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From: Rich <rich@example.invalid>
Newsgroups: sci.crypt
Subject: Re: [file2png] Convert (encrypted) binary data to .png images and back
Date: Tue, 31 Dec 2024 23:15:00 -0000 (UTC)
Organization: A noiseless patient Spider
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Stefan Claas <pollux@tilde.club> wrote:
> Rich wrote:
>> Stefan Claas <pollux@tilde.club> wrote:
>> > Hi all,
>> > 
>> > here is a Python3 implementation of my file2png program, available 
>> > here: https://github.com/706f6c6c7578/file2png
>> > 
>> > Hope you find it useful, when for example uploading such images to 
>> > platforms like X, which does not compress .png images, as I have 
>> > tested!
>> 
>> PNG compression is lossless, so it should not matter if the PNG is 
>> compressed, you should get back the same bytes that went in.
>> 
>> Now, if instead you mean "rescale" (reduce/increase size) or other 
>> transformations (RGBI to RGB or RGB to indexed) then those 
>> transformations may alter the content such that you do not get back 
>> what you put in.
> 
> I remember taht facebook/Meta did something with .png in the past and
> it did not work, but it was not rescaling etc.

There's only one compression algorithm in the PNG standard, zlib, which 
itself is lossless.

But there are a lot of "transformations" that could be done on a PNG 
that would result in modification of the actual image bytes inside.  
Given the potential cost savings to a company with the scale of 
facebook/meta from even a few percentage of saved space on uploaded 
PNG's from "optimizing" them, it would not be surprising to learn they 
were doing something to them that changed the stored bytes.