And how certain can you ever be that you, the rightful owner, will be able to change metadata in the future with your chosen software on your preferred OS? Software comes, software goes. JW's spot on here.
John
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PKI is a standard with Fortune 500 clout. If it's "not available in the future" then there are bigger problems to worry about. Also it does prevent changes to the original content - just try making changes to a secure PDF document.
I guess the "gotcha" is the use of "original" in the paragraphy above. Obviously there's no way of protecting images displayed on the internet - regardless of what protection is put in place to preserve the exif - you just grab the displayed pixels. So in the long run, if there's a way to display the image, there will always be ways to copy it and leave the encrypted content behind. Same problem faced by RIAA et. al.
My recollection is that Canon had some kind of (hardware?) that plugged in to provide authentication that the content of a digital file hadn't been altered to support the presentation of that evidence in court. Which goes to Jonothan's point that you can prove an alternation, but not prevent it. Wouldn't being able to prove the alteration be sufficient?