Py for CTFs

The Python for CTFs Handbook: Scripting Your Way to Victory

Python is an indispensable tool for CTF participants. Its versatility, extensive libraries, and ease of use make it perfect for automating tasks, crafting exploits, and solving a wide range of challenges. This handbook dives into Python's capabilities for CTFs, equipping you to script your way to victory.

I. Core Concepts:

• Variables and Data Types: Integers, floats, strings, lists, dictionaries – the building blocks of Python.

• Control Flow: if, elif, else, for, while – controlling the execution of your code.

• Functions: Reusable blocks of code.

• Modules and Libraries: Pre-built code for specific tasks (e.g., networking, cryptography).

• Object-Oriented Programming (OOP): Classes and objects for more complex programs (optional, but useful).

II. Essential Libraries:

1. requests: For making HTTP requests (web challenges).

   Python

   import requests
   
   response = requests.get("http://example.com")
   print(response.text)
   
   data = {"key": "value"}
   response = requests.post("http://example.com/api", json=data)

2. socket: For low-level network communication.

   Python

   import socket
   
   s = socket.socket(socket.AF_INET, socket.SOCK_STREAM)
   s.connect(("127.0.0.1", 8080))
   s.sendall(b"Hello")
   data = s.recv(1024)
   s.close()

3. struct: For packing and unpacking binary data (exploits, forensics).

   Python

   import struct
   
   packed_data = struct.pack("<i", 12345)  # Pack integer as little-endian
   unpacked_data = struct.unpack("<i", packed_data)[0]

4. hashlib: For hashing algorithms (cryptography).

   Python

   import hashlib
   
   md5_hash = hashlib.md5(b"password").hexdigest()
   sha256_hash = hashlib.sha256(b"password").hexdigest()

5. base64: For base64 encoding/decoding (cryptography, web).

   Python

   import base64
   
   encoded = base64.b64encode(b"data").decode()
   decoded = base64.b64decode(encoded).decode()

6. re: For regular expressions (parsing, web).

   Python

   import re
   
   pattern = r"(\d+)-(\d+)-(\d+)"
   string = "2023-10-27"
   match = re.search(pattern, string)
   if match:
       year, month, day = match.groups()

7. binascii: For converting between binary and ASCII representations.

   Python

   import binascii
   
   hex_data = binascii.hexlify(b"data").decode()
   binary_data = binascii.unhexlify(hex_data)

8. pwn (pwntools): A powerful CTF framework (exploits). Install with: pip install pwntools

   Python

   from pwn import *
   
   r = remote("127.0.0.1", 8080)  # Connect to a remote service
   r.sendline(b"payload")
   shellcode = asm(shellcraft.sh()) # Assemble shellcode
   r.send(shellcode)
   r.interactive()

III. Common CTF Tasks and Examples:

1. Web Requests:

   Python

   import requests
   
   url = "http://example.com/api"
   data = {"param1": "value1", "param2": "value2"}
   
   response = requests.post(url, data=data)  # POST request
   response = requests.get(url, params=data)  # GET request with parameters
   
   if response.status_code == 200:
       print(response.text)
       json_data = response.json()  # If the response is JSON

2. HTML Parsing (Beautiful Soup): Install with: pip install beautifulsoup4

   Python

   from bs4 import BeautifulSoup
   import requests
   
   response = requests.get("http://example.com/page")
   soup = BeautifulSoup(response.content, "html.parser")
   
   title = soup.title.string
   links = [a.get("href") for a in soup.find_all("a")]

3. Regular Expressions:

   Python

   import re
   
   log_file = "access.log"
   with open(log_file, "r") as f:
       for line in f:
           match = re.search(r"IP: (\d+\.\d+\.\d+\.\d+)", line)
           if match:
               ip_address = match.group(1)
               print(ip_address)

4. Encoding/Decoding:

   Python

   import base64
   
   encoded = base64.b64encode(b"flag").decode()
   decoded = base64.b64decode(encoded).decode()
   
   # URL encoding/decoding
   import urllib.parse
   encoded = urllib.parse.quote("value with spaces")
   decoded = urllib.parse.unquote(encoded)

5. Cryptography:

   Python

   import hashlib
   
   password = "password123"
   md5_hash = hashlib.md5(password.encode()).hexdigest()
   
   import base64
   encoded = base64.b64encode(b"secret").decode()

6. File I/O:

   Python

   with open("flag.txt", "r") as f:
       flag = f.read().strip()
   
   with open("output.txt", "w") as f:
       f.write("Result: " + flag)

7. Automation:

   Python

   import subprocess
   
   # Run a command and capture the output
   result = subprocess.run(["ls", "-l"], capture_output=True, text=True)
   print(result.stdout)
   
   # Interact with a program
   process = subprocess.Popen(["./vulnerable_program"], stdin=subprocess.PIPE, stdout=subprocess.PIPE)
   output, _ = process.communicate(b"input\n")
   print(output.decode())

8. Binary Exploitation (pwntools):

   Python

   from pwn import *
   
   # Connect to the target
   r = remote('target_ip', 1337)
   
   # Send a payload
   payload = b"A" * 100 + p64(0x401000) # Example: Overwrite return address
   r.sendline(payload)
   
   # Interactive shell
   r.interactive()

IV. Tips for CTFs:

• Practice: The more you code, the better you'll become. Solve old CTF challenges to build your skills.

• Read Writeups: Learn from other people's solutions.

• Use Libraries: Don't reinvent the wheel. Leverage the power of Python's libraries.

• Debug: Use print() statements or a debugger to understand what your code is doing.

• Be Resourceful: Search for solutions online. The CTF community is a great resource.

• Combine Tools: Python can be used to automate interactions with other tools (e.g., Nmap, SQLMap).