1. A method of embedding binary data in a halftone image, comprising: identifying a halftone image and a binary watermark to be embedded in the image; discovering candidate sites in the image for embedding data; forming the discovered candidate sites into a plurality of overlapping groups; determining parity values for respective groups of candidate sites; and toggling halftone values of candidate sites in the image such that the parity values for the respective groups of candidate sites match corresponding bits in the binary watermark.