I have been thinking about an alternative pricing model but it only works for content creation software. It would be enabled by an OpenPGP library which handles

I have been thinking about an alternative pricing model but it only works for content creation software. It would be enabled by an OpenPGP library which handles signing of files and micropayments. What if the app itself was free but it charged a small fee the first time a new file was saved? The file format doesn't have to be proprietary - a simple chunk of metadata should be able to identify the creator and the files won't need to be tracked by a central authority because the mere existence of the signature in the file would imply that the one-off payment had already been made. The canonical example is a rich Markdown editor. You could get the WYSIWYG conveniences à la Word and the resulting .md file would have a signature indicating that the app had saved it. Subsequent edits by the author wouldn't incur a fee as the signature would match the app instance. But someone else with a copy of the app would incur a fee if they also used it to make changes of their own and their signature would be added to the file. Someone else could edit the file with their favourite text editor. This wouldn't incur a fee. But the signatures would have to be preserved otherwise the original author(s) wouldn't be able to re-edit the file without incurring another payment. Perhaps I'm being dumb but I think these files could be safely pushed to Github with the signatures intact. They'd be as secure as any public key. I can see this working with many file types -- MS Office documents, Adobe Illustrator et al, 3D model files, PDFs, images, movies. The same file could be edited with different apps each saving their own signatures. All that's needed is something which disables the "File/Save" command unless a payment is made. Is anyone aware of any software which works like this?