This looks like a good way to checksum files.

However, I thought of something that might be good for a future version.

Could it be an idea to also provide a context-sensitive menu interface within the Finder?

For instance, if I wanted to compare the checksum with a file, I could copy the checksum value, and then right-click on the file that I want to check.

The context sensitive menu would offer to checksum the file, and because I have a value in the clipboard, the option to compare that value against the file would be available. (As there are different types of checksums, the option would need a sub-menu that specifies the type of checksum being used.)

Making this selection would trigger a background process, or a progress bar. Once the process is complete, a dialogue box would appear and specify if the checksum matched or not.

The same context sensitive menu could be used to generate a checksum, using the same set of sub-menus. This would be default behaviour when holding down a modifier key while engaging the context sensitive menu.

If the clipboard is empty, the full ChecksumJelly application would launch.

Although this might not be the direction intended for the application, it seemed a good enough idea to share.

Maybe the "Target Checksum" checkbox could be dropped by verifying if the text field is empty.
If it is, no target checksum as been given.