The other component of FIDO2, Client to Authenticator Protocol (CTAP), is complementary to WebAuthn. It enables an external authenticator, such as a security key or a mobile phone, to work with browsers that support WebAuthn, and also to serve as an authenticator to desktop applications and web services.
For more detailed information on CTAP, go HERE. This Proposed Standard document describes the Client-to-Authenticator Protocol (CTAP) which uses USB, NFC, and / or BLE for communication between browsers or platforms and FIDO2 authenticators.
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