What is clientDataJSON in WebAuthn?

clientDataJSON is an important component in WebAuthn, required for the communication between a client (like a web browser or mobile app) and a server during user authentication processes. It's integral in both registration and login ceremonies in WebAuthn.

It's a JSON structure that is part of the attestation (for registrations) and assertion (for logins) objects:

"clientDataJSON": {
        "type": "webauthn.create",
        "challenge": "ixK7mvqpx8PyvnBHT9h2iVJxQrR5tOieTWlAOLHCM",
        "origin": "https://www.passkeys-debugger.io",
        "crossOrigin": false
      }

Continue reading for a technical breakdown of the attributes.

The clientDataJSON object in WebAuthn is more than just a data container. It’s a security mechanism ensuring that the authentication process is tied to the original challenge and domain, thus safeguarding against common security threats.‍

• Type: Indicates the WebAuthn operation, either webauthn.create (registration) or webauthn.get (authentication).
• Challenge: A base64url encoded cryptographic challenge from the server.
• Origin: The effective domain of the requester, as identified by the client. This attribute prevents Phishing, since it's unique for every Relying Party.

• Security Focus: By including the domain (origin) and challenge, clientDataJSON ensures that the authentication response is tied to the original request, preventing misuse.
• Encoding and Decoding: For optimal performance, clientDataJSON is converted into an ArrayBuffer for transmission, and then back to a string for server-side validation.
• Browser and Application Integration: For desktop or mobile applications, the CTAP Authenticator API needs to be implemented by libraries or SDKs to build and decode the clientDataJSON Object

clientDataJSON facilitates secure communication between the client and server during WebAuthn registration and authentication processes.

It ensures the authentication process is tied to the original request and domain, preventing replay and phishing attacks.

The main challenge is its conversion to and from an ArrayBuffer for efficient communication, which is typically managed by browsers or requires specific libraries in applications.