If you know you might want to prove something's authenticity later, post the SHA256 somewhere now. E.G. Multiple social media sites. Large and trusted web archives. Cryptocurrency blockchains. The last one's a stronger proof, with lots of money making sure it stays immutable.
Or hash everything you produce/consume. Then hash the list of hashes, and post that.
Or alternatively, counter forgeries by capturing more data. For web, but all data in general. Sensor RAWs to prove an image isn't "AI". Browser and network stack RAM dumps to prove a website's authenticity. Etc. There's what, a couple dozen accelerometers, GPS's, LIDARs etc. on the latest iPhones?
Or hash everything you produce/consume. Then hash the list of hashes, and post that.
Or alternatively, counter forgeries by capturing more data. For web, but all data in general. Sensor RAWs to prove an image isn't "AI". Browser and network stack RAM dumps to prove a website's authenticity. Etc. There's what, a couple dozen accelerometers, GPS's, LIDARs etc. on the latest iPhones?