Re: xorpng

Stefan Claas <pollux@tilde.club> wrote:

Rich wrote:

Stefan Claas <pollux@tilde.club> wrote:

*Please* try to write a .ppm (P6) to .png converter (and back) in C(++)
and see if you can get the *original* data back. The sci.crypt community
and me of course would appreciate your help very much!

 
Since you have not "shown your work" (apologizes if you show it later,
I'm replying as I encounter articles in date order) we can't help you
diagnose your problem.

 

So, to the extent I can tell, you are doing something wrong somewhere.

 
Please try your conversion steps with the encrypted image, I already
have shown:
 
https://jmp.sh/HZM9ML9f
 
It includes a padding byte, needed for ppm so that it is divideable by 3.

That PPM is over-sized.  The image header claims:

  P6
  # Created by GIMP version 2.10.34 PNM plug-in
  480 480
  255

For 480x480 pixels, with a maxval of 255 (1 byte per color) and three
color channels, the binary part of the ppm should be:

480*480*3=691,200 bytes long.

Instead, the binary part of the ppm is 691228 bytes long (28 bytes too
many are present).

So when converting it to a PNG, 28 bytes are lost, which do not return
when converting back from PNG to PPM.