique Schröder, Hoover H. F. Yin
In: Proc. Priv. Enhancing Technol. 2021(3)

A ring signature scheme allows the signer to sign on behalf of an ad hoc set of users, called a ring. The verifier can be convinced that a ring member signs, but cannot point to the exact signer. Ring signatures have become increasingly important today with their deployment in anonymous cryptocurrencies. Conventionally, it is implicitly assumed that all ring members are equally likely to be the signer. This assumption is generally false in reality, leading to various practical and devastating deanonymizing attacks in Monero, one of the largest anonymous cryptocurrencies. These attacks highlight the unsatisfactory situation that how a ring should be chosen is poorly understood.

We propose an analytical model of ring samplers towards a deeper understanding of them through systematic studies. Our model helps to describe how anonymous a ring sampler is with respect to a given signer distribution as an information-theoretic measure. We show that this measure is robust – it only varies slightly when the signer distribution varies slightly. We then analyze three natural samplers – uniform, mimicking, and partitioning – under our model with respect to a family of signer distributions modeled after empirical Bitcoin data. We hope that our work paves the way towards researching ring samplers from a theoretical point of view.

@article{DBLP:journals/popets/RongeELSY21,
author    = {Viktoria Ronge and
Christoph Egger and
Russell W. F. Lai and
Dominique Schr{\"{o}}der and
Hoover H. F. Yin},
title     = {Foundations of Ring Sampling},
journal   = {Proc. Priv. Enhancing Technol.},
volume    = {2021},
number    = {3},
pages     = {265--288},
year      = {2021},
url       = {https://doi.org/10.2478/popets-2021-0047},
doi       = {10.2478/popets-2021-0047},
timestamp = {Thu, 14 Oct 2021 09:26:27 +0200},
biburl    = {https://dblp.org/rec/journals/popets/RongeELSY21.bib},
bibsource = {dblp computer science bibliography, https://dblp.org}
}


The Patient as Genomic Data Manager - Evaluation of the PROMISE App
Lena Griebel, Marc Hinderer, Ali Amr, Benjamin Meder, Marc Schweig, Dominic Deuber, Christoph Egger, Claudia Kawohl, Annika Krämer, Isabell Flade, Dominique Schröder, Hans-Ulrich Prokosch
In: Digital Personalized Health and Medicine - Proceedings of MIE 2020, Medical Informatics Europe, Geneva, Switzerland, April 28 - May 1, 2020
@inproceedings{DBLP:conf/mie/GriebelHAMSD0KK20,
author    = {Lena Griebel and
Marc Hinderer and
Ali Amr and
Benjamin Meder and
Marc Schweig and
Dominic Deuber and
Christoph Egger and
Claudia Kawohl and
Annika Kr{\"{a}}mer and
Dominique Schr{\"{o}}der and
Hans{-}Ulrich Prokosch},
editor    = {Louise Bilenberg Pape{-}Haugaard and
Christian Lovis and
Patrick Weber and
Per Hostrup Nielsen and
Philip Scott},
title     = {The Patient as Genomic Data Manager - Evaluation of the {PROMISE}
App},
booktitle = {Digital Personalized Health and Medicine - Proceedings of {MIE} 2020,
Medical Informatics Europe, Geneva, Switzerland, April 28 - May 1,
2020},
series    = {Studies in Health Technology and Informatics},
volume    = {270},
pages     = {1061--1065},
publisher = {{IOS} Press},
year      = {2020},
url       = {https://doi.org/10.3233/SHTI200324},
doi       = {10.3233/SHTI200324},
timestamp = {Wed, 21 Jul 2021 17:50:05 +0200},
biburl    = {https://dblp.org/rec/conf/mie/GriebelHAMSD0KK20.bib},
bibsource = {dblp computer science bibliography, https://dblp.org}
}


Julian Brost, Christoph Egger, Russell W. F. Lai, Fritz Schmid, Dominique Schröder, Markus Zoppelt
In: CCS '20: 2020 ACM SIGSAC Conference on Computer and Communications Security, Virtual Event, USA, November 9-13, 2020
[PDF | DOI | BibTeX]
@inproceedings{DBLP:conf/ccs/Brost0LSSZ20,
author    = {Julian Brost and
Christoph Egger and
Russell W. F. Lai and
Fritz Schmid and
Dominique Schr{\"{o}}der and
Markus Zoppelt},
editor    = {Jay Ligatti and
Xinming Ou and
Jonathan Katz and
Giovanni Vigna},
title     = {Threshold Password-Hardened Encryption Services},
booktitle = {{CCS} '20: 2020 {ACM} {SIGSAC} Conference on Computer and Communications
Security, Virtual Event, USA, November 9-13, 2020},
pages     = {409--424},
publisher = {{ACM}},
year      = {2020},
url       = {https://doi.org/10.1145/3372297.3417266},
doi       = {10.1145/3372297.3417266},
timestamp = {Fri, 09 Apr 2021 18:39:46 +0200},
biburl    = {https://dblp.org/rec/conf/ccs/Brost0LSSZ20.bib},
bibsource = {dblp computer science bibliography, https://dblp.org}
}


Atomic Multi-Channel Updates with Constant Collateral in Bitcoin-Compatible Payment-Channel Networks
In: Proceedings of the 2019 ACM SIGSAC Conference on Computer and Communications Security, CCS 2019, London, UK, November 11-15, 2019.
[PDF | DOI | BibTeX]
@inproceedings{DBLP:conf/ccs/0001MM19,
author    = {Christoph Egger and
Pedro Moreno{-}Sanchez and
Matteo Maffei},
editor    = {Lorenzo Cavallaro and
Johannes Kinder and
XiaoFeng Wang and
Jonathan Katz},
title     = {Atomic Multi-Channel Updates with Constant Collateral in Bitcoin-Compatible
Payment-Channel Networks},
booktitle = {Proceedings of the 2019 {ACM} {SIGSAC} Conference on Computer and
Communications Security, {CCS} 2019, London, UK, November 11-15, 2019},
pages     = {801--815},
publisher = {{ACM}},
year      = {2019},
url       = {https://doi.org/10.1145/3319535.3345666},
doi       = {10.1145/3319535.3345666},
timestamp = {Thu, 14 Oct 2021 09:58:24 +0200},
biburl    = {https://dblp.org/rec/conf/ccs/0001MM19.bib},
bibsource = {dblp computer science bibliography, https://dblp.org}
}


My Genome Belongs to Me: Controlling Third Party Computation on Genomic Data
Dominic Deuber, Christoph Egger, Katharina Fech, Giulio Malavolta, Dominique Schröder, Sri Aravinda Krishnan Thyagarajan, Florian Battke, Claudia Durand
In: Proc. Priv. Enhancing Technol. 2019(1)

An individual’s genetic information is possibly the most valuable personal information. While knowledge of a person’s DNA sequence can facilitate the diagnosis of several heritable diseases and allow personalized treatment, its exposure comes with significant threats to the patient’s privacy. Currently known solutions for privacy-respecting computation require the owner of the DNA to either be heavily involved in the execution of a cryptographic protocol or to completely outsource the access control to a third party. This motivates the demand for cryptographic protocols which enable computation over encrypted genomic data while keeping the owner of the genome in full control. We envision a scenario where data owners can exercise arbitrary and dynamic access policies, depending on the intended use of the analysis results and on the credentials of who is conducting the analysis. At the same time, data owners are not required to maintain a local copy of their entire genetic data and do not need to exhaust their computational resources in an expensive cryptographic protocol.

In this work, we present METIS, a system that assists the computation over encrypted data stored in the cloud while leaving the decision on admissible computations to the data owner. It is based on garbled circuits and supports any polynomially-computable function. A critical feature of our system is that the data owner is free from computational overload and her communication complexity is independent of the size of the input data and only linear in the size of the circuit’s output. We demonstrate the practicality of our approach with an implementation and an evaluation of several functions over real datasets.

@article{DBLP:journals/popets/DeuberEFMSTBD19,
author    = {Dominic Deuber and
Christoph Egger and
Katharina Fech and
Giulio Malavolta and
Dominique Schr{\"{o}}der and
Sri Aravinda Krishnan Thyagarajan and
Florian Battke and
Claudia Durand},
title     = {My Genome Belongs to Me: Controlling Third Party Computation on Genomic
Data},
journal   = {Proc. Priv. Enhancing Technol.},
volume    = {2019},
number    = {1},
pages     = {108--132},
year      = {2019},
url       = {https://doi.org/10.2478/popets-2019-0007},
doi       = {10.2478/popets-2019-0007},
timestamp = {Tue, 01 Sep 2020 13:13:12 +0200},
biburl    = {https://dblp.org/rec/journals/popets/DeuberEFMSTBD19.bib},
bibsource = {dblp computer science bibliography, https://dblp.org}
}


Russell W. F. Lai, Christoph Egger, Manuel Reinert, Sherman S. M. Chow, Matteo Maffei, Dominique Schröder
In: 27th USENIX Security Symposium, USENIX Security 2018, Baltimore, MD, USA, August 15-17, 2018

Passwords and access control remain the popular choice for protecting sensitive data stored online, despite their well-known vulnerability to brute-force attacks. A natural solution is to use encryption. Although standard practices of using encryption somewhat alleviate the problem, decryption is often needed for utility, and keeping the decryption key within reach is obviously dangerous. To address this seemingly unavoidable problem in data security, we propose password-hardened encryption (PHE). With the help of an external crypto server, a service provider can recover the user data encrypted by PHE only when an end user supplied a correct password. PHE inherits the security features of password-hardening (Usenix Security ’15), adding protection for the user data. In particular, the crypto server does not learn any information about any user data. More importantly, both the crypto server and the service provider can rotate their secret keys, a proactive security mechanism mandated by the Payment Card Industry Data Security Standard (PCI DSS). We build an extremely simple password-hardened encryption scheme. Compared with the state-of-the-art password-hardening scheme (Usenix Security ’17), our scheme only uses minimal number-theoretic operations and is, therefore, 30% - 50% more efficient. In fact, our extensive experimental evaluation demonstrates that our scheme can handle more than 525 encryption and (successful) decryption requests per second per core, which shows that it is lightweight and readily deployable in large-scale systems. Regarding security, our scheme also achieves a stronger soundness property, which puts less trust on the good behavior of the crypto server.

@inproceedings{DBLP:conf/uss/Lai0RCMS18,
author    = {Russell W. F. Lai and
Christoph Egger and
Manuel Reinert and
Sherman S. M. Chow and
Matteo Maffei and
Dominique Schr{\"{o}}der},
editor    = {William Enck and
title     = {Simple Password-Hardened Encryption Services},
booktitle = {27th {USENIX} Security Symposium, {USENIX} Security 2018, Baltimore,
MD, USA, August 15-17, 2018},
pages     = {1405--1421},
publisher = {{USENIX} Association},
year      = {2018},
url       = {https://www.usenix.org/conference/usenixsecurity18/presentation/lai},
timestamp = {Mon, 01 Feb 2021 08:43:20 +0100},
biburl    = {https://dblp.org/rec/conf/uss/Lai0RCMS18.bib},
bibsource = {dblp computer science bibliography, https://dblp.org}
}


Phoenix: Rebirth of a Cryptographic Password-Hardening Service
In: 26th USENIX Security Symposium, USENIX Security 2017, Vancouver, BC, Canada, August 16-18, 2017

Password remains the most widespread means of authentication, especially on the Internet. As such, it is the Achilles heel of many modern systems. Facebook pioneered using external cryptographic services to harden password-based authentication in a large scale. Everspaugh et al. (USENIX Security ’15) provided the first comprehensive treatment of such a service and proposed the PYTHIA PRF-Service as a cryptographically secure solution. Recently, Schneider et al. (ACM CCS ’16) proposed a more efficient solution which is secure in a weaker security model.

In this work, we show that the scheme of Schneider et al. is vulnerable to offline attacks just after a single validation query. Therefore, it defeats the purpose of using an external crypto service in the first place and it should not be used in practice. Our attacks do not contradict their security claims, but instead show that their definitions are simply too weak. We thus suggest stronger security definitions that cover these kinds of real-world attacks, and an even more efficient construction, PHOENIX, to achieve them. Our comprehensive evaluation confirms the practicability of PHOENIX: It can handle up to 50% more requests than the scheme of Schneider et al. and up to three times more than PYTHIA.

@inproceedings{DBLP:conf/uss/Lai0SC17,
author    = {Russell W. F. Lai and
Christoph Egger and
Dominique Schr{\"{o}}der and
Sherman S. M. Chow},
editor    = {Engin Kirda and
Thomas Ristenpart},
title     = {Phoenix: Rebirth of a Cryptographic Password-Hardening Service},
booktitle = {26th {USENIX} Security Symposium, {USENIX} Security 2017, Vancouver,
pages     = {899--916},
publisher = {{USENIX} Association},
year      = {2017},
url       = {https://www.usenix.org/conference/usenixsecurity17/technical-sessions/presentation/lai},
timestamp = {Mon, 01 Feb 2021 08:43:05 +0100},
biburl    = {https://dblp.org/rec/conf/uss/Lai0SC17.bib},
bibsource = {dblp computer science bibliography, https://dblp.org}
}


Global Caching for the Alternation-free μ-Calculus
Daniel Hausmann, Lutz Schröder, Christoph Egger
In: 27th International Conference on Concurrency Theory, CONCUR 2016, August 23-26, 2016, Québec City, Canada
[PDF | DOI | BibTeX]
@inproceedings{DBLP:conf/concur/HausmannSE16,
author    = {Daniel Hausmann and
Lutz Schr{\"{o}}der and
Christoph Egger},
editor    = {Jos{\'{e}}e Desharnais and
title     = {Global Caching for the Alternation-free {$$\mu$$}-Calculus},
booktitle = {27th International Conference on Concurrency Theory, {CONCUR} 2016,
August 23-26, 2016, Qu{\'{e}}bec City, Canada},
series    = {LIPIcs},
volume    = {59},
pages     = {34:1--34:15},
publisher = {Schloss Dagstuhl - Leibniz-Zentrum f{\"{u}}r Informatik},
year      = {2016},
url       = {https://doi.org/10.4230/LIPIcs.CONCUR.2016.34},
doi       = {10.4230/LIPIcs.CONCUR.2016.34},
timestamp = {Sun, 25 Jul 2021 11:47:10 +0200},
biburl    = {https://dblp.org/rec/conf/concur/HausmannSE16.bib},
bibsource = {dblp computer science bibliography, https://dblp.org}
}


Practical Attacks against the I2P Network
In: Research in Attacks, Intrusions, and Defenses - 16th International Symposium, RAID 2013, Rodney Bay, St. Lucia, October 23-25, 2013. Proceedings
[PDF | DOI | BibTeX]
@inproceedings{DBLP:conf/raid/EggerSKV13,
author    = {Christoph Egger and
Johannes Schlumberger and
Christopher Kruegel and
Giovanni Vigna},
editor    = {Salvatore J. Stolfo and
Angelos Stavrou and
Charles V. Wright},
title     = {Practical Attacks against the {I2P} Network},
booktitle = {Research in Attacks, Intrusions, and Defenses - 16th International
Symposium, {RAID} 2013, Rodney Bay, St. Lucia, October 23-25, 2013.
Proceedings},
series    = {Lecture Notes in Computer Science},
volume    = {8145},
pages     = {432--451},
publisher = {Springer},
year      = {2013},
url       = {https://doi.org/10.1007/978-3-642-41284-4\_22},
doi       = {10.1007/978-3-642-41284-4\_22},
timestamp = {Tue, 14 May 2019 10:00:53 +0200},
biburl    = {https://dblp.org/rec/conf/raid/EggerSKV13.bib},
bibsource = {dblp computer science bibliography, https://dblp.org}
}


Facing the Linux 8000 Feature Nightmare
In: Proceedings of ACM European Conference on Computer Systems (EuroSys 2010), Best Posters and Demos Session
[PDF]

Configuration coverage in the analysis of large-scale system software
Reinhard Tartler, Daniel Lohmann, Christian Dietrich, Christoph Egger, Julio Sincero
In: ACM SIGOPS Oper. Syst. Rev. 45(3)

System software, especially operating systems, tends to be highly configurable. Like every complex piece of software, a considerable amount of bugs in the implementation has to be expected. In order to improve the general code quality, tools for static analysis provide means to check for source code defects without having to run actual test cases on real hardware. Still, for proper type checking a specific configuration is required so that all header include paths are available and all types are properly resolved.

In order to find as many bugs as possible, usually a "full configuration" is used for the check. However, mainly because of alternative blocks in form of #else-blocks, a single configuration is insufficient to achieve full coverage. In this paper, we present a metric for configuration coverage (CC) and explain the challenges for (properly) calculating it. Furthermore, we present an efficient approach for determining a sufficiently small set of configurations that achieve (nearly) full coverage and evaluate it on a recent Linux kernel version.

@article{DBLP:journals/sigops/TartlerLDES11,
author    = {Reinhard Tartler and
Daniel Lohmann and
Christian Dietrich and
Christoph Egger and
Julio Sincero},
title     = {Configuration coverage in the analysis of large-scale system software},
journal   = {{ACM} {SIGOPS} Oper. Syst. Rev.},
volume    = {45},
number    = {3},
pages     = {10--14},
year      = {2011},
url       = {https://doi.org/10.1145/2094091.2094095},
doi       = {10.1145/2094091.2094095},
timestamp = {Mon, 26 Oct 2020 08:24:58 +0100},
biburl    = {https://dblp.org/rec/journals/sigops/TartlerLDES11.bib},
bibsource = {dblp computer science bibliography, https://dblp.org}
}



# Student Theses

• Julian Brost: "Threshold Password-Hardened Encryption" (FAU Master Thesis; with Dominique Schröder, Russell Lai)
• Kirthivaasan Puniamurthy: "A proof viewer for State-separating proofs" (Aalto Master Thesis; with Christopher Brzuska, Konrad Kohbrok, Sabine Oechsner)

# Talks

• FAUST Workshop Crypto Hacking

FAUST Workshop, GPN18, ICMP9

• BGP und OSPF – wie das Internet funktioniert

• DNSSEC

• Debian -- jetzt auch ohne Linux