2012-lincoln-bootstrapping
findings extracted from this paper
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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.
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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.
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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 This reactive probing renders unpublished Tor entry points discoverable even when not listed in any directory.
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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.
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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.