chash: the sha1 hash of the compressed file.
While if you manually compress the file with gzip -n -9, you would find the sha1 value is still different with the one in the manifest file. By looking into ips/pkg's source code, it turns out that the head used in ips/pkg is a little different with the one gzip(1) generated,
-0000000 1f 8b 08 00 00 00 00 00 02 ff
+0000000 1f 8b 08 00 00 00 00 00 02 03
