# Nuit Du Hack CTF Quals 2017 You have been terminated### Category: Steganography, 50 points
> You found this strange image on a computer, I heard there is a hidden message in it... Can you find it ?> Hint 1: I heard a cat... Hint 2: There are multiple implementations of this algorithm
### Write-up
We start off with a picture like this.
![Chill out](http://i.imgur.com/s4gzIhh.jpg)
The first hint points us toward an algorithm called [Arnold's Cat Map](https://en.wikipedia.org/wiki/Arnold%27s_cat_map)
As hint 2 mentions, there are multiple implementations and some doens't seem to work.
First off I tried Jason Davies [Catmap](https://www.jasondavies.com/catmap/)But it didn't seem to work. 
After that I found [Zhanxw Cat](https://github.com/zhanxw/cat)
However it's four years old so some modifications had to be done to make it running.
Here's the modified script
```pythonimport PIL.Imagefrom numpy import *#from scipy.misc import lena,imsave#from scipy.misc import imsave
# load imageim = array(PIL.Image.open("o.jpg"))N = im.shape[0]
# create x and y components of Arnold's cat mappingx,y = meshgrid(range(N),range(N))xmap = (2*x+y) % Nymap = (x+y) % N
for i in xrange(N+1): result = PIL.Image.fromarray(im) result.save("cat_%03d.png" % i) im = im[xmap,ymap]```Each frame is saved as an individuall png.
> $ python2.7 cat.py
![25 Hour Fitness](http://i.imgur.com/v3wyAdQ.gif)
> $-->calif0rnia_g0vern0r<--$