Séminaire de Cryptographie

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Laila El Aimani

Design and Analysis of Opaque Signatures

Digital signatures were introduced to guarantee the authenticity and integrity of the underlying messages. However, and in situations where the signed data is commercially or personally sensitive, the universal verification of digital signatures is undesirable, and needs to be limited or controlled. Therefore, mechanisms which share most properties with digital signatures except the universal verification were invented to respond to the aforementioned need; we call such mechanisms ``opaque signatures''. In this talk, we study confirmer signatures where the verification cannot be achieved without the cooperation of a specific entity, i.e. the confirmer, via the so-called confirmation/denial protocols.

Generic constructions of designated confirmer signatures follow one of the following two strategies; either produce a digital signature on the message to be signed, then encrypt the resulting signature, or produce a commitment on the message, encrypt the string used to generate the commitment, and finally sign the latter. In this talk, we revisit both methods and establish the minimal and sufficient assumptions on the building blocks in order to attain secure confirmer signatures. Our study concludes that both paradigms, when used in their basic form, cannot allow a class of encryption schemes which is vital for the efficiency of the confirmation/denial protocols. Next, we propose a variation of both paradigms which thrives on very cheap encryption and consequently leads to efficient confirmer signatures. Indeed, the resulting constructions do not only compete with the dedicated realizations of confirmer/undeniable signatures proposed recently, e.g. \citep{LeTrieuKurosawaOgata2009b,SchuldtMatsuura2010}, but also serve for analyzing the early schemes that have a speculative security. The contents of this talk are parts of the speaker's PhD thesis.