Contact discovery is a crucial component of social applications, facilitating interactions between registered contacts. This work introduces Arke, a novel contact discovery scheme that addresses the limitations of existing solutions in terms of privacy, scalability, and reliance on trusted third parties. Arke ensures the unlinkability of user interactions, mitigates enumeration attacks, and operates without single points of failure or trust. Notably, Arke is the first contact discovery system whose performance is independent of the total number of users and the first that can operate in a Byzantine setting. It achieves its privacy goals through an unlinkable handshake mechanism built on top of an identity-based non-interactive key exchange. By leveraging a custom distributed architecture, Arke forgoes the expense of consensus to achieve scalability while maintaining consistency in an adversarial environment. Performance evaluations demonstrate that Arke provides a throughput of over 1,500 user requests per second at a latency of less than 0.5 seconds in a large geo-distributed setting which would allow privacy-preserving contact discovery for all of the popular messaging applications in one system.

Abstract We consider the problem of proving in zero knowledge that an element of a public set satisfies a given property without disclosing the element, i.e., for some u , “ $$u \in S$$ u ∈ S and P ( u ) holds”. This problem arises in many applications (anonymous cryptocurrencies, credentials or whitelists) where, for privacy or anonymity reasons, it is crucial to hide certain data while ensuring properties of such data. We design new modular and efficient constructions for this problem through new commit-and-prove zero-knowledge systems for set membership , i.e. schemes proving $$u \in S$$ u ∈ S for a value u that is in a public commitment $$c_u$$ c u . We also extend our results to support non-membership proofs , i.e. proving $$u \notin S$$ u ∉ S . Being commit-and-prove, our solutions can act as plug-and-play modules in statements of the form “ $$u \in S$$ u ∈ S and P ( u ) holds” by combining our set (non-)membership systems with any other commit-and-prove scheme for P ( u ). Also, they work with Pedersen commitments over prime order groups which makes them compatible with popular systems such as Bulletproofs or Groth16. We implemented our schemes as a software library, and tested experimentally their performance. Compared to previous work that achieves similar properties—the clever techniques combining zkSNARKs and Merkle Trees in Zcash—our solutions offer more flexibility, shorter public parameters and $$3.7 \times $$ 3.7 × – $$30\times $$ 30 × faster proving time for a set of size $$2^{64}$$ 2 64 .

WAKU-RLN-RELAY is an anonymous peer-to-peer gossip-based routing protocol that features a privacy-preserving spam-protection with cryptographically guaranteed economic incentives. While being an anonymous routing protocol where routed messages are not attributable to their origin, it allows global identification and removal of spammers. It addresses the performance and privacy issues of its counterparts including proof-of-work and reputation-based schemes. Its light computational overhead makes it suitable for resource-limited environments. The spam protection works by limiting the messaging rate of each network participant where rate violation results in financial punishment. We deploy the novel construct of rate-limiting nullifier to enforce the message rate limit. We provide a proof-of-concept implementation of WAKU-RLN-RELAY to prove the efficiency and feasibility of our solution.