Merkle Tree Certificates Proposed to Enable Lightweight Post-Quantum HTTPS in Chrome
Google Chrome developers, working with colleagues from other companies and experts from the IETF PLANTS working group, are preparing the first HTTPS change aimed at solving performance problems expected from post-quantum cryptography. The solution is Merkle Tree Certificates (MTC), compact TLS certificates built on Merkle trees that replace heavy signature chains with lightweight inclusion proofs.
Why post-quantum cryptography needs smaller certificates
Post-quantum cryptographic algorithms typically produce larger keys and signatures than classical schemes, increasing both network traffic and computational load during TLS handshakes. Merkle Tree Certificates address this by having a certificate authority sign only the root of a Merkle tree; each individual MTC then serves as a compact proof that a particular certificate is included in that tree.
In the new model, the amount of authentication data sent to the browser shrinks dramatically compared with conventional X.509 chains, allowing strong post-quantum algorithms to be deployed without slowing down web connections. An additional security benefit is built-in transparency: because every certificate must be recorded in the public Merkle tree, the properties of today’s Certificate Transparency ecosystem become mandatory by default.
How Merkle Tree Certificates work
A Merkle tree is constructed by hashing individual records and then repeatedly hashing pairs of hashes until a single root hash remains. To prove that a specific record belongs to the tree, only the hashes along the path from the record to the root are required. The scheme has been extended to support trees of arbitrary size (not just powers of two) and to allow efficient proofs that one log is a prefix of another.
These properties make MTC suitable for real-world certificate issuance at internet scale while maintaining the auditability required by modern public-key infrastructure.
Chrome deployment timeline
Performance testing of TLS connections using MTC is already being conducted in collaboration with Cloudflare. After validation of the core technology in early 2027, operators of existing Certificate Transparency logs that are trusted by Chrome will be invited to run the first public MTC logs. By the third quarter of 2027, requirements for additional certificate authorities to join a dedicated post-quantum Chrome Root Store supporting only MTC are expected to be finalized. This new root store will operate alongside the current Chrome Root Store.
The field is evolving rapidly, and the outlined schedule may be adjusted as implementation experience grows.
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