FIDO2 refers to the combination of the FIDO Alliance’s specification for Client-to-Authenticator Protocols (CTAP) and the World Wide Web Consortium’s (W3C) Web Authentication (WebAuthn) specification, which together enable users to authenticate to online services from both mobile and desktop environments using an on-device or external authenticator.
WebAuthn defines a standard web API that is implemented by web browsers to enable web applications to use FIDO Authentication. Using WebAuthn, web browsers can invoke the CTAP interface to interact with the authenticators that are embedded in or connected to the host.
CTAP implements a standard interface to hardware authenticators embedded into the host device – for example, a fingerprint sensor – or connected to the host via USB, Bluetooth (BLE) or NFC. CTAP includes two sub-specs – CTAP1 and CTAP2. CTAP2 allows the use of external authenticators (FIDO Security Keys, mobile devices) for authentication on FIDO2-enabled browsers and operating systems over USB, NFC, or BLE for a passwordless, second-factor, or multi-factor authentication experience. CTAP1 enables authentication using existing FIDO U2F devices (such as FIDO Security Keys) on FIDO2-enabled browsers and operating systems over USB, NFC, or BLE for a second-factor experience.
Using WebAuthn and CTAP, FIDO2 supports passwordless, second-factor, and multi-factor user authentication using embedded authenticators (such as biometrics or PINs) or external authenticators (such as FIDO Security Keys, mobile devices, wearables, etc.).
To better understand FIDO2, it is worthwhile explaining FIDO and its other specifications:
FIDO (“Fast IDentity Online”) Alliance is an open industry association launched in February 2013 whose mission is to develop and promote authentication standards that help reduce the world’s over-reliance on passwords. To date, the FIDO Alliance published three sets of specifications in an effort to standardize user authentication:
- FIDO Universal Second Factor (FIDO U2F) provides a standard means for interfacing a second-factor hardware authenticator. This interface is mainly used by Web browsers to allow Web applications to interface with a user’s hardware authenticator. With the release of FIDO2, U2F has been relabeled as CTAP1.
- Client to Authenticator Protocols (CTAP) enables users to authenticate to a Web or native application using an authenticator embedded in the host computer or connected to the host computer. They provide a standardized interface to the authenticator.
- FIDO Universal Authentication Framework (FIDO UAF) defines a framework for users to register their device (i.e. laptop, desktop, mobile) to the online service and select one of the local authentication mechanisms available on the device to authenticate. The online service then selects which locally available authentication mechanism it will accept. For example, users can register their mobile device and select its embedded fingerprint sensor as the means for authenticating to the online service. Other common authentication mechanisms include looking at the camera, speaking into the mic, or entering a PIN. Once registered and accepted by the online service, users can authenticate to the online service using the local authentication action registered instead of using the more traditional username and password options.