Established by the FIDO Alliance, Client to Authenticator Protocol (CTAP) is a specification that describes how an application (such as a browser) and operating system communicate with a compliant authentication device via USB, NFC, or Bluetooth Low Energy (BLE). CTAP1 focuses on a universal second-factor enablement CTAP2 focuses on communication between applications (browsers, operating systems, etc.) and external authenticators. It is a key standard for FIDO2-certified passwordless authentication.