According to the present invention there is provided an encoding data processing apparatus for generating a marked copy of an item of material by introducing code words into a copy of the material item. The code words are arranged to identify a data word from a set of data words having at least two fields. The apparatus comprises a code word generator operable to generate at least two code words having a plurality of code word coefficients, each of the code words being independently generated from a different code word seed, and an encoding processor operable to combine the code word coefficients with the material item. One of the code words is selected from a first set in dependence upon a value of a first of the data fields and one of the code words is selected from a second set in dependence upon a value of a second of the data fields. The values of the first data field are associated with the first set of code words and values of the second data field are associated with the second set of code words, the second set of code words being selected in dependence upon the value of the first data field. By arranging for a value of a second or subsequent data field to be selected in dependence upon a value of the first or a previous data field, then a detecting complexity for unambiguously identifying the data word can be substantially reduced.