Paper 2025/973 Original: eprint.iacr.org/2025/973 ↗

On Proving Equivalence Class Signatures Secure from Non-interactive Assumptions

Abstract

Equivalence class signatures (EQS), introduced by Hanser and Slamanig (AC’14, J.Crypto’19), sign vectors of elements from a bi- linear group. Their main feature is “adaptivity”: given a signature on a vector, anyone can transform it to a (uniformly random) signature on any multiple of the vector. A signature thus authenticates equivalence classes and unforgeability is defined accordingly. EQS have been used to improve the efficiency of many cryptographic applications, notably (delegatable) anonymous credentials, (round-optimal) blind signatures, group signa- tures and anonymous tokens. EQS security implies strong anonymity (or blindness) guarantees for these schemes which hold against malicious signers without trust assumptions. Unforgeability of the original EQS construction is proven directly in the generic group model. While there are constructions from standard assumptions, these either achieve prohibitively weak security notions (PKC’18) or they require a common reference string (AC’19, PKC’22), which reintroduces trust assumptions avoided by EQS. In this work we ask whether EQS schemes that satisfy the original secu- rity model can be proved secure under standard (or even non-interactive) assumptions with standard techniques. Our answer is negative: assum- ing a reduction that, after running once an adversary breaking unforge- ability, breaks a non-interactive computational assumption, we construct efficient meta-reductions that either break the assumption or break class- hiding, another security requirement for EQS.