Xous (microkernel OS for Precursor) has an application called Vault that does FIDO2/U2F as well as password management and USB HID emulation.
https://www.crowdsupply.com/sutajio-kosagi/precursor/updates...
Much more on the dev board side and probably overkill for just this purpose but a really cool device.
Interestingly, Pinephone is in the almost good intersection of hackable and portable: :)
> Just a Linux kernel [...]
That's several orders of magnitude more lines of code than any FIDO authenticator implementation.
> Normally powered off, boots in 2 seconds. :)
Yubikeys boot even faster!
FIDO2 works only with websites that support it.
I mean, if you have the amount of access needed to compromise a USB connection, you have way more than enough access than needed to just yoink the authentication token or encryption key from browser storage.
Maybe I don't have enough money to buy a pinephone just for this purpose but even if I would have, I would personally look more into soldering (esp32?) or some single board chip (SOC?) with a touch screen sounds nice too.
Now I am not kidding, there was this dumb phone which I was using and its cost was like 12.5$ and it had the features of camera, mic , text , messaging, audio, file manager and so much more... Basically it just didn't had a browser or ability to add apps ofc since it wasn't android. But yeah I do believe that something niche could be developed for people like you at maybe 1/10th the price
No way to prompt it for data, or compromise it remotely.
No other features, no OS userspace, no wifi, no adb, no nothing. Just a Linux kernel + a tiny single userspace static binary based on lvgl for UI and libsodium for encryption/storage. Normally powered off, boots in 2 seconds. :)