In brief: Let’s Encrypt uses Merkle-Tree Certificates to quantum-secure the Web-PKI without enlarging TLS handshakes through larger post-quantum signatures.
Let’s Encrypt is deploying Merkle-Tree Certificates to quantum-secure the Web-PKI instead of larger post-quantum signatures. This allows the certificate authority to avoid bloated TLS handshakes and plans initial testing for end of 2026.
Let’s Encrypt has decided on a technical strategy to quantum-secure the Web-PKI: instead of implementing post-quantum signature methods with significantly larger cryptographic signatures, the CA will adopt Merkle-Tree Certificates. This approach enables the construction of quantum-safe certificate chains without disproportionately enlarging handshake size in the TLS protocol.
The problem: established post-quantum signature algorithms such as CRYSTALS-Dilithium or SPHINCS+ produce signatures that are many times larger than classical RSA or ECDSA signatures. Direct migration would lead to noticeable latency delays and increased network load – particularly critical in mobile and low-bandwidth environments.
Merkle-Tree Certificates reduce these signature sizes through structural optimization: instead of a single large signature, they use a tree structure in which smaller signatures are distributed across multiple layers. This keeps the overall size of the certificate and authentication data significantly more manageable.
For CISOs, this is relevant because quantum-securing the PKI will become a central requirement – particularly given harvest-now-decrypt-later risk. Let’s Encrypt certificates protect millions of domains worldwide, so a quantum-safe solution without performance losses is therefore critical infrastructure. The planned tests in Q4 2026 will demonstrate how mature this solution is for production deployment.
Source: www.heise.de · Published June 4, 2026
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