U2F USB key (Yubikey) for 2-factor authentication and Linux authentication

I just bought a pair of Yubikey U2F (Universal 2-Factor) devices (the 5 NFC model, because of the claims that it would work with iPhones). Mostly because I got tired of pulling out my phone, finding the authenticator app, searching for the entry for the appropriate website, and then typing in the number.

I'll get to the iPhone stuff at the end.

But first, using the Yubikey for the second factor works for only a few websites. Also, it depends on your web browser: I tested Chrome (on Linux, macOS, and Chromebook), and Firefox (on Linux and macOS). Chrome and Firefox can deal with reading a U2F key via USB just fine.

Yubico has clear instructions for how to set the keys up: https://www.yubico.com/setup

Among sites which accept U2F hardware keys are Facebook, Google, GitHub, GitLab,  Dropbox, and Twitter (though Twitter does not support multiple U2F keys, which sucks if you lose a key). You browse to the site as usual, type in your password, and it will prompt you to plug in your U2F key and tap the flashing bit with a gold contact sensor, and you're in.

For using the Yubikey as a U2F in Linux, to authenticate for logging in, unlocking the screensaver, and sudo, you will have to install Yubico's U2F PAM module: https://github.com/Yubico/yubico-pam There is more detailed documentation geared towards developers here: https://developers.yubico.com/yubico-pam/ The PAM module works fine in Ubuntu 19.10 Eoan Ermine.

And it works great: plug the Yubikey in first, type in your password and hit Enter, the key starts flashing, touch the flashing bit, and you are in.

On the downside, I would not use this on a server where you need to do management remotely, since you would not be able to plug in a U2F key on an SSH connection.

As for using NFC and iOS: it does not work like I expected it to, nor how the Yubico website led me to expect. If you tap the Yubikey to the iPhone, it will pop up an alert, which if you tap, will open Safari on a "validation" web page hosted at yubico.com.

The websites which work within Chrome and Firefox on a computer (Google, GitHub, etc) do not seem to have a way to read the Yubikey via NFC on iPhone. There is a Lightning + USB-C key (the 5 Ci) but it's expensive ($70 ea.) and I do not know for sure if it will work, since the Google and GitHub mobile websites viewed on iPhone and Android do not even present the option for using U2F keys.

So, at this point, I feel I should have just bought the cheaper non-NFC, and I would have been at the same point.

UPDATE 1 If you use KeePassXC for storing passwords, it can be configured to require a YubiKey. This uses the "challenge-response" feature, which has to be manually set up using the YubiKey Personalization Tool (also available at GitHub). Yubico has a video walkthrough here: https://www.yubico.com/products/services-software/personalization-tools/challenge-response/