How to Enable Post-Quantum TLS on Cloudflare
Step-by-step guide to enable X25519MLKEM768 and post-quantum cryptography on your Cloudflare-protected domains
Cloudflare, one of the world's largest CDN and security providers, supports post-quantum cryptography through hybrid key exchange algorithms. This guide shows you how to enable post-quantum TLS for your Cloudflare-protected domains, strengthening your defences against harvest-now-decrypt-later attacks.
What Post-Quantum Options Does Cloudflare Offer?
Cloudflare supports X25519MLKEM768, a hybrid key exchange mechanism that combines the classical X25519 elliptic curve algorithm with ML-KEM-768 (Module-Lattice-based Key Encapsulation Mechanism). This hybrid approach keeps your connections secure even if one of the two algorithms is later broken.
The hybrid design is what makes this safe to deploy in production. If a weakness is found in the newer ML-KEM component, the classical X25519 still protects the connection — and vice versa. This belt-and-braces approach is the industry consensus for the quantum transition period.
Post-quantum key exchange only happens when both sides support it: the client negotiates it with the server. Modern browsers including recent versions of Chrome, Firefox and Edge already offer X25519MLKEM768, so the client side is increasingly covered for you.
How Cloudflare Handles Post-Quantum TLS
An important thing to understand up front: Cloudflare has been rolling out post-quantum key exchange across its network, and the exact controls and plan availability have changed over time. Rather than reproduce a click-path that may be out of date by the time you read it, the reliable approach is to check Cloudflare's own current documentation for the setting, and then verify the result yourself — which is the part this guide focuses on, because verification doesn't go stale.
There are two layers to think about:
The edge (Cloudflare ↔ your visitors). This is the connection between browsers and Cloudflare's network. Where Cloudflare has enabled post-quantum key exchange, this is the layer that benefits, and it's controlled in your zone's SSL/TLS settings. Look in the SSL/TLS area of the Cloudflare dashboard for the post-quantum (or "PQC") option, and consult Cloudflare's current docs for whether it's on by default for your plan or needs enabling.
The origin (Cloudflare ↔ your server). This is the connection between Cloudflare and your own server. Securing it is a separate task you control directly — see our guide on enabling X25519MLKEM768 on NGINX with OpenSSL. For end-to-end protection, set your SSL/TLS mode to "Full" or "Full (strict)" so the origin leg is encrypted, not just the visitor leg.
Prerequisites
Before checking your post-quantum posture on Cloudflare:
Domain proxied through Cloudflare: Your traffic must actually pass through Cloudflare's network (the proxied/"orange cloud" state in your DNS records). Records set to DNS-only bypass the edge entirely, so edge TLS settings won't apply to them.
SSL/TLS mode: Use "Full" or "Full (strict)". The "Flexible" mode encrypts only the visitor↔Cloudflare leg, leaving the Cloudflare↔origin leg unencrypted — which undermines the point of any TLS hardening.
Check current plan availability: Whether the post-quantum control is exposed, and on which plans, has shifted as Cloudflare expands the rollout. Confirm against Cloudflare's live documentation rather than assuming.
Verifying Your Configuration
Verification is the part that doesn't go stale, so make it your source of truth. To confirm post-quantum key exchange is actually being negotiated:
Browser Developer Tools: Open Chrome or Firefox, navigate to your site, open Developer Tools (F12), go to the Security tab, and check the connection details. Look for "X25519MLKEM768" in the key exchange group.
Command Line with OpenSSL: If you have OpenSSL 3.5.0 or later:
openssl s_client -connect yourdomain.com:443 -groups X25519MLKEM768
A successful connection indicates post-quantum support is active.
- Online Tools: Several online TLS scanners now detect post-quantum algorithms. These tools show which key exchange mechanisms your server offers.
Performance Considerations
Post-quantum algorithms use larger keys and signatures than classical cryptography. X25519MLKEM768 increases the TLS handshake size by approximately 1KB compared to classical X25519 alone. For most websites, this overhead is negligible.
Cloudflare's global network absorbs most of the computational overhead. Your origin server performance remains unchanged unless you also enable post-quantum algorithms there.
Mobile devices and low-bandwidth connections might notice slightly longer initial connection times. However, once established, the connection performs identically to classical TLS.
Timeline and Compliance
As outlined in our comprehensive migration guide, organisations should implement post-quantum cryptography before regulatory deadlines. The CNSA 2.0 framework requires new national security systems to support post-quantum algorithms by January 2027.
Enabling post-quantum TLS on Cloudflare is a straightforward first step in your migration journey. It immediately protects your traffic against future quantum threats while maintaining full compatibility with existing systems.
Scan your domain free with PQScore → to assess your current post-quantum readiness and identify remaining vulnerabilities in your cryptographic infrastructure.