Break the popularity contest crypto

Question

There is a very interesting puzzle on the Programming Puzzles and Code Golf site to Rearrange pixels in an image so it can't be recognized and then get it back. I understand that this is not cryptography, but I wonder how effectively a message could be hidden in such an image and then securely transferred and retrieved.

Consider Alice and Bob who have conspired beforehand to use one of the solutions to the puzzle for Alice to transmit to Bob images of text. The TLA* is tasked with breaking their ad hoc "encryption" (obfuscation), but do not know about Stack Exchange sites. The TLA does not have access to Alice, as she is outside their jurisdiction. Furthermore, being images of text, the whole image is in greyscale.

If the TLA were to intercept an image 'encoded' with one of the solutions to the Code Golf contest, what steps would they take to decrypt it?

_{* TLA: Hypothetical government agency with access to the network.}

Answer 1

Basically this is a transposition cipher. What you would do is looking for pattern and rearrange like with this text example:

http://www.richkni.co.uk/php/crypta/trans0.php

Your example with grey scale text would be easier as the picture on code golf because you could see very easy what is a correct solution.

Answer 2

The Code Golf is essentially to create an unkeyed 1:1 function to transform images. As such, it's highly vulnerable to chosen-plaintext attacks: if an attacker can convince Alice or Bob to transmit a carefully-crafted image, they can build up a map of what pixels get moved to where for that image size.

Further, if the images being transmitted have predictable features (eg. an image of a newspaper clipping will always have a white border), statistical analysis can be used to build up a map without needing to know the details of the images.