@article {18501, title = {Chipping away at censorship firewalls with user-generated content}, journal = {Proc. 19th USENIX Security Symposium, Washington, DC}, year = {2010}, month = {2010///}, abstract = {Oppressive regimes and even democratic governmentsrestrict Internet access. Existing anti-censorship systems often require users to connect through proxies, but these systems are relatively easy for a censor to discover and block. This paper offers a possible next step in the cen- sorship arms race: rather than relying on a single system or set of proxies to circumvent censorship firewalls, we explore whether the vast deployment of sites that host user-generated content can breach these firewalls. To ex- plore this possibility, we have developed Collage, which allows users to exchange messages through hidden chan- nels in sites that host user-generated content. Collage has two components: a message vector layer for embedding content in cover traffic; and a rendezvous mechanism to allow parties to publish and retrieve messages in the cover traffic. Collage uses user-generated content (e.g., photo-sharing sites) as {\textquotedblleft}drop sites{\textquotedblright} for hidden messages. To send a message, a user embeds it into cover traffic and posts the content on some site, where receivers retrieve this content using a sequence of tasks. Collage makes it difficult for a censor to monitor or block these messages by exploiting the sheer number of sites where users can exchange messages and the variety of ways that a mes- sage can be hidden. Our evaluation of Collage shows that the performance overhead is acceptable for sending small messages (e.g., Web articles, email). We show how Collage can be used to build two applications: a direct messaging application, and a Web content delivery system. }, author = {Burnett,S. and Feamster, Nick and Vempala,S.} }