The code and data behind xeiaso.net
blog: anti-RSA post
Signed-off-by: Xe Iaso <me@christine.website>
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blog/move-away-rsa-ssh.markdown
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blog/move-away-rsa-ssh.markdown
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---
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title: How to move away from RSA for SSH keys
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date: 2023-01-04
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tags:
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- OpenSSH
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- RSA
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- ed25519
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- security
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- sre
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- NixOS
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---
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<xeblog-hero ai="Stable Diffusion v1.5" file="volcano-bliss" prompt="a rolling green landscape by makoto shinkai, breath of the wild, active volcano, windows xp bliss, manga style, ((thick outlines))"></xeblog-hero>
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[RSA](https://en.wikipedia.org/wiki/RSA_(cryptosystem)) is one of the most
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widely deployed encryption algorithms in the world. Notably, when you generate
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an SSH key without any extra flags, `ssh-keygen` will default to using RSA:
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```
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root@hiro:~# ssh-keygen
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Generating public/private rsa key pair.
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Enter file in which to save the key (/root/.ssh/id_rsa):
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```
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For a while cryptographers have feared that RSA is vulnerable to a quantum
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computing algorithm known as [Shor's
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Algorithm](https://en.wikipedia.org/wiki/Shor%27s_algorithm). I won't pretend to
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understand it in this article, but the main reason why it's not deployed is that
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the hardware required to attack RSA keys in the wild literally doesn't exist
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yet (think literally tens of generations more advanced than current quantum
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computers).
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A group of researchers have just published [a
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paper](https://arxiv.org/pdf/2212.12372.pdf) that posits that it's likely you
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can break 2048-bit RSA (the most widely deployed keysize) with a quantum
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computer that only uses 372 qubits of computational power. The [IBM
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Osprey](https://newsroom.ibm.com/2022-11-09-IBM-Unveils-400-Qubit-Plus-Quantum-Processor-and-Next-Generation-IBM-Quantum-System-Two)
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has 433 qubits.
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<xeblog-conv name="Cadey" mood="coffee">Note that quantum computers are
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effectively unobtainable (unless you're a research institution or you have a few
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small loans of billions of dollars laying around), require a team of highly
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specialized experts to monitor them 24/7, and aren't really usable to the
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general public. I highly doubt that quantum computers are going to be rolling
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into store shelves any time soon. I also have no idea what I'm talking about
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with quantum computers. Please temper your interpretations of my statements
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appropriately.</xeblog-conv>
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It may be a good time to move away from RSA keys when and where you can. Today
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I'm going to cover how to make SSH keys using
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[ed25519](https://en.wikipedia.org/wiki/EdDSA#Ed25519) keys instead of RSA.
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<xeblog-conv name="Mara" mood="hacker">It's worth noting that RSA has not been
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broken yet, the paper in question
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## Generating new keys
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To generate a new keypair, use the `ssh-keygen` command:
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```
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ssh-keygen -t ed25519
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```
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Make sure to set a password on that key and then you can add it to your SSH
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agent with `ssh-add`. Copy the public key to your clipboard (print it to the
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screen with `cat ~/.ssh/id_ed25519.pub`) and then you can add it to GitHub or
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other services you use.
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<xeblog-conv name="Mara" mood="hacker">Pro tip: you can get a list of machines
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you've SSHed into by reading your `~/.ssh/known_hosts` file. You could use a
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command like this:</xeblog-conv>
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```
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cat ~/.ssh/known_hosts | cut -d' ' -f1 | sort | uniq
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```
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<xeblog-conv name="Mara" mood="happy">This will get you a list of machines that
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you may need to update your SSH key in! Remember that your new key should go to
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the end of `~/.ssh/authorized_keys`!</xeblog-conv>
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## Disabling RSA host keys
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The OpenSSH server will create a keypair for each machine it runs on. By default
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this creates an RSA key as well as an ed25519 key. You can disable this by
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adding the following line to `/etc/ssh/sshd_config`:
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```
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HostKey /etc/ssh/ssh_host_ed25519_key
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```
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<xeblog-conv name="Mara" mood="hacker">In my testing, this was the case for both
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NixOS and Ubuntu. If you want to be sure you're setting the right key, check the
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file for commented-out HostKey instructions. Uncomment whichever one contains
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`ed25519` in it.</xeblog-conv>
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If your SSH configuration file has a `Ciphers`, `HostKeyAlgorithms`,
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`PubkeyAcceptedAlgorithms`, or `CASignatureAlgorithms` setting in it, you may
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want to make sure that any `rsa` cipher or algorithm isn't present in any of
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them. If your distro has an option to change this system wide (such as in [Red
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Hat and
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derivatives](https://access.redhat.com/documentation/en-us/red_hat_enterprise_linux/8/html/security_hardening/using-the-system-wide-cryptographic-policies_security-hardening)),
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you may want to use that.
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<xeblog-conv name="Mara" mood="happy">You may want to have some kind of
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transition period for shared machines before you start rejecting RSA keys
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willy-nilly. This can break people's workflows and SSH-key-in-GPG setups. Talk
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with your users and work on compromises. Something something shill for the
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company supplying the author of this post with the money needed for food
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something something.</xeblog-conv>
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If you want to do this on NixOS, add the following configuration to either your
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`configuration.nix` or something that is imported by your `configuration.nix`:
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```nix
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services.openssh.hostKeys = [{
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path = "/etc/ssh/ssh_host_ed25519_key";
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type = "ed25519";
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}];
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```
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<xeblog-conv name="Mara" mood="hacker">This tells SSH to use only an ed25519
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host key. By default it will also create an RSA key.</xeblog-conv>
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---
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I hope this helps! Systems administration is full of annyoing migrations and
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compromises like this. Good luck out there!
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<xeblog-conv name="Mara" mood="hacker">Also check out [this
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article](https://xeiaso.net/blog/yubikey-ssh-key-storage) on how you can store
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an SSH key on a Yubikey or any other compliant FIDO2 key!</xeblog-conv>