[phaer]

I think the official recommendation is to store a second yubikey in a safe location.

Personally I just generated my key offline (on a tails livecd) and backed it up to two different LUKS-encrypted USB sticks. One of those is stored at my place and another one at a trusted person, in case my flat burns down or so. The yubikey itself only stores subkeys, my master key stays on said USB sticks.

Been using this setup for about 5 years now and it's been working well for me so far. Once a year, I extend my gpg keys expiration time by using on of the USB sticks.

[aborsy]

No need for LUKS usb. Gpg stores private keys encrypted with a password.

At that point, you can store the encrypted key anywhere.

[phaer]

Ah yes good point for the gpg key itself, but I store a few other important secrets there as well ;)

[beagle3]

Does this also work for the WebAuthn / U2F / TOTP use cases, or just the PGP/PIV/SSH ones?

[tialaramex]

No. There is no way to push a value for the symmetric key that probably makes the FIDO (thus U2F / WebAuthn) feature work. You can tell the Yubikey to pick a random new one, effectively wiping your key (and rendering any credentials previously minted with it now invalid) but you can't write one over USB and this was I believe intentional.

[beagle3]

Thanks. I will wait until it’s actually possible to create back up webauthn keys (rather than enlisting two keys with every service).

[opliko]

There is actually at least one fido2 device that supports backing up (mostly), based on this spec from Dicekeys https://github.com/dicekeys/seeding-webauthn

Solokeys (https://solokeys.com/ - v1, don't think the newer v2 does) have a special firmware version that implements this and allows you to use a custom seed - and as such restore a key from it. It only works on non-resident credentials (most commonly used, as the number of RKs is usually very limited) though. The firmware is here https://github.com/conorpp/solo-dicekeys/releases/tag/5.0.0 But it's also shipped in the keys dicekeys sells and I think only their app implements the client side of seeding anyway: https://www.crowdsupply.com/dicekeys/dicekeys