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.
Open communication over the Internet poses a serious threat to countries with repressive regimes, leading them to develop and deploy censorship mechanisms within their networks. Unfortunately, existing censorship circumvention systems face difficulties in providing unobservable communication with their clients; this highly limits their availability as censors can easily block access to circumvention systems that make observable communication patterns. Moreover, the lack of unobservability may pose serious threats to their users. Recent research takes various approaches to tackle this problem, however they introduce new challenges, and the provided unobservability is breakable. In this paper we propose an easy-to-deploy and unobservable censorship-resistant infrastructure, called FreeWave. FreeWave works by modulating a client’s Internet traffic into acoustic signals that are carried over VoIP connections. Such VoIP connections are targeted to a server, the FreeWave server, that extracts the tunneled traffic and proxies them to the uncensored Internet. The use of actual VoIP connections, as opposed to traffic morphing, allows FreeWave to relay its VoIP connections through oblivious VoIP nodes (e.g., Skype supernodes), hence keeping the FreeWave server(s) unobservable and unblockable. In addition, the use of end-to-end encryption, which is supported mandated by most VoIP providers like Skype, prevents censors from distinguishing FreeWave’s VoIP connections from regular VoIP connections. To utilize a VoIP connection’s throughput efficiently we design communications encoders tailored specifically for VoIP’s lossy channel. We prototype FreeWave over Skype, the most popular VoIP system. We show that FreeWave is able to reliably achieve communication throughputs that are sufficient for web browsing, even when clients are far distanced from the FreeWave server. We also validate FreeWave’s communication unobservability against traffic analysis and standard censorship techniques. Keywords-Internet Censorship; Circumvention; VoIP; The Tor network is designed to provide users with low-latency anonymous communications. Tor clients build circuits with publicly listed relays to anonymously reach their destinations. However, since the relays are publicly listed, they can be easily blocked by censoring adversaries. Consequently, the Tor project envisioned the possibility of unlisted entry points to the Tor network, commonly known as bridges. We address the issue of preventing censors from detecting the bridges by observing the communications between them and nodes in their network. We propose a model in which the client obfuscates its messages to the bridge in a widely used protocol over the Internet. We investigate using Skype video calls as our target protocol and our goal is to make it difficult for the censoring adversary to distinguish between the obfuscated bridge connections and actual Skype calls using statistical comparisons. We have implemented our model as a proof-of-concept pluggable transport for Tor, which is available under an open-source licence. Using this implementation we observed the obfuscated bridge communications and compared it with those of Skype calls and presented the results.