Serpentine

Challenge Description

This is a challenge involving a Python script. We will likely have to manipulate the script to display the flag in this challenge.

We first download the provided script using wget <link>. I’ve attached to full script below for reference:

import random
import sys
 
 
 
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)])
 
 
flag_enc = chr(0x15) + chr(0x07) + chr(0x08) + chr(0x06) + chr(0x27) + chr(0x21) + chr(0x23) + chr(0x15) + chr(0x5c) + chr(0x01) + chr(0x57>
 
 
def print_flag():
	flag = str_xor(flag_enc, 'enkidu')
    print(flag)
 
 
def print_encouragement():
	encouragements = ['You can do it!', 'Keep it up!', 
					'Look how far you\'ve come!']
	choice = random.choice(range(0, len(encouragements)))
	print('\n-----------------------------------------------------')
	print(encouragements[choice])
	print('-----------------------------------------------------\n\n')
 
 
 
def main():
 
	print(
'''
	Y
	.-^-.
   /     \      .- ~ ~ -.
  ()     ()    /   _ _   `.                     _ _ _
   \_   _/    /  /     \   \                . ~  _ _  ~ .
	 | |     /  /       \   \             .' .~       ~-. `.
	| |    /  /         )   )           /  /             `.`.
	\ \_ _/  /         /   /           /  /                `'
	 \_ _ _.'         /   /           (  (
					/   /             \  \
				   /   /               \  \
				  /   /                 )  )
				 (   (                 /  /
				  `.  `.             .'  /
					`.   ~ - - - - ~   .'
					   ~ . _ _ _ _ . ~
'''
	)
	print('Welcome to the serpentine encourager!\n\n')
  
	while True:
		print('a) Print encouragement')
		print('b) Print flag')
		print('c) Quit\n')
		choice = input('What would you like to do? (a/b/c) ')
	    
	if choice == 'a':
		print_encouragement()
	  
	elif choice == 'b':
		print('\nOops! I must have misplaced the print_flag function! Check my source code!\n\n')
	  
	elif choice == 'c':
		sys.exit(0)
	  
	else:
		print('\nI did not understand "' + choice + '", input only "a", "b" or "c"\n\n')
	
	
	
	if __name__ == "__main__":
	main()
 

What does the script do?

When we run this program, we are provided with 3 options.

• a) Print encouragement
• b) Print flag
• c) Quit

Options a and c work perfectly when I run the program. But option b(print flag) does not.

Fixing the error

Understanding the error

Noticed something in the script attached above? There’s a function named “print_flag”. However, when we choose option ‘b’ when we run the program, the script prints this line instead:

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

We can run nano serpentine.py to add the print_flag() function if the user chooses option b.

I added the following 2 lines after the elif statement, and ran the code again.

elif choice == 'b':
      print('\nFixed the code now! The flag will be displayed below!\n')
      print_flag()

This allows the script to display the flag correctly when we choose option b.

Flag

picoCTF{7h3_r04d_l355_7r4v3l3d_8e47d128}