From December 8 to 12 2019, ASIACRYPT 2019, the 25th Annual International Conference on the Theory and Application of Cryptology and Information Security will be held in Kobe, Japan. ASIACRYPT is an important international conference for cryptography research. It is sponsored by the International Association for Cryptologic Research (IACR) since 2000, one of its three flagship conferences, and a top-tier conference in Security & Privacy.

In a paper that is going to be presented at ASIACRYPT 2019 in December, we, in cooperation with Mojtaba Khalili and Mohammad Dakhilalian (both Isfahan University of Technology, Iran), have investigated how to construct a specific type of structure-preserving signatures (SPS). SPS are pairing-based signature schemes where all the messages, signatures and public keys are group elements in the source groups of a bilinear group, with numerous applications in public-key cryptography.

The specific type of SPS we are interested in are so called SPS on equivalence classes (SPS-EQ), where a message (in our figure (u,v)) is considered as a projective equivalence class, and a new representative of the same class (in our figure (u^ρ,v^ρ)) can be obtained by multiplying a vector by a scalar ρ. At the same time an existing signature σ can be updated to a signature σ' for the new representative of the class without access to the secret key. 

SPS-EQ have proven to be useful in many cryptographic applications that enhance the privacy of users such as attribute-based anonymous credentials, delegatable anonymous credentials, self-blindable certificates, blind signatures, group signatures, sanitizable signatures, verifiably encrypted signatures, access control encryption or proving the correctness of a shuffle in mix-nets (i.e., for anonymous communication or electronic voting).

Specifically, in the paper we resolve a problem left unsolved since SPS-EQ were introduced at ASIACRYPT 2014. Namely, while previous construction satisfying strong security notions were only known in an idealized model called the (bilinear) generic group model, we present constructions that can be proven to be secure under standard cryptographic assumptions (something that is desirable to gain confidence in the constructions) and in particular the assumption that the so called Decisional Diffie Hellman assumption holds in both source groups of the bilinear group (the by now widely accepted SXDH assumption). While relying on standard assumptions comes at a performance penalty, the proposed construction sill provides good efficiency for practical applications.

The full version of the paper to be published in the procceedings of ASIACRYPT 2019, appearing in the Lecture Notes in Computer Science (LNCS) series by Springer, is freely available on the IACR Cryptology ePrint Archive.

The paper: Mojtaba Khalili, Daniel Slamanig, Mohammad Dakhilalian. Structure-Preserving Signatures on Equivalence Classes From Standard Assumptions. 25th Annual International Conference on the Theory and Application of Cryptology and Information Security - ASIACRYPT 2019, December 8-12 2019, Kobe, Japan