PicoCTF Writeup – fixme2.py

# Information:

CTF Name: PicoCTF

CTF Challenge: fixme2.py

Challenge Category: General Skills

Challenge Points: 100

Beginner picoMini 2022.

# Used Tools:

  • Python3

# Challenge Description:

Fix the syntax error in the Python script to print the flag. Download Python script

Hints:

Hint 1
Are equality and assignment the same symbol?
Hint 2
To view the file in the webshell, do: $ nano fixme2.py
Hint 3
To exit nano press Ctrl and x and follow the on-screen prompts.
Hint 4
The str_xor function does not need to be reverse engineered for this challenge.

# Writeup:

This is a challenge to help learn how to read python code.

As you probably know, in most programming languages to check if 2 values are equal you have to use the symbol ‘==’ and to assign a value to a variable you have to use ‘=’.

Below you have the python3 code:

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import random
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(0x58) + chr(0x18) + chr(0x11) + chr(0x41) + chr(0x09) + chr(0x5f) + chr(0x1f) + chr(0x10) + chr(0x3b) + chr(0x1b) + chr(0x55) + chr(0x1a) + chr(0x34) + chr(0x5d) + chr(0x51) + chr(0x40) + chr(0x54) + chr(0x09) + chr(0x05) + chr(0x04) + chr(0x57) + chr(0x1b) + chr(0x11) + chr(0x31) + chr(0x5f) + chr(0x51) + chr(0x52) + chr(0x46) + chr(0x00) + chr(0x5f) + chr(0x5a) + chr(0x0b) + chr(0x19)
  
flag = str_xor(flag_enc, 'enkidu')
# Check that flag is not empty
if flag = "":
  print('String XOR encountered a problem, quitting.')
else:
  print('That is correct! Here\'s your flag: ' + flag)

After a quick read, I could find in line 22 that there is an equal sign missing.

You could also find out the line of the error by running:

After fixing the problem, the program worked fine.

And the flag is:

Show flag
picoCTF{3qu4l1ty_n0t_4551gnm3nt_4863e11b}

Code scripts:

If you want to checkout the code for the python 3 script with me solution, take a look here.


Thank you very much for reading!

Cheers,

MRegra


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