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From: Richard Heathfield <rjh@cpax.org.uk>
Newsgroups: sci.crypt
Subject: Re: xorpng
Date: Mon, 10 Feb 2025 00:59:04 +0000
Organization: Fix this later
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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).

-- 
Richard Heathfield
Email: rjh at cpax dot org dot uk
"Usenet is a strange place" - dmr 29 July 1999
Sig line 4 vacant - apply within