TLS Certificate Optimization: The Technical Details behind "No Browser Left Behind"

Overview

Back in early December we announced our "no browser left behind" initiative to the world. Since then, we have served well over 500 billion SHA-1 certificates to visitors that otherwise would not have been able to communicate securely with our customers’ sites using HTTPS. All the while, we’ve continued to present newer SHA-2 certificates to modern browsers using the latest in elliptic curve cryptography, demonstrating that one does not have to sacrifice security to accommodate all the world’s Internet users. (If you weren’t able to acquire a SHA-1 certificate before CAs ceased issuing them on 2015/12/31, you can still sign up for a paid plan and we will immediately generate one to serve to your legacy visitors.)

Shortly after we announced these new benefits for our paid Universal SSL customers, we started hearing from other technology leaders who were implementing (or already had implemented) similar functionality. At first glance, the logic to identify incoming connections that only support SHA-1 seems straightforward, but as we spoke with our friends at Facebook, Twitter, and Mozilla, I realized that everyone was taking a slightly different approach. Complicating the matter even further was the fact that at CloudFlare we not only wanted to optimize between SHA-1 and SHA-2, but also between RSA and the newer, but less universally supported ECDSA certificates. Solve the "optimal certificate" question incorrectly, and the TLS handshake will fail — or get explicitly aborted by browsers that have deprecated SHA-1 entirely; solve it correctly, and the client and server will establish the most performant, secure connection available between the two endpoints.

Certificate Optimization Logic

After several trillion requests, we’re confident that our approach works quite well for CloudFlare’s customers and their visitors. If you have taken an alternative approach to implementation, or have found any exceptions/potential refinements to our logic, please chime in below. We remain committed to withdrawing SHA-1 support if, as our CEO said, "a vulnerability is discovered [in our certificate optimization logic] which allows some form of downgrade attack—where a modern browser can be tricked into receiving a certificate signed with an insecure protocol—and the vulnerability cannot be patched".
TLS Handshake

Before your web browser can securely exchange "application data" such as HTTP GET or POST requests and responses with a web server, it must first establish the cryptographic parameters of the secure session. This well-choreographed dance, known as the SSL/TLS handshake, commences as soon as you click, type, or get redirected to a URL containing the "https://" scheme. (The process described below also applies to connections from any user agent — not just browsers—so substitute "mobile app", "command-line utility", or anything else that can communicate via HTTPS.)

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RR
"Through counter-intelligence it should be possible to pinpoint potential trouble-makers ... And neutralize them, neutralize them, neutralize them"