1. 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 being arranged to identify a data word from a set of data words having at least two fields, the apparatus comprising 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, one of the code words being selected from a first set in dependence upon a value of a first of the data fields and one of the code words being selected from a second set in dependence upon a value of a second of the data fields, and an encoding processor operable to combine the code word coefficients with the material item, wherein values of the first of the data fields are associated with the first set of code words and values of the second of the data fields 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.