Decentralized Reliability Estimation for Low Latency Mixnets

Mix networks (mixnets) enable anonymous packet-based communication via multi-hop routing. Measuring the reliability of mix nodes serving as intermediate hops in a way that is decentralized, accurate, public, and compatible with low-latency packet routing, remains, however, a challenge: existing verifiable mixing schemes introduce routing delays ranging from minutes to hours that grow with traffic volume, severely limiting their practicality for large-scale or latency-sensitive applications. We propose a decentralized scheme that provides public reliability estimates for a mixnet's links and nodes without increasing latency for client traffic. The scheme achieves optimal complexity, independent of total network traffic, and enables accurate, low-overhead estimation even at scale. Our approach relies on verifiable measurement packets generated via a novel VRF-based routing primitive. This mechanism produces unforgeable measurement packets indistinguishable from normal traffic, while ensuring that all packet routing choices are consistent with the mixnet's routing policy. Reliability scores are derived from revealed measurement packets. The scheme remains robust under both unreliable and adversarial conditions, producing accurate reliability estimates as long as every honest node has a majority of honest predecessors and successors in the routing graph. We validate our design experimentally, demonstrating its practicality and effectiveness.