Abstract: Invention maintains duplicate files in safe places. A SCAN computer program creates a TOKEN Table of an earlier file. The TOKEN Table reflects the indices of successive segments of the file and the exclusive-or (XR) and Cyclic redundancy check (CRC) products of the characters in each segment. An updated file is compared to the earlier file by comparing the XR and CRC products of segments in the updated file to the XR and CRC products in the TOKEN Table. On detecting matching products for identical segments, the next segments are compared. On mismatch, the segment (window) for the updated file is bumped one character and new XR and CRC products generated and compared. The indices of the TOKEN Table and the offsets from the start of the file of the first characters of the updated file matching segments are set forth in a Match Table.