U2F is part of the FIDO Alliance and is designed to allow online services to augment the security of their existing password infrastructure by adding a strong second factor to user login. The user logs in with a username and password as before. The service can also prompt the user to present a second factor device at any time it chooses.

The strong Multi-Factor Authentication (2FA) allows the service to simplify its Passwords (e.g. 4–digit PIN) without compromising security.

U2F originally developed by Google and Yubico became a FIDO standard alongside the original FIDO UAF.

U2F really has two parts a JavaScript API (now WebAuthn) for Relying Party to browser and a browser to authenticator potion both were called U2F but now the Client To Authenticator Protocol (CTAP)

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