polygl0ts
EPFL's CTF team
A baby captcha just for you.
nc misc.ctf.nullcon.net 6001
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 200 times to output the flag.
The image is a concatenation of 4 characters from the Bill Cipher (a code in Gravity Fall).
Using Google, we found a table with similar character for the Bill Cipher.
The mapping works if we answer manually to the server, so that’s great.
Right now we would like to automate it. The quality of the image seems not enough to do a good enough matching. So with more google search I found the exact images used in the challenge!
From there we just split the catpcha in 4, pick the symbol with the least absolute difference for each of the 4 images.
It’s then the trivial to communicate with the server and get the flag.
The code is written here
The image mapping is stored here
hackim19{Since_you_are_not_a_robot_I_will_give_you_the_flag}