Bitwise Cipher: Decrypting Messages

TL;DR

This guide shows you how to decrypt messages encoded using a simple bitwise cipher. We’ll cover understanding the cipher, identifying the key, and writing a basic Python script to decode your message.

Understanding the Bitwise Cipher

A bitwise cipher works by applying logical operations (AND, OR, XOR) between each character of the plaintext message and a secret key. The most common operation is XOR because it’s reversible – using the same key twice gets you back to the original message.

Step 1: Identifying the Key

Without knowing the key, decrypting is very difficult. Here are some ways to find or guess it:

• Known-Plaintext Attack: If you know a small part of the original message (e.g., “Hello”), XOR that with the corresponding ciphertext to reveal the key segment.
• Frequency Analysis: While less effective on short messages, look for repeating patterns in the ciphertext. These might correspond to common characters or phrases encrypted with the same key segment.
• Brute-Force (for short keys): Try all possible key values within a reasonable range.

Let’s assume we have a ciphertext and suspect the key is a single character.

Step 2: Writing a Python Decryption Script

We’ll use Python because it’s easy to read and has built-in functions for bitwise operations. This script assumes the cipher uses XOR and that both the message and key are strings of ASCII characters.

1. Import necessary libraries: We don’t need any external libraries for this simple example.
2. Define a decryption function: This function will take the ciphertext and key as input and return the decrypted message.

Step 3: The Decryption Function

def decrypt_xor(ciphertext, key):
    plaintext = ''
    key_length = len(key)
    for i in range(len(ciphertext)):
        char_code = ord(ciphertext[i]) ^ ord(key[i % key_length])
        plaintext += chr(char_code)
    return plaintext

Explanation:

• ord() converts a character to its ASCII code.
• ^ performs the XOR operation on the ASCII codes.
• chr() converts an ASCII code back to a character.
• i % key_length ensures that the key repeats if it’s shorter than the ciphertext.

Step 4: Example Usage

Let’s say our ciphertext is “Hx0bx1ax05x1c” and we suspect the key is “A”.

ciphertext = "Hx0bx1ax05x1c"
key = "A"
decrypted_message = decrypt_xor(ciphertext, key)
print(f"Decrypted message: {decrypted_message}")

This should output the decrypted message. If it doesn’t make sense, try a different key.

Step 5: Handling Different Key Lengths

The script above works well for repeating keys. For longer, non-repeating keys, ensure the key is at least as long as the ciphertext to avoid errors. If you have a key file:

with open("keyfile.txt", "r") as f:
    key = f.read().strip()

Step 6: Dealing with Non-ASCII Characters

If your message contains characters outside the ASCII range, you might need to use a different encoding (e.g., UTF-8). Modify the script to encode and decode using the appropriate encoding:

def decrypt_xor_utf8(ciphertext, key):
    plaintext = ''
    key_length = len(key)
    for i in range(len(ciphertext)):
        char_code = ord(ciphertext[i].encode('utf-8')) ^ ord(key[i % key_length].encode('utf-8'))
        plaintext += chr(char_code).decode('utf-8')
    return plaintext

Remember to adjust the encoding in both ord() and chr() calls.