The mod_freedom Apache module hooks into the HTTP 404 ErrorDocument handler and steganographically embeds encrypted NET payloads in image responses to valid RP requests, while returning normal content to all other clients. Using Identity-Based Encryption (IBE, Boneh-Franklin) keyed on the server's hostname eliminates any need for out-of-band public-key distribution and allows deployment on thousands of volunteer webservers without mutual trust.

DEFIANCE's Address-Change Signaling (ACS) requires each client to contact a sequence of IP addresses with precise timing (per-user wait and window parameters) and a one-time passphrase derived from NET provisioning. Connections arriving out of order, outside the timing window, or lacking the correct passphrase receive only innocuous content, so a censor probing a suspected address block finds only normal commodity servers.

§4 defense

A balls-and-bins analysis shows that an adversary conducting N full rounds of a rate-limited rendezvous protocol discovers only 63% of a pool of N entry points; full coverage requires N ln N rounds (the coupon collector's bound). Concretely, with three 8-hour shifts of 100 humans performing 60-minute CAPTCHA+proof-of-work challenges, an adversary discovers ~2,400 entry points per day, exhausting a static pool of 10,000 addresses in roughly 19 days.

§3 evaluation

The Chinese Great Firewall was observed conducting two follow-up probes for each outbound TCP/443 connection: the first with garbage binary data (target unknown) and the second specifically performing an SSL negotiation, an SSL renegotiation, and successfully building a one-hop Tor circuit to confirm the bridge. This reactive probing renders unpublished Tor entry points discoverable even when not listed in any directory.

§1 detection

NET payloads are wrapped in three nested layers — (1) steganographic encoding plus transport encryption with a factory digital signature, (2) proof-of-life (CAPTCHA), and (3) proof-of-work (computational puzzle) — so that even an adversary who harvests many payloads cannot decode them faster than gateway addresses can be rotated. The payload format is explicitly extensible to add harder challenges as adversaries improve.

§3.3 defense

InterSecLab's 76-page analysis of the Geedge/MESA leak (based on nine months of indexing and translating >100,000 documents) characterizes the Tiangou Secure Gateway (TSG) product line as a commercially deployable detection stack that combines deep packet inspection, real-time mobile subscriber monitoring, active probing, ML-based traffic classifiers, and granular per-region rule sets. TSG is not a research prototype — leaked documentation includes deployment timelines and client government interactions for Kazakhstan, Ethiopia, Pakistan, Myanmar, and one unnamed country, with censorship rules explicitly tailored to each region.

2025-interseclab-internet-coup §3–§5 (InterSecLab report) dpiactive-probingml-classifiersni-blockingip-blockingtraffic-shapefully-encrypted-detect

Justice for Myanmar documents that Geedge Networks supplied Myanmar's military junta with GFW-derived surveillance and censorship infrastructure under Belt and Road frameworks following the February 2021 coup. The deployed system (Tiangou Secure Gateway / TSG) incorporates the same DPI, active-probing, and ML-classifier capabilities as the domestic Chinese GFW, giving Myanmar one of the most technically capable censorship systems in Southeast Asia.

2025-jfm-silk-road-surveillance §3, §5 dpisni-blockingip-blockingtraffic-shapeml-classifieractive-probing

Snowflake's sustained operation in heavily censored regions demonstrates that WebRTC must remain accessible to users, which in turn requires that TURN servers remain unblocked to support NAT traversal for peer-to-peer WebRTC connections. This transitive unblockability makes TURN service providers viable rendezvous channels for the Bridge Distribution Problem.

2025-vilalonga-extended §3.2 Bridge Distribution Problem ip-blockingactive-probing

TURN servers used by major applications such as Facebook Messenger for media relay are hypothesized to be less likely blocked in censored regions due to collateral damage to legitimate WebRTC traffic. Providers like Cloudflare, Metered Video, and ExpressTURN supply geographically distributed TURN infrastructure that can be used without any special configuration by a censorship evasion system.

2025-vilalonga-extended §1 Introduction & Background dpiactive-probingip-blocking

Traffic splitting across N TURN proxies (1 ≤ N ≤ M) is hypothesized to resist active probing because each TURN server responds to probing requests identically to a regular TURN server, providing no distinguishing signal. Additionally, proxy ephemerality combined with splitting allows on-the-fly migration to new proxies when existing ones are blocked, maintaining connectivity even under partial blocking.

2025-vilalonga-extended §3.3 Censorship Evasion active-probingip-blockingdpi

The paper defines Unauthenticated Push (UP) channels as a distinct archetype from signaling/rendezvous channels, characterized by three properties: strictly unidirectional delivery, no client authentication or account association required, and higher bandwidth (kilobytes to megabytes) to support software updates rather than just minimal proxy-address exchanges. This design deliberately shifts operational-security burden onto senders to approach receiver anonymity.

2025-vines-extended §1–§3 ip-blockingactive-probing