| 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 “drop sites” 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.
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