Let’s Encrypt Unveils Merkle Tree Certificates to Secure the Web Against Quantum Threats

### Executive Summary Let’s Encrypt describes Merkle Tree Certificates (MTCs), a post-quantum Web PKI design that replaces large serialized certificate signature chains with compact Merkle-tree batch signatures and inclusion proofs, enabling quantum-resistant authentication without bloating TLS handshakes; Chrome, Cloudflare, and the IETF PLANTS working group are testing and standardizing the approach with staging targeted for late 2026 and production in 2027, and the report recommends enabling hybrid post-quantum key exchange today.