A censor hosting Skype supernodes can perform passive traffic-flow analysis on relayed streams even without breaking encryption, since supernode-relayed conversations expose traffic metadata. However, with thousands of supernodes in the Skype network, the probability that any censor-controlled supernode relays a specific SkyF2F tunnel is low, making large-scale correlation high-cost.

Website fingerprinting attacks that match file sizes and access patterns against a database of known sites remain applicable to SkyF2F, but are limited to the granularity of 512-byte fixed-size stream cells, since streams are multiplexed within a single tunnel circuit. The authors note this is less effective than against SafeWeb, where full request/response sizes are directly observable.

§V.A detection

Because Skype relies on a central login server, it is technically possible for a censor to block Skype, but the paper observes that blocking widely-deployed services like Skype or Google inflicts real economic harm, making it a credible deterrent. Additionally, Skype's proprietary, closed-source protocol and P2P architecture make it harder to characterize and selectively filter than open protocols.

§V.C defense

SkyF2F's friend-to-friend service model, where a server publishes its appid only to trusted contacts rather than publicly, provides significant resistance to both sybil attacks (malicious censor-controlled servers) and DoS exhaustion attacks. A censor posing as a client can establish many tunnels to exhaust a public server's resources; restricting service to a trusted friend list eliminates most of this attack surface.

§V.A defense

SkyF2F tunnels censored traffic through Skype's encrypted overlay network, forcing the censor into an all-or-nothing dilemma: blocking SkyF2F requires blocking Skype entirely, which causes actual economic damage to businesses and users who depend on it. Because Skype users are identified by pseudonym and all messages are routed to overlay addresses rather than Internet addresses, IP-based blocking, DNS filtering, port blocking, and keyword filtering are all rendered ineffective.

§V.A defense

A three-stage detection pipeline exploiting the "dual-role" behavioral fingerprint of single-IP circumvention relays achieved 23.2% recall (96/414 ground-truth relays) with a 0.18% false-positive rate against 97,651 benign TLS servers, for an overall accuracy of 99.5%. The ground-truth set covered OpenVPN, WireGuard, and SOCKS relays identified in a 17 TB single-day backbone trace (WIDE Project, April 9, 2025).

2026-almutairi-server §3.4, Table 1 traffic-shapedpiflow-correlation

Multi-censor simulations show that single-censor-optimized distribution strategies perform suboptimally in realistic multi-region deployments. When two networks have different censor strategies (e.g., one optimal, one zig-zag), the distributor cannot detect that a proxy is blocked until all censors have blocked it; this leaves clients without reachable proxies despite the proxy appearing "available" from the distributor's view. The authors conclude that "single-censor evaluation does not accurately predict more realistic deployment performance." A zig-zag censor in one region with 0.25 weight caused 44.4% collateral damage while reducing proxy lifetime to a median of 4 steps.

2026-fares-game §6, Table 4 traffic-shapeflow-correlation

The zig-zag traffic analysis attack (confirmed supported in Geedge TSG leak) rapidly enumerates all static proxy pools. With ζ_watch ∈ {4, 6} steps and a best-quality classifier (ρ_TP=0.99, ρ_FP=0.001), almost total proxy enumeration and user blockage occurs well before step 300. Even ζ_watch=2 leaves ~50% of users blocked. Collateral damage is high across all settings when ζ_watch ≥ 4: eventually ~50% of innocent servers are also blocked. However, Snowflake-style ephemeral proxies resist zig-zag effectively: reachability remains above 95% after 360 steps because churn prevents the censor from expanding its known proxy set beyond agents' direct assignments.

2026-fares-game §5.1, Fig 3, Fig 4 traffic-shapeflow-correlation

Per-flow RTTdiff detection rates are only ~20% because the majority of proxy flows connect to CDN-cached content (Cloudflare, Google, Fastly) that sits within 5ms of the proxy, suppressing the discrepancy. However, aggregating across flows per website visit yields detection rates exceeding 70%—and from the abstract, approximately 80% of top-5K domains generate at least one detectable flow—with half of those detections made within the first 60 packets. This means an adversary can reliably expose client and proxy IPs after just a few website visits.

2025-xue-discriminative §VI-C-2, Table II traffic-shapeflow-correlation

IMAP/SSL traffic on port 993 constitutes less than 1% of total ISP traffic but accounts for nearly one third of all false positives in the RTTdiff exploit, because IMAP's non-RESTful multi-connection pattern violates the request-response correlation assumption. The overall per-flow FPR is bounded at 0.6–0.7% (on par with GFW's estimated FPR against fully-encrypted proxies), but implementing a pre-filter to whitelist IMAP traffic reduces the FPR by approximately one third, making the fingerprint substantially more precise.

2025-xue-discriminative §VI-C-3, Table III traffic-shapeflow-correlation

Cross-layer RTT discrepancy (RTTdiff) is a protocol-agnostic fingerprint that exploits an inherent architectural property of all proxy setups: transport-layer sessions terminate at the proxy while application-layer sessions remain end-to-end. Evaluation across 10 proxy protocols—including VMess, Shadowsocks, VLESS, Trojan, XTLS-Vision, and obfs4-wrapped SOCKS—shows near-identical detection rates for all except obfs4, confirming the fingerprint is not tied to any specific obfuscation scheme. At FPR=0.01, per-website detection rates exceed 70% across all tested client and proxy location combinations.

2025-xue-discriminative §I, §VI-C traffic-shapeflow-correlation