An increasing number of countries and companies routinely block or
monitor access to parts of the Internet. To counteract these
measures, we propose Infranet, a system that enables clients to
surreptitiously retrieve sensitive content via cooperating Web servers
distributed across the global Internet. These Infranet servers
provide clients access to censored sites while continuing to host
normal uncensored content. Infranet uses a tunnel protocol that
provides a covert communication channel between its clients and
servers, modulated over standard HTTP transactions that resemble
innocuous Web browsing. In the upstream direction, Infranet clients
send covert messages to Infranet servers by associating meaning to the
sequence of HTTP requests being made. In the downstream
direction, Infranet servers return content by hiding censored data in
uncensored images using steganographic techniques. We describe the
design, a prototype implementation, security properties, and
performance of Infranet. Our security analysis shows that Infranet
can successfully circumvent several sophisticated censoring
techniques.
Infranet consists of a requester and responder
communicating over a covert tunnel. A requester, running on a user's
computer, uses the tunnel to request censored content. Upon receiving
the request, the responder, a standard public Web server running
Infranet software, retrieves the sought content from the Web and
returns it to the requester via the tunnel.
In the upstream direction, the Infranet tunnel protocol modulates
covert messages on standard HTTP requests for uncensored content.
This is done using a confidentially negotiated function that maps URLs
to message fragments, which compose requests for censored content.
The requester and responder communicate via a channel with far greater
bandwidth from the responder to the requester than vice versa.
Because the responder serves many Infranet users' requests for hidden
content, it can maintain the frequency distribution of hidden
messages. A requester typically wants to send a message from this
distribution.
Software
Work on Infranet is proceeding quickly. To see the latest source code and releases, visit our project page at Sourceforge.
I. Clarke, O. Sandbert, B. Wiley, and T. Hong.
Freenet: A distributed anonymous information storage and retrieval
system.
In Proceedings of the Workshop on Design Issues in Anonymity and
Unobservability, Berkeley, CA, July 2000.
R. Dingledine, M. Freedman, and D. Molnar.
The Free Haven Project: Distributed anonymous storage service.
In Proceedings of the Workshop on Design Issues in Anonymity and
Unobservability, Berkeley, CA, July 2000.
D. Martin and A. Schulman.
Deanonymizing users of the SafeWeb anonymizing service.
In Proc. 11th USENIX Security Symposium, San Francisco, CA,
August 2002.
M. Waldman and D. Mazières.
Tangler: A censorship-resistant publishing system based on document
entanglements.
In Proceedings of the 8th ACM Conference on Computer and
Communications Security, Philadelphia, PA, November 2001.
M. Waldman, A. Rubin, and L. Cranor.
Publius: A robust, tamper-evident, censorship-resistant, web
publishing system.
In Proc. 9th USENIX Security Symposium, pages 59-72, Denver,
CO, August 2000.
