An encoding data processing apparatus is described which is operable to generate protected compression encoded data representative of protected media signals. The apparatus comprises a data analyser which is operable to receive compression encoded data representative of the media signals and to select low perceptibility data bits in the compression encoded data which when altered would have a relatively low perceivable effect on the media signals recovered from compression decoding the compression encoded data after the selected bits had been altered. The bits are selected in accordance with predetermined criteria. The apparatus also comprises an information validation engine which is operable to form a normalised version of the compression encoded data by setting the selected data bits in the compression encoded data to a predetermined value and to generate a secure hash value of the normalised version of the compression encoded data. The information validation engine then forms the protected compression encoded data by replacing the selected data bits of the normalised compression encoded data with the data bits of the secure hash value. By generating a secure hash of the normalised bitstream rather than the original bitstream, subsequent authentication of the bitstream to verify that the bitstream has not been changed becomes possible without increasing the size of the bitstream.