Your checksum is a unique string of code that functions as a fingerprint of the data in the file,
that you need to generate from the zip file that you send in to the authorities.
If you make any changes to the report (no matter how small), you will need to generate your checksum once again as the checksum will be changed. It will be used to verify the origin of your report.
Unfortunately we can't recommend a software for this purpose and advice you to talk to your local IT-department for further assistance with this.