Symmetric Key Cracking: Finding the Correct Key

TL;DR

This guide shows you how to use a brute-force algorithm to crack a symmetric key. It explains how to write a program that tries every possible key until it finds the correct one, and importantly, how to detect when the correct key has been found.

How to Crack a Symmetric Key with Brute Force

1. Understand the Basics
• A symmetric key is used for both encryption and decryption. If you have the encrypted data, finding the key unlocks it.
• Brute force means trying every possible key combination. This can take a very long time depending on the key length.
• The longer the key, the more combinations there are, and the harder it is to crack.
2. Choose a Programming Language

Python is often used for this because of its simplicity and libraries. However, any language can be used.

3. Write the Brute Force Code

Here’s an example in Python:

import itertools

def crack_key(encrypted_message, key_length):
    characters = 'abcdefghijklmnopqrstuvwxyz0123456789'
    for combination in itertools.product(characters, repeat=key_length):
        key = ''.join(combination)
        decrypted_message = decrypt(encrypted_message, key) # Replace with your decryption function
        if is_valid_plaintext(decrypted_message): # Replace with your validation function
            return key
    return None

4. Implement the Decryption Function

You’ll need a function that takes the encrypted message and a potential key, and attempts to decrypt it. The specific implementation depends on the encryption algorithm used.

Example (replace with your actual decryption logic):

def decrypt(encrypted_message, key):
  # Your decryption code here
  return decrypted_message

5. Implement a Plaintext Validation Function

This is the most important part. You need a way to determine if the decrypted message is likely to be valid plaintext. This could involve:

• Checking for common words or phrases.
• Using a dictionary of known words.
• Checking if the decrypted text contains printable characters.
• If you know something about the expected content, check for that (e.g., dates, names).

Example:

def is_valid_plaintext(text):
  # Check if text contains only printable characters and has a reasonable length.
  if all(char.isprintable() for char in text) and 50 < len(text) < 200:
    return True
  else:
    return False

6. Run the Code

Call the crack_key function with your encrypted message and key length.

encrypted_message = "your_encrypted_message"
key_length = 4
correct_key = crack_key(encrypted_message, key_length)
if correct_key:
  print("Key found!")
else:
  print("Key not found.")

7. Detecting the Correct Key

The is_valid_plaintext function is crucial. The brute-force algorithm stops when this function returns True, indicating a likely correct key.

8. Important Considerations
• Key Length: Brute forcing longer keys (e.g., 12+ characters) is often impractical without significant computing resources.
• Algorithm Complexity: The time it takes to crack a key increases exponentially with the key length.
• Hashing: If the data has been hashed instead of encrypted, brute-forcing won’t work directly; you need to try and find a collision (a different input that produces the same hash).
• cyber security: Brute force attacks are often detectable by intrusion detection systems.