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Security Fundamentals & Crypto · Advanced

Client to Authenticator Protocol (CTAP)

FIDO protocol for communication between a client device and an external authenticator over USB, NFC, or Bluetooth.

Definition

CTAP lets a browser or OS talk to roaming authenticators such as security keys and phones. CTAP2 supports FIDO2 features like resident (discoverable) credentials and user verification via PIN or biometrics; CTAP1 corresponds to legacy U2F. Hybrid transport lets a phone act as an authenticator for a nearby computer.

Example

Scanning a QR code to use a phone's passkey on a laptop uses CTAP hybrid transport.

Exam tip

CTAP connects authenticators; WebAuthn connects browsers to websites.

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