Ciscopack is an IOS binary image packing and unpacking program capable of calculating a correct checksum for these images for Unix bases systems. Cisco is using it's own modified version of the ZIP archiver, and if you don't have it, simply supplement the unpacked image body with an empty bit (0xFF) in your favorite hex editor, if the image length is a fraction after packing with ZIP (see the length in the tool output in brackets before "words"). Then use ciscopack.pl to repack the image and the correct checksum would be calculated. Please read the --help output for the options available. Don't forget that you need ZIP installed to use ciscopack.pl.
