357 ripe 2014 telex tapdance

Anticensorship in the Network Infrastructure Eric Wustrow University of Michigan

Background | Internet Censorship

Pervasive censorship Substantial censorship Selective censorship Changing situation Little or no censorship 2

Threat Model

Censor … controls client’s network, but not external network … blocks according to a blacklist … allows HTTPS connections to non-blocked sites 3

Telex | Overview

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Telex | Overview

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Telex | Overview

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Telex | Overview

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Telex | Overview

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Telex | Overview

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Prototype | Test Deployment Single Telex Station on lab-scale “ISP” at Michigan Hosted sites NotBlocked.telex.cc

Blocked.telex.cc

Unobjectionable content *

Simulated censored site only reachable via Telex

Inline Blocking Asymmetric flows

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Telex v2: Passive tap

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New architecture -- passive ISP tap Client

ISP Proxy

Server

TLS Handshake Plaintext: “ G Ciphertext: “\x ET / HTTP/1.1\r\nX-Ignore : 95\x1f\x6b\x2 7\xe2 … Ta \x81\x28\x66 …” g: \xc8\x3f\x22 …” ack=X] , Y = q e s [ ACK , len=M] X = k c a , Y = OK” [seq Y X O R P “ : t Plaintex

ACK [seq=X ack != Y? , ack=Y+M] Plaintext: “G ET http://bloc ked.com/ …” [seq=X, ack= Y+M] tml> ….” h < …

K O /1.1 200 P T T H “ : t x Plainte

New architecture -- passive ISP tap • Pros – No inline blocking required, only passive tap – Works with asymmetric flows (client -> server) • Cons – Censor can use active attacks • (though we can use “active defenses”)

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Anticensorship in the Network Infrastructure • Future work – Looking for ISPs willing to help • Technical feedback • Prototype deployment

– Strategies for optimal deployment – Improving traffic analysis defense

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Anticensorship in the

Network Infrastructure

https://telex.cc Eric Wustrow Colleen M. Swanson Scott Wolchok Ian Goldberg J. Alex Halderman