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AG Codes and Kryptographie Show image information

AG Codes and Kryptographie

Jakob Juhnke


Codes and Cryptography

Research Assistant

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Fully-Featured Anonymous Credentials with Reputation System

K. Bemmann, J. Blömer, J. Bobolz, H. Bröcher, D. Diemert, F. Eidens, L. Eilers, J. Haltermann, J. Juhnke, B. Otour, L. Porzenheim, S. Pukrop, E. Schilling, M. Schlichtig, M. Stienemeier, in: Proceedings of the 13th International Conference on Availability, Reliability and Security - ARES '18, 2018

Practical, Anonymous, and Publicly Linkable Universally-Composable Reputation Systems

J. Blömer, F. Eidens, J. Juhnke, in: Lecture Notes in Computer Science, Springer International Publishing, 2018, pp. 470-490


Anonymous and Publicly Linkable Reputation Systems

J. Blömer, J. Juhnke, C. Kolb, in: Proceedings of the 18th International Conference on Financial Cryptography and Data Security (FC), 2015, pp. 478--488

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Reputation systems are used to compute and publish reputation scores for services or products. We consider reputation systems where users are allowed to rate products that they purchased previously. To obtain trustworthy reputations, they are allowed to rate these products only once. As long as users rate products once, they stay anonymous. Everybody is able to detect users deviating from the rate-products-only-once policy and the anonymity of such dishonest users can be revoked by a system manager. In this paper we present formal models for such reputation systems and their security. Based on group signatures presented by Boneh, Boyen, and Shacham we design an efficient reputation system that meets all our requirements.

Short Group Signatures with Distributed Traceability

J. Blömer, J. Juhnke, N. Löken, in: Proceedings of the Sixth International Conference on Mathematical Aspects of Computer and Information Sciences (MACIS), 2015, pp. 166-180


Group signatures, introduced by Chaum and van Heyst [15], are an important primitive in cryptography. In group signature schemes every group member can anonymously sign messages on behalf of the group. In case of disputes a dedicated opening manager is able to trace signatures - he can extract the identity of the producer of a given signature. A formal model for static group signatures schemes and their security is defined by Bellare, Micciancio, and Warinschi [4], the case of dynamic groups is considered by Bellare, Shi, and Zhang [5]. Both models define group signature schemes with a single opening manager. The main difference between these models is that the number of group members in static schemes is fixed, while in dynamic schemes group members can join the group over time.


An Approach for Determining Optimal Contrast in Visual Cryptography

J. Juhnke, H. Lefmann, V. Strehl, Fortschritt-Berichte VDI, 2013

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