Abstract: A system for watermarking an image file selects coefficients using a selection procedure that is kept secret, and assigns the selected coefficients to coefficient pairs. The difference between the coefficients of the pairs is then used to generate multi-bit raw signature values that characterize the authentic image at different locations. To detect an unauthorized alteration after the image file has been watermarked, coefficient pairs are selected using the same secret procedure that was originally used to generate the raw signature values. The difference between the coefficients of the pairs is then checked against the raw signature values derived from the original image file. The raw signature values derived from the authentic image file may be placed in the header of the file or in a separate file. Alternatively, they may be embedded in host coefficients that are selected in accordance with a procedure that is kept secret.