--- title: "Post-Quantum Encryption in Access Server: How to Turn It On" description: OpenVPN Access Server supports hybrid post-quantum key agreement (ML-KEM + X25519) to block harvest-now, decrypt-later attacks. See requirements and setup. image: https://blog.openvpn.net/hubfs/682dae00-4625-4a3b-966c-c6c5dea44829.png --- - [Blog](https://blog.openvpn.net) - [Access Server](https://blog.openvpn.net/tag/access-server) # OpenVPN Access Server Is Ready for Post-Quantum Encryption. Here’s How to Turn It On. Nov 3, 2026 • 7 min read ![Illustration of a quantum computer beside a workstation and a server](https://blog.openvpn.net/hubfs/682dae00-4625-4a3b-966c-c6c5dea44829.png) Share - - - - Post-Quantum Encryption in Access Server: How to Turn It On 8:54 By Jason Ng ## [OpenVPN Access Server](https://openvpn.net/access-server/) supports hybrid post-quantum key agreement (X25519MLKEM768) to protect VPN traffic from harvest-now, decrypt-later attacks. In most supported configurations, it turns on automatically when Access Server and the client both use OpenSSL 3.5 or newer. Today's encryption standards are more than a match for today's computers. While this is reassuring, tomorrow's a different story. In a "harvest now, decrypt later" attack, an attacker can record encrypted sessions today and hold them until a quantum computer is powerful enough to decrypt them. The data you'll still need to protect ten years from now and more is the data you're encrypting today, which is why post-quantum cryptography has moved from being just a research topic to top of mind as a security roadmap priority. Below, we break down how the quantum threat works, how the industry is answering it, and how [OpenVPN Access Server](https://openvpn.net/access-server/) can be easily configured to protect your VPN traffic with post-quantum encryption today. ## Why quantum computers threaten today’s encryption Most encrypted connections, including every VPN session, start with key agreement (also called key exchange): the two sides agree on the session keys that encrypt everything after. Today, that step relies on asymmetric cryptography (also called public key cryptography). For OpenVPN, as for most encrypted connections, that means Diffie-Hellman (DH) or its elliptic-curve form, ECDH. However, with a large enough quantum computer running Shor's algorithm, that math can be broken, and with it, the security of the traffic an attacker saved. The good news is that the ciphers that scramble the data itself, like AES-256, hold up well against quantum attacks. That makes key agreement the weak link, which is why post-quantum protection starts there. For a closer look at how quantum computing changes the security landscape, read[Quantum Computing and the Future of Cybersecurity](https://blog.openvpn.net/quantum-safe-vpn). ## The risk starts today with harvest now, decrypt later Quantum computers that can break today's key agreement don't exist yet, and attackers don't need them. Instead, they only need to capture encrypted traffic now and store it until they do. So the real question isn't when quantum computers arrive. It's how long your data needs to stay confidential. Intellectual property, financial records, health data, and government information can stay sensitive for a decade or more. If that data travels over an encrypted connection today, it's already in scope. ## How hybrid post-quantum key agreement closes the gap In August 2024, NIST published[FIPS 203](https://nvlpubs.nist.gov/nistpubs/fips/nist.fips.203.pdf), standardizing ML-KEM (formerly known as Kyber), a key encapsulation mechanism designed to resist quantum attacks. OpenSSL 3.5 then added support for hybrid key agreement and prefers X25519MLKEM768 when both client and server support it. X25519MLKEM768 pairs ML-KEM with X25519, the classical elliptic-curve Diffie-Hellman (ECDH) key exchange built on Curve25519. Hybrid means you aren't betting on a single algorithm. ML-KEM is new and hasn't yet had the decades of scrutiny classical algorithms have, so X25519 stays in as a proven backstop. To recover the session keys, an attacker would have to break both X25519 and ML-KEM. Hybrid key agreement is one piece of a bigger migration. For the full plan,[Post-Quantum Cryptography and Your VPN](https://blog.openvpn.net/post-quantum-cryptography-vpn/) walks through each step. ## How OpenVPN Access Server delivers post-quantum encryption OpenVPN wasn't starting from zero. For years, TLS Crypt has wrapped the entire TLS handshake, key agreement included, in a pre-shared symmetric key. That protection has a catch though: if the TLS Crypt key ever leaks, it breaks down completely, so it isn’t the right fit for every deployment. Post-quantum key agreement adds a NIST-standardized layer that doesn’t depend on keeping a shared key secret. Access Server now supports X25519MLKEM768, the hybrid approach covered above. In most supported configurations, it just works: when Access Server and the VPN client both run OpenSSL 3.5 or newer, they use post-quantum key agreement automatically. How fast you move is up to you. Use post-quantum key agreement wherever clients support it, run it alongside classical key agreement while you migrate, or require it for every connection once you're ready. To use post-quantum key agreement in Access Server, you need: - An Access Server on Linux with OpenSSL 3.5 or newer. As of October 2026, that includes Ubuntu 26.04 LTS, Debian 13, and RHEL 9 and 10 and compatible distributions. Other distributions may add support as they move to OpenSSL 3.5, so check your OS version before you start. - An OpenVPN Community client that uses OpenSSL 3.5 or newer, such as OpenVPN GUI for Windows, Tunnelblick for macOS, or OpenVPN 2 for Linux One caveat: as of October 2026, the OpenVPN Connect app (v3) doesn't support OpenSSL 3.5. If your users rely on Connect, keep classical key agreement available so no one gets locked out. Once it's on, the Access Server logs show which key agreement each connection used, so you can confirm it's working. Ready to set it up? The[Access Server post-quantum tutorial](https://openvpn.net/as-docs/v3/tutorials/tutorial--post-quantum-cryptography.html) walks through every step, from migration options to FIPS configuration. ## The quantum threat isn't here yet, but your data is Harvest-now, decrypt-later attacks target the traffic you send today. With OpenVPN Access Server, you can start protecting it now: test in a non-production environment, roll out in compatibility mode, and require post-quantum key agreement once your clients are ready. Post-quantum encryption is also becoming part of zero trust and security requirements in regulated industries. Getting ahead of it now keeps your VPN from becoming the gap in your architecture. [Read the Access Server post-quantum tutorial](https://openvpn.net/as-docs/v3/tutorials/tutorial--post-quantum-cryptography.html) ## Still have questions about post-quantum encryption in Access Server? #### Does OpenVPN Access Server support post-quantum cryptography? Yes. Access Server supports hybrid post-quantum key agreement with X25519MLKEM768 when it and the OpenVPN client both use OpenSSL 3.5 or newer. In FIPS mode, Access Server supports SecP256r1MLKEM768 and SecP384r1MLKEM1024. #### Which post-quantum groups does Access Server support? X25519MLKEM768 is the default for both Access Server and OpenVPN clients that use OpenSSL 3.5 or newer. If you need a different group, Access Server also supports the hybrid groups SecP256r1MLKEM768 and SecP384r1MLKEM1024, and the ML-KEM-only groups MLKEM512, MLKEM768, and MLKEM1024. In FIPS mode, use SecP256r1MLKEM768 or SecP384r1MLKEM1024. Clients don’t use these groups by default, so FIPS deployments need extra client configuration. #### Is post-quantum encryption on by default in Access Server? Yes, as long as both sides support it. When Access Server and the OpenVPN client both use OpenSSL 3.5 or newer, they negotiate post-quantum key agreement automatically in most supported configurations. You only need the tls-groups directive if you want to require post-quantum key agreement or use a specific group. #### Does the OpenVPN Connect app support post-quantum encryption? Not yet. OpenVPN Connect v3 doesn't support OpenSSL 3.5, so it connects with classical key agreement. For post-quantum sessions today, use OpenVPN GUI for Windows, Tunnelblick for macOS, or OpenVPN 2 for Linux. #### Will requiring post-quantum encryption block some users? Yes. If you allow only X25519MLKEM768, clients that can't negotiate it fail to connect. Use tls-groups "X25519MLKEM768:X25519" to offer post-quantum key agreement while keeping classical clients connected. #### Should I enable post-quantum encryption now? If your VPN carries data that must stay confidential for years, start now: harvest-now, decrypt-later attacks target traffic captured today. Begin in compatibility mode, confirm the key agreement in your logs, then require it once your clients support it. #### Does Access Server support post-quantum certificates? Not yet, and for VPN connections that isn’t a risk today. Certificates prove the identity of the server and client during the handshake, and today’s certificate cryptography can’t be broken yet. By the time a quantum computer can break it, today’s certificates will have long since expired or been revoked, and impersonating a client or server after the fact doesn’t work. Post-quantum certificates matter more for uses like code signing, where a signature has to stay unforgeable for years. ### Ready to see how OpenVPN can help protect your people and AI agents? Try the self-hosted Access Server solution or the managed CloudConnexa service for free, no credit card required. [See Which One is Right for You](https://openvpn.net/product-comparison/?_gl=1*1hi46u*_ga*NDgyODEwNDkyLjE3NzIxMjIzNDE.*_ga_E45Z33NTV7*czE3Nzc5ODY2ODIkbzExMiRnMSR0MTc3Nzk4ODMyMSRqNTUkbDAkaDE4NzIyNTczNDc.*_fplc*dVltNzJ6YmZ0bFZTdld4NjRtWVpVWXclMkJEV3AzZGhPUlRZaUppdWNxQkRjOER3MFN0VW84JTJCdEJGdnRDTmVYNlI5NlJBdTRLZ0VGOTNHMHc0U3l3bVFsR3NzQXk5RzJIQVdzNHE0QVJHcSUyQlVKUlRsNTM0S1RWZERyZ0V4NDNnJTNEJTNE*_gcl_au*MTM5NjEwNTIxNy4xNzcyMTIyMzQx*_ga_SPGM8Y8Y79*czE3Nzc5ODY2ODEkbzExMyRnMSR0MTc3Nzk4ODMyMyRqNTMkbDAkaDA.) ## Related posts from OpenVPN ### Subscribe for Blog Updates ```json { "@context" : "https://schema.org", "@type" : "BlogPosting", "author" : { "@type" : "Person", "name" : "Jason Ng", "url" : "https://blog.openvpn.net/author/jason-ng" }, "dateModified" : "2026-11-03T17:00:00.907Z", "datePublished" : "2026-11-03T17:00:00.000Z", "headline" : "Post-Quantum Encryption in Access Server: How to Turn It On", "image" : [ "https://blog.openvpn.net/hubfs/682dae00-4625-4a3b-966c-c6c5dea44829.png" ], "mainEntityOfPage" : { "@id" : "https://blog.openvpn.net/access-server-post-quantum-encryption", "@type" : "WebPage" }, "publisher" : { "@type" : "Organization", "logo" : { "@type" : "ImageObject", "url" : "https://blog.openvpn.net/hubfs/Dark=True%20Medium.png" }, "name" : "OpenVPN" } } ```