Research Interests

My research explores the boundaries of cryptography and computational complexity theory on the one hand as well as cryptography and security on the other hand. The theoretical part of my research investigates the hardness of instantiating a primitive or a cryptographic protocol, while the applied side of my research focuses on efficient instantiates. A complete list of my publications is available here.

Foundations of Cryptography

Research in this area focuses on the minimal assumptions that are needed to realize a certain primitive as well as the round complexity of cryptographic protocols.
  • “Impossibility of Blind Signatures from One-Way Permutations” (with Jonathan Katz and Arkady Yerukhimovich). TCC 2011
  • “On the Impossibility of Three-Move Blind Signature Schemes” (with Marc Fischlin). Eurocrypt 2011
  • “Round Optimal Blind Signatures” (with S. Garg, V. Rao, A. Sahai, and D. Unruh). CRYPTO 2011
  • “Uniqueness is a Different Story: Impossibility of Verifiable Random Functions from Trapdoor Permutations” (with Dario Fiore)

Public-Key Cryptography

Research in this area focuses on security models and efficient constructions.
  • How to Aggregate the CL Signature”. ESORICS 2011
  • Confidential Signatures and Deterministic Signcryption” (with Alexander W. Dent, Marc Fischlin, Mark Manulis, Martijn Stam). PKC 2010
  • “Security of Verifiably Encrypted Signatures and a Construction Without Random Oracles” (with Markus Rückert). Pairings 2009

Malleable Signature Schemes

A “malleable” signature schemes allows a third party to modify parts of the message without contradicting their verifiability and unforgeability. In the case of sanitizable signature scheme only designated parties can modify the message while redactable signature schemes allow anybody to change parts of the message.
  • “Security of Sanitizable Signatures Revisited” (with C. Brzuska, M. Fischlin, T. Freudenreich, A. Lehmann, M. Page, J. Schelbert, and F. Volk). PKC 2009
  • “Unlinkability of Sanitizable Signatures” (with C. Brzuska, M. Fischlin, and A. Lehmann). PKC 2010
  • “Sanitizable Signatures: How to Partially Delegate Control for Authenticated Data” (with C. Brzuska, M. Fischlin, and A. Lehmann) BIOSIG 2009
  • “Redactable Signatures for Tree-Structured Data: Definitions and Construction” (with C. Brzuska, H. Busch, Ö. Dagdelen, M. Fischlin, M. Franz, S. Katzenbeisser, M. Manulis, C. Onete, A. Peters and B. Poettering. ACNS 2010

Private-Key Cryptography

Research in this area focuses on security models and efficient constructions.
  • “History-Free Aggregate Message Authentication Codes” (with O. Eikemeier, M. Fischlin, J.-F. Goetzmann, A. Lehmann, P. Schröder, D. Wagner). SCN 2010

Hash Functions

Non-malleability of a cryptographic primitive is a fundamental security property which ensures some sort of independence of cryptographic values. Boldyreva et al. study the notion of non-malleability for hash functions presenting a simulation based definition. In the following paper, we give a more handy approach to non-malleability of hash functions. Our notion avoids simulators completely and is applicable to strengthened Merkle-Damgard and it suffices for the security of the Bellare-Rogaway encryption scheme.
  • “Expedient Non-Malleability Notions for Hash Functions” (with Paul Baecher and Marc Fischlin) RSA 2011

CAPTCHA

A CAPTCHA is an automated challenge and response program to distinguish humans from computers. In this paper, we survey the state-of-the-art of currently deployed CAPTCHAs and show that they are rather weak.
  • “CAPTCHAs: The Good, the Bad, and the Ugly” (with P. Baecher, L. Gordon, M. Fischlin, and M. Lützow). Sicherheit 2010