1. A method of embedding watermark data in a two colour rasterized image, said method comprising: dividing said image into a plurality of blocks; for each of said plurality of blocks, assessing whether a defined pixel within said block may be altered to store a bit of said data within said block without altering the visual attributes of said block in a manner to be materially perceptible to the human eye, to determine a set of said plurality of blocks suited to store a bit of said data; storing a bit of said data in each of said plurality of blocks assessed as suited to store a bit of said data.