First, develop cameras that timestamp and digitally sign all the photos they take. Second, use your time machine to go back to 1800 and ensure all historical p

First, develop cameras that timestamp and digitally sign all the photos they take. Second, use your time machine to go back to 1800 and ensure all historical photographs are taken with such a camera. Then when you see something that purports to be a historical photograph, you simply validate the signature and timestamp.