Paper 2026/1831 Original: eprint.iacr.org/2026/1831 ↗

Silent-Share: Decoupling Hidden Threshold Matching from Pairing Operations via Group-Valued Oblivious Key-Value Stores

Abstract

Matchmaking encryption (ME) enables bilateral access control with private policies, but existing pairing-based constructions tie receiver-side authorization cost to the policy size. This is especially problematic when one party holds a large hidden policy while the other holds only a small attribute set. We present Silent-Share, a bilateral hidden-policy threshold access-control protocol that decouples policy representation from pairing-based authorization. The construction combines a one-sided hidden-threshold policy-based key encapsulation mechanism (PB-KEM) with a sparse group-valued oblivious key-value store (GOKVS). The GOKVS compactly encodes policy-dependent group elements, so a receiver holding attribute set $\mathcal{A}$ performs exactly $2|\mathcal{A}|$ pairings, independent of the policy size $|\mathcal{P}|$ and threshold $d$. Total decapsulation additionally incurs a hidden-threshold reconstruction cost, characterized separately. Two independent one-sided instances are composed and bound with AES-GCM to realize bilateral authorization. We prove one-sided KEM confidentiality and policy hiding in the random-oracle model under a hidden common exponent assumption, and extend these guarantees to the bilateral composition. Our implementation on BN254 shows that, when the correct $d$-subset is provided, one-sided decapsulation for $|\mathcal{A}|=10$ takes about $394$ ms, dominated by pairing operations. The pairing-based authorization layer remains flat as $|\mathcal{P}|$ grows from $50$ to $800$, confirming the policy-size independence. The hidden-threshold reconstruction cost is reported separately and can dominate when $|\mathcal{A}|$ is large. Encapsulation is approximately $2$--$3\times$ faster than fuzzy matchmaking encryption across the tested parameter range.