Further reading
An article by The Verge on hardware security keys: https://www.theverge.com/ 2019/ 2/ 22/ 18235173/ the-best-hardware-security-keys-yubico-titan-key-u2f [Tip: hold Ctrl and click a link to open it in a new tab. (Hide tip)]
Fido Alliance administers and develops standards for hardware security keys: https://fidoalliance.org/