A data processing apparatus includes an encrypting apparatus for encrypting data in units of an encryption block having a predetermined data length. A processing apparatus is also provided for performing predetermined processing on data in units of a processing block having a data length of a whole multiple of the predetermined length of the encryption block. A controller is also provided for writing the encrypted data in a storage medium so that the data positioned in the same encryption block is also positioned in the same processing block. The controller also reads the data from the storage means in units of the processing block when the data is to be read out.