Sanitizable signatures are signature schemes which support delegation of modification rights. The signer can allow a third party, the sanitizer, to perform certain admissible modifications to a signed message and then to update the signature, in such a way that basic security properties like unforgeability or accountability are preserved. Recently, Camenisch et al. (PKC 2017) devised new schemes with the previously unattained invisibility property, which says that an outsider is unable to tell which message blocks the sanitizer is allowed to modify. Subsequently, Beck et al. (ACISP 2017) gave an even stronger version of this notion and constructions achieving it. In this talk we characterize the invisibility property in both forms by showing that invisible sanitizable signatures are equivalent to IND−CPA-secure encryption schemes, and strongly invisible sanitizable signatures are equivalent to IND−CCA2-secure encryption schemes. The equivalence is established by proving that invisible (resp. strongly invisible) sanitizable signature schemes yield IND−CPA-secure (resp. IND−CCA2-secure) public-key encryption schemes and that, vice versa, we can build (strongly) invisible sanitizable signatures given a corresponding encryption scheme.
Patrick Harasser graduated with an M.Sc. in Mathematics from the University of Trento (Italy) and the University of Tübingen (Germany) in March 2016. In March 2017 he joined the “Cryptography and Complexity Theory” group at Darmstadt University of Technology, lead by Prof. Dr. Marc Fischlin, as a Ph.D. student. He has since been working on CROSSING Project P2.