Little over a year ago I ordered 2 Yubico U2F security keys, through the Github offer. When they arrrived, I was immediately annoyed by the fact that only Chrome (and Chromium) supported the U2F standard. At that time, my main browser, did not support U2F at all, and the feature request had been open for a year, with no real progress. Sigh.
So those keys disappeared in a drawer... Until a friend told me about the U2F addon for Firefox. Dug up the keys, installed the Firefox addon, et voila: working U2F in Firefox. Yay!
As there aren't many websites that support U2F, one of the keys is mostly just hanging on my keychain, doing nothing. But every now and then I need it, because I logged out, cleared browser content, or used a new PC.
And then suddenly, on that new PC, it no longer worked. In Chromium, I get "unknown error occurred" as soon as I plug in the key. The LED on the key blinks once, and pushing the button toggles the LED state. That's not good! The LED should be blinking until you push the button.
I remember seeing this problem before, but I'm pretty sure I used it without a problem afterwards. Maybe it broke, false contact, or bad USB port? To be sure, I dug up the 2nd key from secure storage, and tried that one. Same result. WTF?
Curious why both keys no longer worked, I opened the Yubico support site. Turns out they have a FAQ for using U2F on Linux, and ... For the key to work, you need a udev rule. You can find the udev rule in the FAQ, in case it doesn't come with your distro. On Gentoo, it is installed as part of the libu2f-host package. After installing that package and running "udevadm control --reload", both keys started working.
Guess I forgot about installing the libu2f-host package on my other computer... But boy, do I feel stupid now!