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What is CTAP (Client-to-Authenticator-Protokoll)?

CTAP (Client-to-Authenticator Protocol) is an open standard developed by the FIDO Alliance that defines the communication protocol between a client (such as a web browser or a mobile app) and an authenticator (such as a hardware security key) for passwordless authentication. CTAP is designed to be used in conjunction with other FIDO standards, such as U2F and FIDO2, to provide a secure and easy-to-use alternative to traditional password-based authentication.

CTAP defines a set of commands and responses that enable the client-side software to interact with the authenticator and perform cryptographic operations for authentication. The commands include operations such as registering a new credential, authenticating an existing credential, and retrieving metadata about the authenticator.

CTAP is based on public-key cryptography, where the authenticator generates a public-private key pair and uses it to sign and verify cryptographic operations. The client-side software generates a challenge that the authenticator must sign with its private key to prove its authenticity. The signed response is then sent back to the client-side software for verification, ensuring that the user is in possession of the authenticator and providing a strong form of two-factor authentication.

CTAP provides several benefits over traditional password-based authentication. It is resistant to phishing attacks because the user must physically interact with the authenticator to complete the authentication process. It is also resistant to man-in-the-middle attacks because the signed response can only be verified by the relying party that generated the original challenge. Additionally, CTAP provides a consistent user experience across different websites and applications that support the standard, making it easy to use and adopt.

CTAP has been adopted by several major online services, including Google, Microsoft, and Dropbox, and is supported by popular web browsers such as Chrome, Firefox, and Edge.

The FIDO Alliance continues to develop and promote the CTAP standard, along with other passwordless authentication standards, to improve online security and user experience.