FIDO2U2F

FIDO2U2F describes authenticators and deployments that implement both CTAP1 (U2F) and CTAP2 (FIDO2), ensuring compatibility between WebAuthn-based services and legacy U2F applications.

FIDO2 consists of WebAuthn, a Web API for registering and using credentials, and CTAP2, the protocol between the browser or platform and the authenticator. U2F is the earlier two-factor standard based on public-key cryptography that predated FIDO2. U2F originated in the mid-2010s and many services still support it for legacy compatibility alongside FIDO2.

Many devices support both CTAP1 and CTAP2, allowing users to log into sites that support WebAuthn or U2F. Credentials are public-private key pairs; the private key remains on the authenticator and is used to sign challenges issued by the relying party. WebAuthn enables passwordless logins, strong second-factor authentication, and phishing-resistant authentication, while U2F provided enhanced security for second factors before WebAuthn became common.

Adoption spans browsers, operating systems, and hardware keys from vendors such as Yubico, Feitian, and others, enabling widespread support for passwordless experiences and resilient two-factor security. The FIDO2-U2F approach often yields backward compatibility with older services while extending capability to WebAuthn-based workflows.

Security considerations include protection against phishing, credential theft, and replay attacks, as well as practical concerns about loss, recovery, and backup strategies. In practice, many organizations deploy FIDO2 and U2F in tandem to maximize compatibility and security across diverse platforms and applications.