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.
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.