FIDO2Applet VS YubiKey-Guide

I believe the Solokey meets your definition. The hardware schematics are open, as is the software running on it.

The Precursor is also open hardware and software.

If you trust any smartcard at all running a Javacard-compatible operating system, there's also https://github.com/BryanJacobs/FIDO2Applet .

And of course if you're truly paranoid you can get a FPGA and implement a hardware security key on that. The overall security posture would likely be weaker, but you could be confident, hopefully, that nobody has put some kind of backdoor into the hardware you designed yourself to run atop a generic array of logic gates.

If you get a smartcard, you can install https://github.com/BryanJacobs/FIDO2Applet on it to make it into a FIDO2 authenticator. You can install a GPG and a PIV applet too.

A Yubikey is just a proprietary smartcard with a bunch of apps installed and some HID emulation (pretending to be a keyboard, which you likely do not want).

Instead, most people generate keypair(s) on an airgapped machine and write them to two Yubikeys. Or write subkeys to a single Yubikey and keep a backup in encrypted form. See https://github.com/drduh/YubiKey-Guide

I'd recommend using the Yubikey as a GPG smartcard[1]. The private key stays on the Yubikey. I also use it for ssh. But make sure you have a backup key or two, just in case the primary Yubikey gives out. FIDO2 and all other regular Yubikey functionality still works with it.

[1]: https://github.com/drduh/YubiKey-Guide

The excellent guide by drduh should be mentioned here: https://github.com/drduh/YubiKey-Guide — I've been using this approach for years to store my OpenPGP keys on Yubikeys and use them for SSH.

I don't generate my keys on devices. That lets me be flexible and keep backups, as well as use the same keys on multiple physical devices. Using a single yubikey is a bad idea, as you're bound to eventually lose it or break it. Hasn't happened to me yet in 5 years, but I expect it to happen.

I followed this guide to generate a master key and three subkey.

See https://github.com/drduh/YubiKey-Guide . Also google for OpenPGP card specifications, they will answer your question

Regarding GPG/SSH keys, there is a great howto: https://github.com/drduh/YubiKey-Guide

I'd be lying if I said I understood all of that. I'm definitely going to do some research ahaha. Someone else on this post sentence this like which they said was very good guide. https://github.com/drduh/YubiKey-Guide

When storing the key on a Yubikey, however, all you need is just the PIN in order to use the key. It can contain numbers, letters, symbols, etc. and can be quite long, so you can treat it as a passphrase for all intents and purposes. There are actually two PIN codes: one regular one, for a read access, and another administrative one, for write access for when you want to modify the gpg applet settings or the key on the Yubikey. There is also a limit to how many times in a row you can enter pins incorrectly, after which the gpg applet gets locked and the only thing you can do is reset it, erasing the PGP keys. See https://github.com/drduh/YubiKey-Guide for more information.

Try follow this guide from DrDuh https://github.com/drduh/YubiKey-Guide/blob/master/README.md