If that's what you're trying to do, you needn't be so devious. Instead, I would suggest using a PGP-type signature and putting that in the "comment" field of the GIF file. When you validate the GIF for authenticity, you can strip the comment out and evaluate the image for integrity. The GIF comment field should be able to contain a regular signature, which is really just text.

This way, if the image is modified, the comment signature will not check out, and since the signature is based on a private key that they don't have, they can't forge a new one. You could achieve the same thing with SHA1 or MD5 using a long and secure "passphrase" as well.

JPEG, as I have tried to emphasise, is a very bad idea since the compression will pretty much destroy any subtle fingerprinting you do on a bit level. A more sophisticated "watermarking" technique is required in that case, and these are generally non-trivial to implement, as they often involve things like "fuzzy logic" to detect partial patterns, or to correct damage done by the JPEG compression.

In reply to Re^3: Gif steganography by tadman
in thread Gif steganography by Anonymous Monk

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