International Association for Cryptologic Research

Ph.D. Database

The aim of the IACR Ph.D. database is twofold. On the first hand, we want to offer an overview of Ph.D. already completed in the domain of cryptology. Where possible, this should also include a subject classification, an abstract, and access to the full text. On the second hand, it deals with Ph.D. subjects currently under investigation. This way, we provide a timely map of contemporary research in cryptology. All entries or changes need to be approved by an editor. You can contact them via phds (at)


Jean Monnerat (#549)
Name Jean Monnerat
Personal Homepage
Topic of his/her doctorate. Short Undeniable Signatures: Design, Analysis, and Applications
Category public-key cryptography
Keywords undeniable signatures, short signatures
Ph.D. Supervisor(s) Serge Vaudenay
Year of completion 2006

Digital signatures are one of the main achievements of public-key cryptography and constitute a fundamental tool to ensure data authentication. Although their universal veri?ability has the advantage to facilitate their veri?cation by the recipient, this property may have undesirable consequences when dealing with sensitive and private information. Motivated by such considerations, undeniable signatures, whose veri?cation requires the cooperation of the signer in an interactive way, were invented.

This thesis is mainly devoted to the design and analysis of short undeniable signatures. Exploiting their online property, we can achieve signatures with a fully scalable size depending on the security requirements. To this end, we develop a general framework based on the interpolation of group elements by a group homomorphism, leading to the design of a generic undeniable signature scheme. On the one hand, this paradigm allows to consider some previous undeniable signature schemes in a uni?ed setting. On the other hand, by selecting group homomorphisms with a small group range, we obtain very short signatures.

After providing theoretical results related to the interpolation of group homomorphisms, we develop some interactive proofs in which the prover convinces a veri?er of the interpolation (resp. non-interpolation) of some given points by a group homomorphism which he keeps secret. Based on these protocols, we devise our new undeniable signature scheme and prove its security in a formal way. We theoretically analyze the special class of group characters on Z_n^* After studying algorithmic aspects of the homomorphism evaluation, we compare the efficiency of different homomorphisms and show that the Legendre symbol leads to the fastest signature generation. We investigate potential applications based on the speci?c properties of our signature scheme. Finally, in a topic closely related to undeniable signatures, we revisit the designated con?rmer signature of Chaum and formally prove the security of a generalized version.

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