This is speculation but generally rules like this follow some sort of incident. e.g. Someone responds to a FOI request and accidentally discloses more information than desired due to metadata. So a blanket rule is instituted not to use a particular format.
replyMaybe they know more than we do. It may be possible to tamper with files at a deeper level. I wonder if it is also possible to use some sort of tampered compression algorithm that could mark images much like printers do with paper.
replyAnother guess is that perhaps the step is a part of a multi-step sanitation process, and the last step(s) perform the bitmap operation.
I'm not sure about computer image generation but you can (relatively) easily fingerprint images generated by digital cameras due to sensor defects. I'll bet there is a similar problem with PC image generation where even without the EXIF data there is probably still too much side channel data leakage.
replyImage metadata is the wild west of structured text. The developer of the foremost tool for dealing with it (exiftool) has made 'remove metadata' feature but still disclaims that it is not able to remove everything.
replyHow could that be possible? Isn't JPEG a fairly straightforward container for JFIF+metadata?
reply"Fairly straightforward" is incorrect. Not an authority to describe in more detail, but the most tricky blocker I'm aware of are these proprietary "MakerNote" tags from camera manufacturers, which are (often undocumented) binary blobs. exiftool might not even know what's in there, let alone how to safely remove it without corrupting the file.
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