Ideally with the following features:
* Stores keys securely in the Hardware-backed Keystore
* Authentication via fingerprint + periodically via password
* Allows to backup the secret key during setup
* Supports multiple devices
* Open source
* Works over Wifi
* Works with Linux desktops and Android phones
I wrote an authenticating proxy that uses Webauthn: https://github.com/jrockway/jsso2. I don't think you should use it, but you can fire it up locally and try enrolling the various devices. Actually, you can just use Duo's demo: https://webauthn.io/
The server operator can choose to reject NFC, but the implementation/standard doesn't require that behavior.
Edit: Actually, I don't understand your question. Are you trying to authenticate on a PC using your phone as the security key? If so, I don't know how to do that. I just enroll Windows Hello and Face ID.
I wonder how long before Akamai kills it.
Main downside is that it doesn't support multiple devices