Masayuki Abe
NTT Social Informatics Laboratories & Kyoto University

Masaya Nanri
Kyoto University

Octavio Perez Kempner
NTT Social Informatics Laboratories

Mehdi Tibouchi
NTT Social Informatics Laboratories & Kyoto University

Keywords: Mercurial Signatures, Equivalence Class Signatures,Threshold Signatures, Class-Hiding, and Anonymous Credentials.

Abstract

Mercurial signatures are an extension of equivalence class signatures that allow malleability for the public keys, messages, and signatures within the respective classes. Unfortunately, the most efficient construction to date suffers from a weak public key class-hiding property, where the original signer with the signing key can link the public keys in the same class. This is a severe limitation in their applications, where the signer is often considered untrustworthy of privacy.

This paper presents two-party and multi-party interactive threshold mercurial signatures that overcome the above limitation by eliminating the single entity who knows the signing key. For the general case, we propose two constructions. The first follows the same interactive structure as the two-party case, avoiding complex distributed computations such as randomness generation, inversion, and multiplication, and even eliminates the need for private communication between parties. The second is based on a blueprint for general multi-party computation using verifiable secret sharing, but adopting optimizations.

We show applications in anonymous credential systems that individually fit the two-party and multi-party constructions. In particular, in the two-party case, our approach provides stronger privacy by completely removing the trust in the authorities. We also discuss more applications, from blind signatures to multi-signatures and threshold ring signatures.

Finally, to showcase the practicality of our approach, we implement our interactive constructions and compare them against related alternatives.