A method of and an apparatus for implementing real-time transaction specific watermarking to generate a transaction specific digitally watermarked data file is described. The method comprises two stages. Prior to receiving (52; 102) a transaction request for the data file: the data file is divided (34; 84) into a plurality of sections; and a set of pre-calculated elementary watermarks is created (86). On receiving (52; 102) a transaction request for the supply of the data file: the method comprises calculating (56; 106) a transaction specific watermark; selecting (64; 114) pre-calculated elementary watermarks equivalent to constituent portions of the transaction specific watermark by comparing the constituent portions of the transaction specific watermark with the pre-calculated units of data; and constructing (66; 116, 118) a transaction specific watermarked data file by assembling the selected pre-calculated elementary watermarks together with corresponding ones of the plurality of sections.