Authnkey is a credential provider that lets you use FIDO2 security keys (like YubiKey, SoloKey, Nitrokey, or Token2) for passkey authentication on Android. Android's built-in WebAuthn support has a significant gap: it does not support CTAP2 over NFC. This means you cannot use PIN-protected passkeys or discoverable credentials with NFC security keys — you're limited to basic U2F tap-to-authenticate. Authnkey fills this gap by implementing the full CTAP2 protocol. The app registers as a credential provider, so any app or browser that supports the Android Credential Manager API will offer Authnkey as an option when passkeys are requested. This also works on devices without Google Play Services, since Authnkey implements the CTAP2 protocol directly rather than relying on Google's FIDO2 API. Supported features: - Passkey creation and authentication - PIN verification - Discoverable credentials - Multiple accounts per site - NFC and USB transports No root required.