Path: ...!news.roellig-ltd.de!news.mb-net.net!open-news-network.org!2.eu.feeder.erje.net!feeder.erje.net!newsfeed.bofh.team!paganini.bofh.team!not-for-mail
From: Stefan Claas <pollux@tilde.club>
Newsgroups: sci.crypt
Subject: Re: xorpng
Date: Sun, 5 Jan 2025 20:50:11 +0100
Organization: To protect and to server
Message-ID: <vlenpl$rjtu$1@paganini.bofh.team>
References: <vl243l$3jkpe$1@paganini.bofh.team> <vlc7ia$k8so$5@dont-email.me> <vlc80o$ipdu$1@paganini.bofh.team> <vlc891$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>
Mime-Version: 1.0
Injection-Date: Sun, 5 Jan 2025 19:50:13 -0000 (UTC)
Injection-Info: paganini.bofh.team; logging-data="905150"; posting-host="P19FF9OOayxo5XAXCSdPUg.user.paganini.bofh.team"; mail-complaints-to="usenet@bofh.team"; posting-account="9dIQLXBM7WM9KzA+yjdR4A";
User-Agent: flnews/1.3.0pre29 (for GNU/Linux)
Cancel-Lock: sha1:Evg7C9kKTxAErCKCBtBCX6rSkGc=
X-Ed25519-Sig: 66d1b6ea01b8c1d9c7d7559fb7a8cdb89edc920df6cd9b87e455efda2a21d89d
 a245be3521eb9f8e050a053ef08fff1922ff20fc551b1e8640bedc2867858707
X-Date: It's Sun Sep 11450 08:50:11 PM CET 1993, the September that never ends.
X-Ed25519-Pub: c0ffee5a36e581eb10f60b2831b3cdb955d2e7ef680dd282a8d43ad8b84b357a
X-Notice: Filtered by postfilter v. 0.9.3
Bytes: 4033
Lines: 53

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.

-- 
Regards
Stefan