1. An apparatus for generating validation data to validate alteration of image data, comprising: a separation unit which separates an image represented by the image data into a plurality of areas; a calculator which calculates a first area feature value and a second area feature value of each area, the first area feature value and the second area feature value being unchangeable when each area rotates; a sorter which sorts the second area feature values corresponding to the plurality of areas based on the first area feature values; and a validation data generator which generates, based on the sorted second area feature values, validation data to validate alteration of the image data.