METHOD OF PRODUCING A CHECKPOINT WHICH DESCRIBES A BOX FILE AND A METHOD OF GENERATING A DIFFERENCE FILE DEFINING DIFFERENCES BETWEEN AN UPDATED FILE AND A BASE FILE

A checkpoint which describes a base file is produced by dividing the base file into a series of segments; generating for each segment a segment description; and creating from the generated segment descriptions a segment description structure as the checkpoint. The segment descriptions represent segments of the base file at a minimum level of resolution sufficient to represent distinctly the segment. A difference file which defines differences between an updated file and the base file is produced by generating at different levels of resolution segment descriptions for segments in the updated file and comparing the generated segment descriptions with segment descriptions in the checkpoint to identify matching and non-matching segments. Data identifying segments in the updated file that match segments in the base file and data representing portions of the updated file at a minimum level of resolution sufficient to represent distinctly the portion are stored as the difference file.