This invention provides a technique of preventing determination of image alteration when digital image data has undergone, e.g., rotation without any substantial change in contents. To do this, an area separation processing unit separates image data into areas. For each of the separated areas, an area feature value calculator calculates an area feature value independent of the coordinate information of the image. An area order sorter sorts the separated areas in accordance with the calculated area feature values. A validation data generation processing unit generates validation data based on the sort result.