A watermarking system comprises an encoding data processor operable to generate at least one marked version of an original item of material by introducing one of a predetermined set of code words into a copy of the original material item. The code words are generated from a pseudo-random number generator initialised with a seed value. The seed value is used to identify the marked material item, from the code word produced from the seed value. The watermarking system also includes a detecting data processor operable to identify a particular marked copy of the material item by recovering a code word from the marked material item and correlating all code words in the set of code words with the recovered code word. The detecting data processing apparatus also includes a registration processor operable to associate samples of the original material item with corresponding samples of the marked material item to which corresponding code word coefficients may have been added. By registering the original material item with the marked material item, a likelihood of correctly identifying the version of the marked material item is improved, reducing a false negative detection probability.