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.

Choose a Programming Language

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

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

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

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

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.")

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.

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.