Data security devices are provided which store user data and interact with terminal devices to provide information about the stored user data. Security device has memory for storing user data, an interface for transmission of data communications connectable to a data communications network, and a controller. The controller processes a request from the terminal device for information about said user data by first generating a message. The message is generated to permit verification, using secret data, that the message was generated by the controller. The controller sends the message to the terminal device for communication to a publication entity for publication of the message. The controller then receives from the terminal device a cryptographic construction. The controller checks validity of the cryptographic construction for said message, and subsequent supply of the information requested about the user data to the terminal device is then dependent on said cryptographic construction.