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Serpentine

  • Description: Find the flag in the Python script!
  • Difficulty: Medium

🔎 Solution​

This challenge provides us with a Python source file.

When executed with the b option (intended to print the flag), the program reports that the print_flag function has been misplaced.

a) Print encouragement
b) Print flag
c) Quit
What would you like to do? (a/b/c) b

Oops! I must have misplaced the print_flag function! Check my source code!

Inspecting the source code, we find 2 key components defined at the beginning:

  • str_xor, a helper function performing an XOR operation.
def str_xor(secret, key):
    #extend key to secret length
    new_key = key
    i = 0
    while len(new_key) < len(secret):
        new_key = new_key + key[i]
        i = (i + 1) % len(key)        
    return "".join([chr(ord(secret_c) ^ ord(new_key_c)) for (secret_c,new_key_c) in zip(secret,new_key)])
  • print_flag, which is supposed to output the flag string.
def print_flag():
  flag = str_xor(flag_enc, 'enkidu')
  print(flag)

However, within the main function, print_flag is never actually invoked. This means the solution is straightforward: we simply need to modify the code so that the function is called.

The placement of the call doesn't matter - it could be before the while loop or within 1 of the conditional branches. In this case, I inserted the call under the b) Print flag option.

elif choice == 'b':
      print_flag()

Running the program again after this adjustment successfully reveals the flag.

What would you like to do? (a/b/c) b
picoCTF{7h3_r04d_l355_7r4v3l3d_8e47d128}

🚩Flag​

picoCTF{7h3_r04d_l355_7r4v3l3d_8e47d128}