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.
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. 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.