Measuring the Adoption of TLS Encrypted Client Hello Extension and Its Forebear in the Wild.

The Transport Layer Security (TLS) protocol was introduced to solve the lack of security and privacy in the early versions of the world wide web. However, even though it has substantially evolved over the years, certain features still present privacy issues. One such feature is the Server Name Indication (SNI) extension, which allows multiple web servers to reside behind a provider hosting multiple domains with the same IP address; at the same time it allows third parties to discover the domains that end users visit. In the last few years, the Encrypted Server Name Indication (ESNI) Internet draft is being developed by the Internet Engineering Task Force (IETF); this encrypted variant of the extension was renamed to Encrypted Client Hello (ECH) in latest versions. In this paper, we measure the adoption of both these versions, given that they have substantial differences. By analyzing the top 1M domains in terms of popularity, we identify that only a small portion, less than 19%, supports the privacy-preserving ESNI extension and practically no domain supports ECH. Overall, these results demonstrate that there is still a long way to go to ensure the privacy of end users visiting TLS-protected domains which are co-located behind a common Internet-facing server.