Re: xorpng

Richard Heathfield <rjh@cpax.org.uk> wrote:

On 05/01/2025 06:17, Rich wrote:

Stefan Claas <pollux@tilde.club> wrote:

Rich wrote:
>

If instead you mean some kind of "special, PNG aware, encryptor that
only encrypted the bitmap data of a PNG", but left the file as
otherwise a proper PNG image structure, then that is slightly tricky
(and an algorithm that is only useful for PNG's alone).

>
Yes, this is what I mean.

 
Which brings up the question of: why?
 
Why go to the trouble to create an encryptor that is specalized for
just encrypting the internal bitmap data within a PNG, leaving the rest
as a PNG file, when a generic "byte stream" encryptor will encrypt the
entire PNG with no extra effort?

 
Sorry to come late to the party.
 
I can think of two places where image formats can be useful in
cryptography.
 
Firstly, lossless image formats can be used to steganographically
hide small ciphertexts in the low bits without seriously
degrading the image.
 
Secondly, if you want an eyeball on how tangled your bits are,
rip off the image's metadata, encrypt the bitmap data, bolt the
metadata back on, and visually inspect the resulting image and
look for patterns (or, if you used Tux - of course you did - any
remnants of penguin).

Yes, all useful, and all also trivial to do with the netpbm image
formats.  Pad to a square/rectangular size if need be, slap on two
lines of text for metadata at the front, then view the image with an
image viewer.

Stefan appeared to be making a program that, given an actual working
PNG image file, would root around in the PNG file format itself, and
encrypt just the internal bitmap data.