An apparatus for reading or writing user data stored block-wise in encrypted form on a storage medium. The apparatus includes a command interface for receiving and interpreting a read (or write) command. The read command includes a user data information specifying which user data are to be read. A reader reads the user data and a related encryption indicator from a block of the storage medium. The encryption indicator indicates if said user data is encrypted or not. The reader further reads a related key data identifier specifying which key data to use for decrypting the user data if said encryption indicator indicates that said user data are encrypted. A decryptor decrypts the user data using the key data.