An apparatus for storing or reading data in a memory array of a transponder and a corresponding transponder, read/write device and program element is described. Therein, a data file system for storing data within the memory array is defined by a predetermined protocol. The storing additional data includes checking whether a memory size of the application data file is larger than the memory size indicated by the application data length indicator; and storing second application data in a partial memory area of the application data file not occupied by the first application data. Thereby, memory areas which, according to the predetermined protocol, are not used can be used for new applications, data can be hidden in these areas such that they can not be read by protocol compliant reader devices and the data structure read or written is compatible with the former predetermined protocol.