Paper 2026/402

Conditionally Linkable Attribute-Based Signatures

Abstract

Attribute-Based Signatures (ABS) enable users to authenticate messages under expressive attribute policies while remaining anonymous. Existing ABS variants, however, treat linkability as a static, system-wide property: signatures are either always unlinkable, as in standard ABS, or globally linkable, as in traceable or accountable extensions. This rigid dichotomy fails to capture scenarios where correlation should arise only under explicitly declared conditions. This work introduces Conditionally Linkable Attribute-Based Signatures (CLABS), a framework extending ABS with programmable, context-dependent linkability. Each certified user with attribute $x$ is associated with a linking set $L_x$ over a public context space $\mathcal{T}$. For each context $\tau\in\mathcal{T}$, a public function $f_\tau$ specifies how attributes are compared. Two signatures are publicly linkable if and only if $\tau\in L_x\cap L_{x'}$ and $f_\tau(x)=f_\tau(x')$; otherwise they remain unlinkable. This enables selective, verifiable correlation without central trust and with leakage limited to the opt-in bit. We formalize the syntax and security notions of CLABS, capturing conditional linkability and context-aware anonymity, thereby ensuring privacy and verifiable linkage under voluntary participation. CLABS unifies global unlinkability and fine-grained, context-specific linkage within a single formal framework. We realize CLABS generically using three modular components: a pseudorandom function for deterministic tag generation, a conventional signature for attribute certification, and a signature of knowledge (SoK) proving correct tag computation and Boolean policy satisfaction without revealing $x$. Finally, we instantiate CLABS under standard lattice assumptions in the quantum random oracle model (QROM), achieving post-quantum security while supporting arbitrary Boolean policies. The techniques we employ to prove circuit satisfiability and tag correctness may be of independent interest.