Security keys are little USB devices that generate one-time tokens in place of the six-digit codes from authenticator apps. Google and Facebook both promote security keys as a better way to keep your account safe. Keys are available that work over USB, Bluetooth, and NFC, so they can be used with a smartphone or tablet in addition to a PC. Not all keys will generate both U2F and OTP codes, but the cheapest of the company’s line-up isn’t compatible with OTP.”]