Books

Papers

• Finding Low-Weight Polynomial Multiples Using the Rho Method. To appear at Africacrypt 2022.

• A New Approach for finding Low-Weight Polynomial Multiples. Inscrypt 2021. Full version.

• Toward Practical Group Encryption (with Marc Joye). ACNS 2013: 237-252. Full version

• Efficient Group Signatures in the Standard Model (with Olivier Sanders). ICISC 2012: 410-424. Full version

• Generic Constructions for Verifiable Signcryption. ICISC 2011: 204-218. Full version

• Reselling Digital Content (with Yona Raekow). ARES 2010: 391-396.

• Efficient Confirmer Signatures from the "Signature of a Commitment" Paradigm. ProvSec 2010: 87-101. Full version

• Anonymity from Public Key Encryption to Undeniable Signatures. AFRICACRYPT 2009: 217-234.

• On Generic Constructions of Designated Confirmer Signatures. INDOCRYPT 2009: 343-362. Full version

• Exploring Subliminal Channels in Pairing-Based Signatures (with Yona Raekow). WEWoRC 2009

• Toward a Generic Construction of Universally Convertible Undeniable Signatures from Pairing-Based Signatures. INDOCRYPT 2008: 145-157. Full version

• Gradually Convertible Undeniable Signatures (with Damien Vergnaud). ACNS 2007: 478-496. Full version

• Finding Low Weight Polynomial Multiples Using Lattices (with Joachim von zur Gathen). Poster session of the LLL+ 25 conference. Full version

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Patents

• Group Signature Methods and Devices (with Olivier Sanders)

• Group Encryption Methods and Devices (with Marc Joye)

• Signcryption Methods and Devices

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PhD thesis

• Title: Design and Analysis of Opaque Signatures (thesis, talk)

• Defense date: 29 April 2011

• PhD committee:

  • Prof. Dr. Joachim von zur Gathen, supervisor (b-it, Universität Bonn)

  • Prof. Dr. Marek Karpinski (Universität Bonn)

  • Prof. Dr. Alexander Markowetz (Universität Bonn)

  • Prof. Dr. Kenny Paterson (Royal Holloway, University of London)

  • Prof. Dr. Jean-Jacques Quisquater (Université Catholique de Louvain)

• Abstract:

Digital signatures were introduced to guarantee the authenticity and integrity of the underlying messages. A digital signature scheme comprises the key generation, the signature, and the verification algorithms. The key generation algorithm creates the signing and the verifying keys, called also the signer’s private and public keys respectively. The signature algorithm, which is run by the signer, produces a signature on the input message. Finally, the verification algorithm, run by anyone who knows the signer’s public key, checks whether a purported signature on some message is valid or not. The last property, namely the universal verification of digital signatures is undesirable in situations where the signed data is commercially or personally sensitive. Therefore, mechanisms which share most properties with digital signatures except for the universal verification were invented to respond to the aforementioned need; we call such mechanisms “opaque signatures”. In this thesis, we study the signatures where the verification cannot be achieved without the cooperation of a specific entity, namely the signer in case of undeniable signatures, or the confirmer in case of confirmer signatures; we make three main contributions.

We first study the relationship between two security properties important for public key encryption, namely data privacy and key privacy. Our study is motivated by the fact that opaque signatures involve always an encryption layer that ensures their opacity. The properties required for this encryption vary according to whether we want to protect the identity (i.e. the key) of the signer or hide the validity of the signature. Therefore, it would be convenient to use existing work about the encryption scheme in order to derive one notion from the other.

Next, we delve into the generic constructions of confirmer signatures from basic cryptographic primitives, e.g. digital signatures, encryption, or commitment schemes. In fact, generic constructions give easy-to-understand and easy-to-prove schemes, however, this convenience is often achieved at the expense of efficiency. In this contribution, which constitutes the core of this thesis, we first analyze the already existing constructions; our study concludes that the popular generic constructions of confirmer signatures necessitate strong security assumptions on the building blocks, which impacts negatively the efficiency of the resulting signatures. Next, we show that a small change in these constructionsmakes these assumptions drop drastically, allowing as a result constructions with instantiations that compete with the dedicated realizations of these signatures.

Finally, we revisit two early undeniable signatures which were proposed with a conjectural security. We disprove the claimed security of the first scheme, and we provide a fix to it in order to achieve strong security properties. Next, we upgrade the second scheme so that it supports a desirable feature, and we provide a formal security treatment of the new scheme: we prove that it is secure assuming new reasonable assumptions on the underlying constituents.

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