picoCTF writeup : keygenme-py

Challenge Details

Name : keygenme-py

Category: Reverse Engineering

Difficulty: Beginner

Description

We are just given a file keygenme.py and expected to find the flag from there

First Look

If you open the python file in an editor, you will see the first part of the flag.

key_part_static1_trial = "picoCTF{1n_7h3_|<3y_of_"
key_part_dynamic1_trial = "xxxxxxxx"
key_part_static2_trial = "}"

We only have to find the dynamic1_trial part of the key.

Solution

If you further investigate, you will see a check_key function, which checks for the key i.e. the flag in this case.

I created a custom python program printing the characters one by one the program was checking for.

import hashlib

username_trial = "SCHOFIELD" # given
num_code = [4, 5, 3, 6, 2, 7, 1, 8] #extracted from challenge
hashed  = ""
for num in num_code:
    hashed += hashlib.sha256(username_trial.encode()).hexdigest()[num]

print(hashed)

This program gives the remaining part of the flag.

The final flag we get is : picoCTF{1n_7h3_|<3y_of_e584b363}