1. A watermarking method for a picture identification document, said method comprising the steps of: extracting facial features from a facial image on the picture identification document; generating a watermark key from the extracted facial features wherein said facial features used to generate the watermark key are an Eigenface representation of the facial features and the watermark key generated is an Eigenface vector of the facial features; generating a watermark using the watermark key, wherein the watermark key is distinct from the watermark; and embedding the watermark on the picture identification document.