Tags: misc captcha 


nullcon HackIM 2019: Captcha Forest Harder

## Description

A harder captcha for you if you felt the first one was too easy

`nc miscc.ctf.nullcon.net 6002`

The server sent us a `png` image in hexadecimal. We have to solve the captcha and output 4 corresponding characters.

This challenge need to be solved 120 times out of 200 tries to output the flag.

The image is a concatenation of 4 characters from the Bill Cipher (a code in Gravity Fall).
![The code mapping](gravityfallsdecode.png)

This is the harder problem of [Captcha Forest](../Captcha-forest)

The difference is that the image has rotation and scaling over the characters.

#### Example of Captcha with rotation and scaling


## Solution

This is a similiar solution as the previous one. We had the luck to find good enough character to match the code. [source](https://www.dcode.fr/gravity-falls-bill-cipher)

Instead of just testing our mapping symbol with the captcha, we combine it with multiple rotations and scalings.

With a family of 30 different rotations {-45, -42, ..., 0 , 3, ... 45} and 5 different scalings, we reach around 95% of accuracy in the captcha. It is enough to recover the flag.

## Code

The code is written [here](script2.py)

The image mapping is stored [here](mapping)

## Flag


Original writeup (https://github.com/ctf-epfl/writeups/tree/master/nullcon19/Captcha-forest-harder).