U2F is  a two-factor authentication protocol that uses a physical key to verify the user’s identity. Here’s how it works:

  1. The user enters his or her username and password on the website.
  2. The website sends a challenge to the user’s U2F key.
  3. The user presses the button on the U2F key to answer the challenge.
  4. The U2F key sends a cryptographic signature back to the website.
  5. The website verifies the signature and grants access to the user.

The U2F key can be connected via USB, NFC or Bluetooth and can be used for various services. It’s already supported in Chrome, Firefox, and Opera for Google, Facebook, Dropbox, and GitHub accounts.