Paper 2025/1982 Original: eprint.iacr.org/2025/1982 ↗

On Evaluating Anonymity of Onion Routing

Abstract

Anonymous communication networks (ACNs) aim to thwart an adversary, who controls or observes chunks of the communication network, from determining the respective identities of two communicating parties. We focus on low-latency ACNs such as Tor, which target a practical level of anonymity without incurring an unacceptable transmission delay. While several definitions have been proposed to quantify the level of anonymity provided by high-latency, message-centric ACNs (such as mix-nets and DC-nets), this approach is less relevant to Tor, where user–destination pairs communicate over secure overlay circuits. Moreover, existing evaluation methods of traffic analysis attacks on Tor appear somewhat ad hoc and fragmented. We propose a fair evaluation framework for such attacks against onion routing systems by identifying and discussing the crucial components for evaluation, including how to consider various adversarial goals, how to factor in the adversarial ability to collect information relevant to the attack, and how these components combine to suitable metrics to quantify the adversary's success.