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# How to Verify Post-Quantum SSH on Your Ubuntu 26.04 Server
- URL: https://trendboxgeek.com/blog/ubuntu-post-quantum/
- Published: 2026-09-21T05:36:23.000Z
- Updated: 2026-09-21T05:36:23.000Z
- Author: Pankajbhai Chavda
- Tags: #blog-col

In Ubuntu 26.04 LTS, a major change has been quietly introduced behind the scenes: post-quantum SSH key exchange is now turned on by default.

If you are spinning up a fresh 26.04 server in your homelab, you are running OpenSSH 10.2\. That means your SSH sessions are automatically negotiating a hybrid key exchange, where the NIST-standardized ML-KEM-768 is paired with the classical X25519 we have been using for years. If the post-quantum math has a flaw, X25519 saves you. If quantum computers break X25519 in ten years, ML-KEM holds the line.

Why do we care about this today? Because of "harvest now, decrypt later." State actors and ISPs are already recording encrypted traffic. They cannot read this data, but when quantum computers get good enough, they will decrypt that backlog. If you handle long-lived data and post-quantum KEX protects your session today, they still won't be able to read it a decade from now.

It is free future-proofing. Here is how to check if your new servers are using it and how to lock them down if you want to be strict.

## Verify Your Client Support

Before doing anything with the server, first check your local machine. If you are using an old Windows PuTTY build or an outdated jump box, you are just going to fall back to the old classical handshake anyway.

Ask your local SSH client what it supports:

ssh -Q kex | grep mlkem

**Expected Output:**

![Output of verification.](https://trendboxgeek.com/content/images/2026/09/Screenshot-From-2026-09-21-10-57-37.png)

If you see `mlkem768x25519-sha256`, you are good. For that you need OpenSSH 10.0 or higher.

## Watch the Live Handshake

The easiest way to prove this works is just to watch the negotiation happen in real time. Just pass the verbose flag and grep for the algorithm:

ssh -vv user@your-ubuntu-server true 2>&1 | grep -i "kex: algorithm:"

You should see the exact same output:

debug1: kex: algorithm: mlkem768x25519-sha256

If you see `curve25519-sha256` instead, somebody in the connection is outdated. Time to update your client.

## Verify the Server Config in Memory

Ubuntu 26.04 enables this out of the box, but I always like to check what sshd is loading into memory. This shows you the exact priority list of KEX algorithms your server is offering.

sudo sshd -T | grep kexalgorithms

On a clean 26.04 install, you will see `mlkem768x25519-sha256` sitting right at the front of that list.

## Enforce a Strict Post-Quantum Policy

The default Ubuntu config is smart. It prefers post-quantum but falls back to classical crypto, so you don't lock out your old automation scripts.

But if you are securing a sensitive node and want to explicitly drop any client that can't speak post-quantum, you can enforce it. Don't edit `sshd_config` directly; drop a file in the override directory.

sudo nano /etc/ssh/sshd_config.d/99-pqc-only.conf

Add this one line to force only the hybrid post-quantum algorithms.

# Require hybrid post-quantum key exchange
KexAlgorithms mlkem768x25519-sha256,sntrup761x25519-sha512@openssh.com

Restart the service:

sudo systemctl restart ssh

Test your connection immediately. If you just locked yourself out because your daily driver is too old, you will have to use your Proxmox console to delete that file.

Post-quantum crypto is not just theory anymore. It is a practical, zero-cost default sitting right there in the new LTS. Verify your endpoints, update your jump boxes, and take the win.