Here's the situation: We provide a way for customers to view a document, say a spreadsheet, on a display screen in a public area. To do so, they use our content management software, web-based, and upload the file. It goes to our servers, gets modified into the proper display format, and returned to the display.
Most of the time this is fine, but some companies don't want the spreadsheet to leave their network. In that case, the only solution now is to give them an on-premise version of our software, which has limited functionality and is expensive. So if we could encrypt it - and prove it at any time by verifiable 3rd-party sources - we could use our standard web-based solution AND provide the security.
What do you think would be a good way to do that?