Document Marking and Identification Using Both Line and Word Shifting

We continue our study of document marking to deter illicit dissemination. An experiment we performed reveals that the distortion on the photocopy of a document is very diierent in the vertical and horizontal directions. This leads to the strategy that marks a text line both vertically using line shifting and horizontally using word shifting. A line that is marked is always accompanied by two unmarked control lines one above and one below. They are used to measure distortions in the vertical and horizontal directions in order to decide whether line or word shift should be detected. Line shifts are detected using a centroid method that bases its decision on the relative distance of line cen-troids. Word shifts are detected using a correlation method that treats a proole as a waveform and decides whether it originated from a waveform whose middle block has been shifted left or right. The maximum likelihood detectors for both methods are given.