PicoCTF 2019: whats-the-difference (Points 200)

Can you spot the difference? kitters cattos. They are also available at /problems/whats-the-difference... on the shell server

In order to easily solve this challenge, I've used xxd and cut to generate an ascii hexdump of the images:

➜  Downloads xxd kitters.jpg | cut -d ' ' -f 11 > kittens_text.txt

➜  Downloads xxd cattos.jpg  | cut -d ' ' -f 11 > cattos_text.txt

Since I wanted to do a per character diff I wrote the following Python script:

def main():
    new = open("cattos_text.txt")
    old = open("kittens_text.txt")
    old_file = old.readlines()
    new_file = new.readlines()
    print("Loaded lines", len(old_file), len(new_file))
    for line in zip(old_file, new_file):
        for number, old_char  in enumerate(line[0]):
            if old_char != line[1][number]:
                print(line[1][number], end="")

main()

And finally run the script:

➜  Downloads python diff.py
Loaded lines 212919 212919
picoCTF{th3yr3_a5_d1ff3r3nt_4s_bu773r_4nd_j311y_*}%