Coding Challenge for MI5

Re:Coding Challenge for MI5 by jamesbrownuhh on Sunday September 11, 2016 @01:14PM (#12474379)

OK, so, the obvious "congratulations" messages aside, what's really in that file?

Example: Take the PNG, convert it to another lossless format. Then convert that lossless format back to PNG.

Different file size, right? OK, the comment accounts for about 120 bytes.. but what about the rest? Why is it that much bigger?

"As I read, numbers I see. 'Twould be a great shame not to count this art among the great texts of our time" - count this art among the great texts... it's a one time pad, isn't it.

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Re:Coding Challenge for MI5 by Retr0spectrum on Sunday September 11, 2016 @01:49PM (#12474586)

The alpha channel?

Edit: You're right though. Using imagemagick to clone it like so:

   convert puzzle.png puzzle2.png

results in an image that's at least 40% smaller. It might just be better compression, but there might be another layer of steganography going on.

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Re:Coding Challenge for MI5 by maffydub on Monday September 12, 2016 @04:55AM (#12478135)

I unpacked the chunks and there doesn't look to be much more (or it's subtle).

The IDAT chunk (which makes up most of the file) is DEFLATE-d but uses full color (rather than palettized color, which would have saved lots of space, and might be how you'd get 40% compression). It also doesn't use any row filtering.

I checked incase something was hidden in the DEFLATE encoding itself (e.g. using sub-optimal block encoding selection) but this round-trips exactly, so I don't think it can be.

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Re:Coding Challenge for MI5 by basemntunivrse on Sunday September 11, 2016 @06:15PM (#12476021)

"As I read, numbers I see."

If we move the comma, this becomes: "As I read Numbers, I see"...

OTP on the https://en.wikipedia.org/wiki/Book_of_Numbers, perhaps?

There may well be a lot more to this puzzle than first meets the eye!

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