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+---
+title: "FIDO2 hardware keys as your root of trust"
+date: "2026-02-11"
+---
+
+Storing private keys as FIDO2 discoverable secrets on hardware devices like a YubiKey is really convenient while keeping great security.
+One example of this is SSH keys:
+`*-sk` type keys like `ed25519-sk` or `ecdsa-sk` keep a secret key in the hardware device,
+and short private key "shim" on disk,
+treating the hardware key as a second factor of authentication.
+
+Personally, the more interesting use case is to skip the shim stored on disk and keep the entire secret key on the hardware.
+This is called a *resident key* and does not require any state on the local machine to authenticate.
+Unlike storing keys in the PIV slot of a Yubikey,
+which requires custom software like [yubikey-ssh-agent](https://github.com/FiloSottile/yubikey-agent) on the host to retrieve,
+FIDO2 support is built in to openssh (since 8.2) and systemd (systemd-cryptenroll)!
+
+Having builtin support with modern SSH is especially convenient,
+for authenticating from hosts you might not use every day.
+Something like yubikey-ssh-agent is probably not installed on normal systems,
+but the regular ssh-agent distributed with openssh >8.2 is much more likely to work.
+Of course, I would only use this on "semi-trusted" machines,
+where I am not paranoid of key loggers or weird memory shenanigans.
+
+I replaced my per-machine SSH keys with 3 `ed25519-sk` keys stored on a few keys I had laying around.
+This lets me access my internet-facing servers without managing keys on each host,
+and each SSH server only has to authorize these 3 keys to know its me.
+When I take my laptop around I can just grab a key and leave the "backups" at home,
+since any of them work for authentication.
+
+# Followup on full disk encryption
+
+Given `systemd-cryptenroll` [supports FIDO2 tokens](https://wiki.archlinux.org/title/Systemd-cryptenroll#FIDO2_tokens),
+I think proper disk encryption is worth exploring again.
+Previously, I disabled it since entering passwords for both LUKS and my user account felt tedious,
+and storing a key in the machines TPM without a PIN didn't feel very secure,
+since I didn't have to authenticate myself in any way.
+
+With disk encryption keyed by a passphrase as well as a hardware token,
+I can have the security of full disk encryption with the convenience of fast authentication.
+Buying hardware keys for each device seems a bit pricey,
+so I feel re-using hardware keys across machines would be a worthwhile compromise.
+
+I'm also experimenting with bootstrapping identities + encryption on some local servers I run at home,
+where otherwise stateless machines can decrypt secrets on boot using their hardware key,
+and unplugging the key makes the hosts dumb again,
+but I'll save that for another post 👀.