Cryptology ePrint Archive: Report 2017/011

Chameleon-Hashes with Ephemeral Trapdoors And Applications to Invisible Sanitizable Signatures

Jan Camenisch and David Derler and Stephan Krenn and Henrich C. Pöhls and Kai Samelin and Daniel Slamanig

Abstract: A chameleon-hash function is a hash function that involves a trapdoor the knowledge of which allows one to find arbitrary collisions in the domain of the function. In this paper, we introduce the notion of chameleon-hash functions with ephemeral trapdoors. Such hash functions feature additional, i.e., ephemeral, trapdoors which are chosen by the party computing a hash value. The holder of the main trapdoor is then unable to find a second pre-image of a hash value unless also provided with the ephemeral trapdoor used to compute the hash value. We present a formal security model for this new primitive as well as provably secure instantiations. The first instantiation is a generic black-box construction from any secure chameleon-hash function. We further provide three direct constructions based on standard assumptions. Our new primitive has some appealing use-cases, including a solution to the long-standing open problem of invisible sanitizable signatures, which we also present.

Category / Keywords: public-key cryptography / chameleon-hash functions, chameleon-hash functions with ephemeral trapdoors, trapdoor commitments, sanitizable signatures, invisible sanitizable signatures

Original Publication (with major differences): IACR-PKC-2017

Date: received 9 Jan 2017, last revised 3 Mar 2017

Contact author: ksa at zurich ibm com

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Note: clarifications and typos

Version: 20170303:094243 (All versions of this report)

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