A detecting data processing apparatus determines whether one or more code words of a predetermined set of code words are present in a suspected version of a material item. The apparatus comprises a registration processor operable to associate samples of the suspected version with samples of a copy of the original material item, a recovery processor and a detection processor. The recovery processor is operable to generate a recovered code word by comparing the registered copy of the original and the suspect material items. The detection processor is operable to detect one or more code words from a correlation between the recovered code word and the code words from the set. The registration processor is operable to form at least one reduced-bandwidth-version of the material item and to associate the suspected version and the copy of the material item in accordance with a comparison between the reduced-bandwidth-versions of the suspected version and the original material item.