Providing reliable and surreptitious communications is difficult in the presence of adaptive and resourceful state level censors. In this paper we introduce Tithonus, a framework that builds on the Bitcoin blockchain and network to provide censorship-resistant communication mechanisms. In contrast to previous approaches, we do not rely solely on the slow and expensive blockchain consensus mechanism but instead fully exploit Bitcoin's peer-to-peer gossip protocol. We develop adaptive, fast and cost effective data communication solutions that camouflage client requests into inconspicuous Bitcoin transactions. We propose solutions to securely request and transfer content, with unobservability and censorship resistance, and free, pay-per-access and subscription based payment options. When compared to state-of-the-art Bitcoin writing solutions, Tithonus reduces the cost of transferring data to censored clients by 2 orders of magnitude and increases the goodput by 3 to 5 orders of magnitude. We show that Tithonus client initiated transactions are hard to detect, while server initiated transactions cannot be censored without creating split world problems to the Bitcoin blockchain.
@cite_16 further proposed domain fronting, that sets up circumvention proxies on web services that share IP addresses with other benign services. While blocking all such IPs (including CDN IPs) is possible to a powerful censor, it would block access to content considered benign and even useful to the censor.
@cite_16
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We describe "domain fronting," a versatile censorship circumvention technique that hides the re- mote endpoint of a communication. Domain fronting works at the application layer, using HTTPS, to com- municate with a forbidden host while appearing to com- municate with some other host, permitted by the cen- sor. The key idea is the use of dierent domain names at dierent layers of communication. One domain appears on the "outside" of an HTTPS request—in the DNS re- quest and TLS Server Name Indication—while another domain appears on the "inside"—in the HTTP Host header, invisible to the censor under HTTPS encryp- tion. A censor, unable to distinguish fronted and non- fronted trac to a domain, must choose between allow- ing circumvention trac and blocking the domain en- tirely, which results in expensive collateral damage. Do- main fronting is easy to deploy and use and does not re- quire special cooperation by network intermediaries. We identify a number of hard-to-block web services, such as content delivery networks, that support domain-fronted connections and are useful for censorship circumvention. Domain fronting, in various forms, is now a circumven- tion workhorse. We describe several months of deploy- ment experience in the Tor, Lantern, and Psiphon cir- cumvention systems, whose domain-fronting transports now connect thousands of users daily and transfer many terabytes per month. The cached Internet content served by content delivery networks (CDN) comprises a large fraction of today's Internet traffic, yet, there is little study on how real-world censors deal with blocking forbidden CDN-hosted Internet content. We investigate the techniques used by the Great Firewall of China to block CDN-hosted content, and demonstrate that blocking CDN content poses unique technical and non-technical challenges to the censors. We therefore design a client-side circumvention system, CacheBrowser, that leverages the censors' difficulties in blocking CDN content. We implement CacheBrowser and use it to unblock CDN-hosted content in China with a download latency significantly smaller than traditional proxy-based circumvention systems like Tor. CacheBrowser's superior quality-of-service is thanks to its publisher-centric approach, which retrieves blocked content directly from content publishers with no use of third-party proxies. CDNBrowsing is a promising approach recently proposed for censorship circumvention. CDNBrowsing relies on the fact that blocking content hosted on public CDNs can potentially cause the censors collateral damage due to disrupting benign content publishers. In this work, we identify various low-cost attacks against CDNBrowsing, demonstrating that the design of practically unobservable CDNBrowsing systems is significantly more challenging than what thought previously. We particularly devise unique website fingerprinting attacks against CDNBrowsing traffic, and discover various forms of information leakage in HTTPS that can be used to block the previously proposed CDNBrowsing system. Motivated by the attacks, we design and implement a new CDNBrowsing system called CDNReaper, which defeats the discovered attacks. By design, a CDNBrowsing system can browse only particular types of webpages due to its proxy-less design. We perform a comprehensive measurement to classify popular Internet websites based on their browsability by CDNBrowsing systems. To further increase the reach of CDNBrowsing, we devise several mechanisms that enable CDNBrowsing systems to browse a larger extent of Internet webpages, particularly partial-CDN webpages. We present results from measurements of the filtering of HTTP HTML responses in China, which is based on string matching and TCP reset injection by backbone-level routers. This system, intended mainly for Internet censorship, is a national-scale filter based on intrusion detection system (IDS) technologies. Our results indicate that the Chinese censors discontinued this HTML response filtering for the majority of routes some time between August 2008 and January 2009 (other forms of censorship, including backbone-level GET request filtering, are still in place). In this paper, we give evidence to show that the distributed nature of this filtering system and the problems inherent to distributed filtering are likely among the reasons it was discontinued, in addition to potential traffic load problems. When the censor successfully detected a keyword in our measurements and attempted to reset the connection, their attempt to reset the connection was successful less than 51 of the time, due to late or out-of-sequence resets. In addition to shedding light on why HTML response filtering may have been discontinued by the censors, we document potential sources of uncertainty, which are due to routing and protocol dynamics, that could affect measurements of any form of censorship in any country. Between a single client IP address in China and several contiguous server IP addresses outside China, measurement results can be radically different. This is probably due to either traffic engineering or one node from a bank of IDS systems being chosen based on source IP address. Our data provides a unique opportunity to study a national-scale, distributed filtering system.