Welcome to provsec@CISPA!

We are a research group at the CISPA Helmholtz Center for Information Security, where we work on the foundations of modern cryptography through the lens of provable security. We analyze cryptographic schemes used in practice to obtain strong and concrete security guarantees, and we design new cryptographic schemes and protocols motivated by real-world applications. This often involves developing new formal models, security notions, and proof techniques.

Our research spans a broad range of topics in applied and theoretical cryptography. In particular, we work on authenticated key exchange and secure messaging, public-key encryption and signature schemes with advanced functionalities, post-quantum cryptography, anonymous and privacy-preserving cryptographic protocols, and the theory of tight security.

Team

Doreen Riepel

Dr. Doreen Riepel

Faculty

 Xiangyu Liu

Dr. Xiangyu Liu

Postdoc

 Subham Das

Subham Das

PhD Student
Visitors and Alumni

Visitors

Alumni

Weidan JiRemote Intern, Jun–Dec 2025

Agni DattaIntern, Jun–Aug 2025

News

2026

Aug

Our work on anonymous AKEMs got accepted at ASIACRYPT 2026 🎉 The paper will be up soon!

Jul

Three papers accepted at CRYPTO 2026 🎉 — check out our new results on updatable KEMs, anamorphic encryption and JIT-OPRFs. Thanks to all co-authors!

May

Together with Martijn Stam, Doreen organizes ProTeCS, a workshop on Proofs and Proof Techniques for Cryptographic Security, as an affiliated event at EUROCRYPT 2026 in Rome. We are very happy to have Bart Mennink and Mike Rosulek as invited speakers!

Apr

Doreen and Sabrina Kunzweiler launch PELICAN 🐦, an Équipe Associée funded by Inria to improve the provable security of isogeny-based cryptographic protocols. The funding supports mutual visits between our groups.

Mar

Xiangyu joins the PKC 2027 program committee. 📋

Feb

Leveled Isogeny Problems with Hints accepted at PKC 2026 🎉 Great work Subham!

more

Publications

Authors are listed alphabetically by last name, except where indicated by an asterisk (*). When available, slides and videos of conference talks are linked. Talks presented by co-authors are marked with a circle (°).

Preprints
K-Linkable Ring Signatures and Applications in Generalized Voting

Wonseok Choi, Xiangyu Liu, Yanxin Pang, Lirong Xia, Vassilis Zikas

IACR Cryptology ePrint Archive, 2025

eprint
Traceable Ring Signatures Revisited: Extended Definitions, O(1) Tracing, and Efficient Log-Size Constructions

Xiangyu Liu

IACR Cryptology ePrint Archive, 2025

eprint
Snake Mackerel: An Isogeny-Based AKEM Leveraging Randomness Reuse

Jonas Janneck, Jonas Meers, Massimo Ostuzzi, Doreen Riepel

IACR Cryptology ePrint Archive, 2025

eprint
2026
Symmetric and Asymmetric Anonymous Authenticated KEM(to appear)

Benedikt Auerbach, Doreen Riepel, Paul Rösler, Lea Thiemt, Julian Thomas

International Conference on the Theory and Application of Cryptology and Information Security (ASIACRYPT)

eprint code
A Qualitative Study on Ethics Perspectives in Cryptography Research(to appear)

*Maria Hellenthal, Thorsten Helfer, Doreen Riepel, Annabelle Walle, Jannik Zeiser, Til Dralle, Sascha Fahl

ACM Conference on Computer and Communications Security (CCS)

Just-in-Time-OPRFs and a Modular Framework for Fast Private Set Intersection

Mihir Bellare, Rishabh Ranjan, Doreen Riepel

Annual International Cryptology Conference (CRYPTO)

eprint springer slides°
Lattice-Based Updatable KEM for Group Messaging

Joël Alwen, Georg Fuchsbauer, Marta Mularczyk, Doreen Riepel

Annual International Cryptology Conference (CRYPTO)

eprint springer slides
A Unified Treatment of Anamorphic Encryption

Wonseok Choi, Daniel Collins, Xiangyu Liu, Roy Stracovsky, Vassilis Zikas

Annual International Cryptology Conference (CRYPTO)

eprint
Leveled Isogeny Problems with Hints

Subham Das, Riccardo Invernizzi, Péter Kutas, Jonas Meers

IACR International Conference on Practice and Theory of Public-Key Cryptography (PKC)

eprint springer
Secure Cloud Storage: Modularization, Network Adversaries and Adaptive Corruptions

Jonas Janneck, Doreen Riepel

Annual International Conference on the Theory and Applications of Cryptographic Techniques (EUROCRYPT)

eprint springer slides° video°
The Algebraic Isogeny Model: A General Model with Applications to SQIsign and Key Exchanges

Marius A. Aardal, Andrea Basso, Doreen Riepel

Annual International Conference on the Theory and Applications of Cryptographic Techniques (EUROCRYPT)

eprint springer slides°
2025
Anonymous Authentication and Key Agreement, Revisited

*Yanqi Zhao, Xiangyu Liu, Min Xie, Xiaoyi Yang, Jianting Ning, Baodong Qin, Haibin Zhang, Yong Yu

Annual Computer Security Applications Conference (ACSAC)

eprint ieee
Blockchain Governance via Sharp Anonymous Multisignatures

Wonseok Choi, Xiangyu Liu, Vassilis Zikas

ACM Conference on Advances in Financial Technologies (AFT)

eprint lipics
ABE Cubed: Advanced Benchmarking Extensions for ABE Squared

*Sven Argo, Marloes Venema, Doreen Riepel, Tim Güneysu, Diego F. Aranha

IACR Transactions on Cryptographic Hardware and Embedded Systems (CHES)

eprint tches code slides° video°
Intermundium-DL: Assessing the Resilience of Current Schemes to Discrete-Log-Computation Attacks on Public Parameters

Mihir Bellare, Doreen Riepel, Laura Shea

IACR International Conference on Practice and Theory of Public-Key Cryptography (PKC)

eprint springer slides° video°
Public-Algorithm Substitution Attacks: Subverting Hashing and Verification

Mihir Bellare, Doreen Riepel, Laura Shea

IACR International Conference on Practice and Theory of Public-Key Cryptography (PKC)

eprint springer slides° video°
Verifiable and Provably Secure Machine Unlearning

*Thorsten Eisenhofer, Doreen Riepel, Varun Chandrasekaran, Esha Ghosh, Olga Ohrimenko, Nicolas Papernot

IEEE Conference on Secure and Trustworthy Machine Learning (SaTML)

arxiv ieee code
2024
The Concrete Security of Two-Party Computation: Simple Definitions, and Tight Proofs for PSI and OPRFs

Mihir Bellare, Doreen Riepel, Rishabh Ranjan, Ali Aldakheel

International Conference on the Theory and Application of Cryptology and Information Security (ASIACRYPT)

eprint springer slides° video°
Count Corruptions, Not Users: Improved Tightness for Signatures, Encryption and Authenticated Key Exchange

Mihir Bellare, Doreen Riepel, Stefano Tessaro, Yizhao Zhang

International Conference on the Theory and Application of Cryptology and Information Security (ASIACRYPT)

eprint springer slides video
Tightly-Secure Group Key Exchange with Perfect Forward Secrecy

Emanuele Di Giandomenico, Doreen Riepel, Sven Schäge

International Conference on the Theory and Application of Cryptology and Information Security (ASIACRYPT)

eprint springer slides° video°
On the Tight Security of the Double Ratchet

Daniel Collins, Doreen Riepel, Si An Oliver Tran

ACM Conference on Computer and Communications Security (CCS)

eprint acm slides
ISABELLA: Improving Structures of Attribute-Based Encryption Leveraging Linear Algebra

Doreen Riepel, Marloes Venema, Tanya Verma

ACM Conference on Computer and Communications Security (CCS)

eprint acm code slides°
Key Exchange with Tight (Full) Forward Secrecy via Key Confirmation

Jiaxin Pan, Doreen Riepel, Runzhi Zeng

Annual International Conference on the Theory and Applications of Cryptographic Techniques (EUROCRYPT)

eprint springer slides video
CCA Secure Updatable Encryption from Non-Mappable Group Actions

Jonas Meers, Doreen Riepel

Post-Quantum Cryptography (PQCrypto)

eprint springer slides video
2023
Tightly-Secure Authenticated Key Exchange

Doreen Riepel

PhD Thesis, Ruhr University Bochum

pdf rub
Multi-User CDH Problems and the Concrete Security of NAXOS and HMQV

Eike Kiltz, Jiaxin Pan, Magnus Ringerud, Doreen Riepel

Topics in Cryptology – CT-RSA

eprint springer slides
Generic Models for Group Actions

Julien Duman, Dominik Hartmann, Eike Kiltz, Sabrina Kunzweiler, Jonas Lehmann, Doreen Riepel

IACR International Conference on Practice and Theory of Public-Key Cryptography (PKC)

eprint springer slides° video°
No More Reviewer #2: Subverting Automatic Paper-Reviewer Assignment using Adversarial Learning

*Thorsten Eisenhofer, Erwin Quiring, Jonas Möller, Doreen Riepel, Thorsten Holz, Konrad Rieck

USENIX Security Symposium

arxiv usenix code
2022
Strongly Anonymous Ratcheted Key Exchange

Benjamin Dowling, Eduard Hauck, Doreen Riepel, Paul Rösler

International Conference on the Theory and Application of Cryptology and Information Security (ASIACRYPT)

eprint springer slides° video°
Group Action Key Encapsulation and Non-Interactive Key Exchange in the QROM

Julien Duman, Dominik Hartmann, Eike Kiltz, Sabrina Kunzweiler, Jonas Lehmann, Doreen Riepel

International Conference on the Theory and Application of Cryptology and Information Security (ASIACRYPT)

eprint springer slides° video°
FABEO: Fast Attribute-Based Encryption with Optimal Security

Doreen Riepel, Hoeteck Wee

ACM Conference on Computer and Communications Security (CCS)

eprint acm code slides
Password-Authenticated Key Exchange from Group Actions

Michel Abdalla, Thorsten Eisenhofer, Eike Kiltz, Sabrina Kunzweiler, Doreen Riepel

Annual International Cryptology Conference (CRYPTO)

eprint springer slides video
2021
Authenticated Key Exchange and Signatures with Tight Security in the Standard Model

Shuai Han, Tibor Jager, Eike Kiltz, Shengli Liu, Jiaxin Pan, Doreen Riepel, Sven Schäge

Annual International Cryptology Conference (CRYPTO)

eprint springer slides video
Analysing the HPKE Standard

Joël Alwen, Bruno Blanchet, Eduard Hauck, Eike Kiltz, Benjamin Lipp, Doreen Riepel

Annual International Conference on the Theory and Applications of Cryptographic Techniques (EUROCRYPT)

eprint springer cryptoverif slides° video°
Tightly-Secure Authenticated Key Exchange, Revisited

Tibor Jager, Eike Kiltz, Doreen Riepel, Sven Schäge

Annual International Conference on the Theory and Applications of Cryptographic Techniques (EUROCRYPT)

eprint springer slides video

BibTeX