+A challenge-response key is kind of like multifactor authentication, except you don't really I<authenticate>
+to a KDBX database because it's not a service. Specifically it would be the "what you have" component. It
+assumes there is some device that can store a key that is only known to the owner of a database. A challenge
+is made to the device and the response generated based on the key is used as the raw key.
+
+Inherets methods and attributes from L<File::KDBX::Key>.
+
+This is a generic implementation where a responder subroutine is provided to provide the response. There is
+also L<File::KDBX::Key::YubiKey> which is a subclass that allows YubiKeys to be responder devices.
+