U2F

U2F is an earlier FIDO standard that uses a hardware security key as a phishing-resistant second factor alongside a password. The key signs a challenge that includes the site's origin, so codes cannot be replayed on a fake site. It has largely been folded into FIDO2 and WebAuthn but remains widely supported.

• YubiKey 5C NFC → Hardware security key supporting FIDO2, U2F, TOTP, and passkeys. Phishing-resistant in a way app codes are not, but buy two; losing your only key locks you out.

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