Wouldn't you need a reference picture to compare two cameras? Or would you be able to determine which camera took a specific picutre if you get hold of it and can take test pictures? Or can you just compare two completely separate pictures and determine if they are taken with the same camera? If you can then that means protestors/anarchists who intend on documenting what is going on and want to stay anonymos should buy a sort of "burner" camera that they do not use for anything else.