How to move away from RSA for SSH keys

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RSA is one of the most widely deployed encryption algorithms in the world. Notably, when you generate an SSH key without any extra flags, ssh-keygen will default to using RSA:

root@hiro:~# ssh-keygen
Generating public/private rsa key pair.
Enter file in which to save the key (/root/.ssh/id_rsa):

For a while cryptographers have feared that RSA is vulnerable to a quantum computing algorithm known as Shor's Algorithm. I won't pretend to understand it in this article, but the main reason why it's not deployed is that the hardware required to attack RSA keys in the wild literally doesn't exist yet (think literally tens of generations more advanced than current quantum computers).

A group of researchers have just published a paper that posits that it's likely you can break 2048-bit RSA (the most widely deployed keysize) with a quantum computer that only uses 372 qubits of computational power. The IBM Osprey has 433 qubits.

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Note that quantum computers are effectively unobtainable (unless you're a research institution or you have a few small loans of billions of dollars laying around), require a team of highly specialized experts to monitor them 24/7, and aren't really usable to the general public. I highly doubt that quantum computers are going to be rolling into store shelves any time soon. I also have no idea what I'm talking about with quantum computers. Please temper your interpretations of my statements appropriately.

It may be a good time to move away from RSA keys when and where you can. Today I'm going to cover how to make SSH keys using ed25519 keys instead of RSA.

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It's worth noting that RSA has not been broken yet, the paper in question describes a theoretical attack. Quantum computers are nowhere near good enough for this yet.

Generating new keys

To generate a new keypair, use the ssh-keygen command:

ssh-keygen -t ed25519

Make sure to set a password on that key and then you can add it to your SSH agent with ssh-add. Copy the public key to your clipboard (print it to the screen with cat ~/.ssh/ and then you can add it to GitHub or other services you use.

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Pro tip: you can get a list of machines you've SSHed into by reading your ~/.ssh/known_hosts file. You could use a command like this:

cat ~/.ssh/known_hosts | cut -d' ' -f1 | sort | uniq
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This will get you a list of machines that you may need to update your SSH key in! Remember that your new key should go to the end of ~/.ssh/authorized_keys!

Disabling RSA host keys

The OpenSSH server will create a keypair for each machine it runs on. By default this creates an RSA key as well as an ed25519 key. You can disable this by adding the following line to /etc/ssh/sshd_config:

HostKey /etc/ssh/ssh_host_ed25519_key
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In my testing, this was the case for both NixOS and Ubuntu. If you want to be sure you're setting the right key, check the file for commented-out HostKey instructions. Uncomment whichever one contains ed25519 in it.

If your SSH configuration file has a Ciphers, HostKeyAlgorithms, PubkeyAcceptedAlgorithms, or CASignatureAlgorithms setting in it, you may want to make sure that any rsa cipher or algorithm isn't present in any of them. If your distro has an option to change this system wide (such as in Red Hat and derivatives), you may want to use that.

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You may want to have some kind of transition period for shared machines before you start rejecting RSA keys willy-nilly. This can break people's workflows and SSH-key-in-GPG setups. Talk with your users and work on compromises. Something something shill for the company supplying the author of this post with the money needed for food something something.

If you want to do this on NixOS, add the following configuration to either your configuration.nix or something that is imported by your configuration.nix:

services.openssh.hostKeys = [{
  path = "/etc/ssh/ssh_host_ed25519_key";
  type = "ed25519";
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This tells SSH to use only an ed25519 host key. By default it will also create an RSA key.

I hope this helps! Systems administration is full of annyoing migrations and compromises like this. Good luck out there!

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Also check out this article on how you can store an SSH key on a Yubikey or any other compliant FIDO2 key!

Facts and circumstances may have changed since publication. Please contact me before jumping to conclusions if something seems wrong or unclear.

Tags: OpenSSH, RSA, ed25519, security, sre, NixOS