Yubikey is a hardware-based 2-Factor Authentication system.
You can only use this form of two-factor authentication if you have a Yubikey hardware device - specifically one that will generate One Time Passwords (OTPs).
This plugin option will let you add Yubikey One Time Password authentication to your WordPress login form.
In effect, when combined with email-based two-factor authentication, it creates a multi-factor authentication system.
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