2026-wang-who-carries-tor
findings extracted from this paper
Country-level analysis shows significant sensitivity of AS-level transit dependency to the distribution of relay capacity across ASes within each country, implying that a censor controlling a dominant national transit AS gains disproportionately high visibility into Tor traffic if domestic relay capacity is concentrated in that AS's customer cone.
The authors propose a diversity-aware relay placement heuristic derived from bandwidth-weighted AS Hegemony scores to guide future mitigation of transit concentration. By incorporating consensus bandwidth weighting into hegemony calculations, the methodology reflects actual Tor traffic patterns rather than relay counts alone.
SURF (AS1103) surged to over 50% exit hegemony in 2024, meaning a single AS gained visibility into the majority of bandwidth-weighted Tor exit traffic. This represents a dramatic concentration event in an otherwise persistent but distributed chokepoint landscape spanning 2015–2025.
A longitudinal study of Tor transit dependency from 2015 to 2025 reveals persistent concentration in a small number of transit ASes, indicating that AS-level chokepoints enabling flow-correlation attacks are a structural rather than transient property of the Tor network.