Businesses and government agencies need a way to authenticate the images they exchange electronically. A classic solution is to hide a "digital watermark" in a color or gray-scale image by making imperceptible changes in the colors or brightnesses of individual pixels. But for black text on a white background, this approach doesn't work so well: flipping even one white pixel to black on a field of white can produce a visible irregularity. Now University of Maryland information scientist Min Wu is perfecting software that adds secret data to such documents by subtly altering pixels in small blocks along the edges of letters, where the changes are virtually unnoticeable. Each block encodes a single binary digit; together, the digits constitute a signature that can be extracted to prove an image hasn't been forged or altered. If it has, the hidden signature doesn't appear, or becomes mush. Wu says officials at Princeton University, where she began work on the software as a PhD student, are talking with potential licensees about putting it to work in commercial document-verification systems.