Re: xorpng

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