2016-douglas-salmon
findings extracted from this paper
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Salmon simulations show that a censor with agents comprising 1% of 10,000 users can block at most 4A servers (one block per agent per full group) against a system with 1,000–2,000 servers; server groups with a hard cap of M=10 users that fill entirely with legitimate users before any agent joins become permanently invincible to server discovery. The censor's optimal strategy is to ensure each agent is always alone in its group at the time of joining, which requires knowing the user arrival rate — information Salmon withholds by not publishing user statistics.
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Without recommendation-tree grouping logic, a censor starting agents at trust level 6 who each recommend 1–2 additional agents (requiring 4–5 months of waiting) can cut off over 95% of users even at agent percentages in the 15–30% range, as shown in Figure 6. With recommendation-tree grouping enforced, the same attack at equivalent agent fractions produces dramatically lower service disruption because agents cluster among themselves rather than spreading across innocent user groups.
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Salmon's trust-level mechanism (7 discrete levels; promotion from level n to n+1 requires 2^(n+1) days; banning triggered when suspicion exceeds T=1/3) reduces the fraction of users cut off by an attacking censor by more than 3× relative to rBridge under the same agent-percentage conditions. Simulations with 10,000 users (1–10% censor agents) and 1,000–2,000 servers show that trust levels keep high-seniority innocent users isolated from newer users where agents concentrate.
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A single harvesting script running for 9 days on one free Amazon EC2 instance verified 3,101 working VPN Gate servers by testing 44,039 IP addresses, demonstrating that VPN Gate's collective defense mechanism — which relies on detecting automated scanning patterns — can be fully bypassed by routing successive queries through previously verified VPN servers. This result implies that a censor could, with no collateral damage, essentially completely shut down VPN Gate by blocking all verified servers.
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Salmon's defense against the active zig-zag attack — where a censor blocks a known server to force users onto new ones and watches for correlated reassignments — requires both per-user authentication (unique login credentials per server so unauthorized probes receive a plausible HTTPS page) and traffic camouflage. Without authentication, the server must respond as a functioning proxy to any connection, fully exposing itself to the censor; without camouflage, even a rejected connection may reveal the server's nature.