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From: Rich <rich@example.invalid>
Newsgroups: sci.crypt
Subject: Re: xorpng
Date: Mon, 6 Jan 2025 03:44:43 -0000 (UTC)
Organization: A noiseless patient Spider
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Chris M. Thomasson <chris.m.thomasson.1@gmail.com> wrote:
> On 1/5/2025 1:25 AM, Stefan Claas wrote:
>> 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?
>> 
>> To make more content as allowed postable on social media, like X.
>> 
> 
> Well, posting a png to say, facebook, well... It's probablly going to 
> turn it into a jpg... This can ruin the embedded ciphertext in the png 
> image...

ebay seems to recently be converting jpeg's into webp's after one 
uploads them, which would also alter the stored bitmap (as webp is 
lossy).  Although given the volume of image data they handle, if webp 
is even 5% smaller than jpegs, it becomes a huge storage savings for 
them.