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From: Rich <rich@example.invalid>
Newsgroups: sci.crypt
Subject: Re: xorpng
Date: Mon, 10 Feb 2025 02:19:15 -0000 (UTC)
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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.