Mark H. Wood wrote:

I didn't know where else to turn, for folks who might be able to point
me at standards for or discussion of embedding crypto signatures in
image formats, to detect tampering with the image.

I do not know any, but like to add my POV. Let's say GnuPG could
digitally sign a .png image, i.e. inserting the signature steganographically in the image and later a user could verify the steganographically embedbed
signature. What happens if Eve uses Photoshop and does a slightly image
correcting and re-saves the image? It would IMHO give a user then
an invalid signature or none.

Sending images over the Internet, say from an authorized photostudio
(passport photos etc.) can only be savely transmitted (openly) IMHO if
the photostudio would embedd the image in an, for example, digitally
signed .pdf, containing an eIDAS[1] signature, guaranteeing globally
that the image in the .pdf was signed by an authorized photosudio and
not manipulated by a middleman, while in transfer.

[1] eIDAS is the Digital Signature Standard in the EU for .pdf documents,
which can be verified with the free Adobe Reader.

Regards
Stefan




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