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From: Rich <rich@example.invalid>
Newsgroups: sci.crypt
Subject: Re: xorpng
Date: Sun, 5 Jan 2025 21:07:07 -0000 (UTC)
Organization: A noiseless patient Spider
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References: <vl243l$3jkpe$1@paganini.bofh.team>   <vlc891$k8s5$2@dont-email.me> <vlc8om$k8s5$3@dont-email.me> <vlc9d8$irra$1@paganini.bofh.team> <vlcahc$ks00$1@dont-email.me> <vlcbki$j00g$1@paganini.bofh.team> <vlccrh$lb6a$1@dont-email.me> <vlchr0$j921$1@paganini.bofh.team> <vlcivh$md8n$2@dont-email.me> <vlcjan$oal1$2@paganini.bofh.team> <vld86b$tdna$1@dont-email.me> <vldj6q$pqvr$2@paganini.bofh.team> <vldk3q$psnq$1@paganini.bofh.team> <vlecdg$13phn$2@dont-email.me> <vlenpl$rjtu$1@paganini.bofh.team>
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Stefan Claas <pollux@tilde.club> wrote:
> Rich wrote:
>> Stefan Claas <pollux@tilde.club> wrote:
>> > 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.
>> > 
>> > I.e, first you put data with file2png in a .png and then encrypt 
>> > it to finally post it.  I can do this now with my xorpic program, 
>> > but I thought a solution with AES-GCM or XChaCha20+ploy1305 is 
>> > better.
>> 
>> The "path" I outlined in my previous post, where you utilize the 
>> netpbm image format as your 'intermediary' would allow you to use 
>> any generic encryption routine you like, while also allowing you to 
>> convert the encrypted binary data to/from an image format of your 
>> choice (well, your choice within the set of other formats for which 
>> NetPBM has to/from converters available).
>> 
>> This frees you from having to understand the internal structure of 
>> the various image formats.  You just work with the netpbm format (a 
>> raw binary bit/pixel block) for the encrypt/decrypt/padding 
>> operations, and delegate all the "image format" complexity to the 
>> netpbm library.
> 
> Thank you!  My ppmenc tool works nicely, here are the test images:
> 
> https://jmp.sh/HZM9ML9f
> 
> The big problem I face when converting the encryypted image to .png 
> and back a diff shows a difference and the decryption fails.
> 
> Maybe someone can figure out what to do, so that a converted .ppm can 
> be posted online , for viewers/readers and then can be converted back 
> to the original .ppm, which shows no difference.

We can't read your mind over Usenet so can you show how you converted 
the encrypted image to a png and back.