Now we're getting somewhere
We are not getting anywhere. The topic was and is NOT if a photographer can verify if his/her image has been modified. No additional info is necessary (checksum, digest, key) if the original is available.
The topic is, how an image can be proven as being the original to a third party
, for example in court. In other words: how can one prove, that an image is original, not compared to another version, but on its own.
It is possible to encode a digest like MD5 using an asymmetric key; however, the private part of that key has to be preserved somewhere. It can be stored in the camera firmware, in which case only the manufacturer knows the key - as long as no-one makes the effort to find it
Doing the same in Photoshop, etc. is a dead-born idea. What should that prove? That I have successfully processed the image in Photoshop?
Methink some posters don't understand the difference between watermarking and authentication.