1. An apparatus for generating a marked copy of an item of material by introducing code words into a copy of the item, the code words being arranged to uniquely identify a data word from a predetermined set of data words each of the data words having at least two fields, the apparatus comprising: means for generating at least two of the code words having a plurality of code word coefficients, each of the at least two of the code words being independently generated from a different code word seed, one of the at least two of the code words being selected from a first set in a dependence upon a value of a first of the at least two fields and another one of the at least two code words being selected from a second set in a dependence upon a value of a second of the at least two fields, values of the first of the at least two fields being associated with the first set and values of the second of the at least two fields being associated with the second set, the second set being selected in a dependence upon the value of the first of the at least two fields, and means for combining the plurality of code word coefficients with the item.