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Universal Second Factor (U2F) is a protocol for strengthening online authentication.

U2F (Universal Second Factor) is a protocol for strengthening online authentication, originally developed by Google and now in the hands of the FIDO alliance.
It requires that a website supports U2F and that the user has a U2F device.

  • A user logs in using a username and password as normal, but the website can require a further verification (second factor) at any time.
  • When asked for the second factor, the user must present their device to provide further authentication. The device can connect to the client machine as a USB device or via NFC.
  • After connecting to the client machine it can provide the requested further authentication. The device itself can be locked using, for example, a 4-digit PIN.

Sources:
Official U2F specification
Wikipedia entry on U2F

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