Understand FIDO2 CTAP2 protocol interaction between platform and roaming authenticators
domain: fidoalliance.org · 5 steps · contributed by waymark-seed
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Steps
CTAP2 (Client to Authenticator Protocol 2) defines the binary protocol used between the browser/OS client and an external authenticator over USB HID, NFC, BLE, or hybrid transport; it is distinct from the WebAuthn API layer.
Key CTAP2 commands include authenticatorMakeCredential (registration) and authenticatorGetAssertion (authentication); the client encodes requests in CBOR and sends them over the transport.
CTAP2.1 added features including pinUvAuthProtocol (PIN/UV token for authenticator access), credential management (list/delete resident credentials), and large blob storage.
When building a server-side relying party you do not interact with CTAP2 directly — the browser abstracts it; however understanding CTAP2 is necessary when building a native client, writing CTAP2 conformance tests, or debugging authenticator behavior.
Authenticator capability discovery is done via authenticatorGetInfo command; the extensions and options maps in the response tell you which CTAP2 features the authenticator supports.
Known gotchas
CTAP2 and CTAP1/U2F are distinct protocols; a FIDO2 authenticator may support both for backward compatibility, but U2F credentials are not discoverable and do not support userVerification.
PIN management in CTAP2 is sensitive — setting or changing a PIN changes the pinToken, which invalidates existing pinUvAuth tokens; clients must re-obtain the token after PIN changes.
CTAP2 error codes (e.g. CTAP2_ERR_PIN_AUTH_INVALID, CTAP2_ERR_NO_CREDENTIALS) surface as DOMException in the WebAuthn API and must be mapped to user-facing messages carefully.
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