The use of TLS in Censorship Circumvention Sergey Frolov Eric Wustrow University of Colorado Boulder University of Colorado Boulder
[email protected] [email protected] Abstract—TLS, the Transport Layer Security protocol, has TLS [54], and adoption continues to grow as more websites, quickly become the most popular protocol on the Internet, already services, and applications switch to TLS. used to load over 70% of web pages in Mozilla Firefox. Due to its ubiquity, TLS is also a popular protocol for censorship Given the prevalence of TLS, it is commonly used by circumvention tools, including Tor and Signal, among others. circumvention tools to evade Internet censorship. Because However, the wide range of features supported in TLS makes censors can easily identify and block custom protocols [30], it possible to distinguish implementations from one another by circumvention tools have turned to using existing protocols. what set of cipher suites, elliptic curves, signature algorithms, and TLS offers a convenient choice for these tools, providing other extensions they support. Already, censors have used deep plenty of legitimate cover traffic from web browsers and packet inspection (DPI) to identify and block popular circumven- other TLS user, protection of content from eavesdroppers, and tion tools based on the fingerprint of their TLS implementation. several libraries to choose from that support it. In response, many circumvention tools have attempted to mimic popular TLS implementations such as browsers, but this However, simply using TLS for a transport protocol is technique has several challenges. First, it is burdensome to keep not enough to evade censors. Since TLS handshakes are not up with the rapidly-changing browser TLS implementations, and encrypted, censors can identify a client’s purported support for know what fingerprints would be good candidates to mimic.