EN, Google Online Security Blog

Cultivating a robust and efficient quantum-safe HTTPS

Posted by Chrome Secure Web and Networking Team

Today we’re announcing a new program in Chrome to make HTTPS certificates secure against quantum computers. The Internet Engineering Task Force (IETF) recently created a working group, PKI, Logs, And Tree Signatures (“PLANTS”), aiming to address the performance and bandwidth challenges that the increased size of quantum-resistant cryptography introduces into TLS connections requiring Certificate Transparency (CT). We recently shared our call to action to secure quantum computing and have written about challenges introduced by quantum-resistant cryptography and some of the steps we’ve taken to address them in earlier blog posts.

To ensure the scalability and efficiency of the ecosystem, Chrome has no immediate plan to add traditional X.509 certificates containing post-quantum cryptography to the Chrome Root Store. Instead, Chrome, in collaboration with other partners, is developing an evolution of HTTPS certificates based on Merkle Tree Certificates (MTCs), currently in development in the PLANTS working group. MTCs replace the heavy, serialized chain of signatures found in traditional PKI with compact Merkle Tree proofs. In this model, a Certification Authority (CA) signs a single “Tree Head” representing potentially millions of certificates, and the “certificate” sent to the browser is merely a lightweight proof of inclusion in that tree.

Why MTCs?

MTCs enable the adoption of robust post-quantum algorithms without incurring the massive bandwidth penalty of classical X.509 certificate chains. They also decouple the security strength of the corresponding cryptographic algorithm from the size of the data transmitted to the user. By shrinking the authentication data in a TLS handshake to the absolute minimum, MTCs aim to keep the post-quantum web as fast and seamless as today’s internet, maintaining high performance even as we adopt stronger security. Finally, with MTCs, transparency is a fundamental property of issuance: it is impossible to issue a certificate without including it in a public tree. This means the security properties of today’s CT ecosystem are included by default, and without adding extra overhead to the TLS handshake as CT does today.

Chrome’s MTC Propagation Plan

Chrome is already experimenting with MTCs with real internet traffic, and we intend to gradually build out our deployment such that MTCs provide a robust quantum-resistant HTTPS available for use throughout the internet.

Broadly speaking, our rollout spans three distinct phases.

  • Phase 1 (UNDERWAY): In collaboration with Cloudflare, we are conducting a feasibility study to evaluate the performance and security of TLS connections relying on MTCs. To ensure a seamless and secure experience for Chrome users who might encounter an MTC, every MTC-based connection is backed by a traditional, trusted X.509 certificate during this experiment. This “fail safe” allows us to measure real-world performance gains and verify the reliability of MTC issuance without risking the security or stability of the user’s connection.