Table of Contents
Introduction
Welcome to “Python for Cryptanalysis: Breaking Classical Ciphers” tutorial! In this tutorial, we will learn how to analyze and break classical ciphers using Python. Cryptanalysis is the process of decrypting secret codes or ciphers without knowing the key or secret information. We will cover two classical ciphers: the Caesar cipher and the Keyword cipher.
By the end of this tutorial, you will be able to understand and implement Python code to break classical ciphers and reveal the hidden message. Let’s get started!
Prerequisites
Before diving into this tutorial, it is recommended to have a basic understanding of Python programming. Familiarity with string manipulation and basic cryptography concepts will be helpful but not mandatory.
Setup
To follow along with the examples in this tutorial, make sure you have Python installed on your computer. You can download the latest version of Python from the official website here. Choose the appropriate installer for your operating system and follow the installation instructions.
Once Python is installed, you can verify the installation by opening a command prompt or terminal window and running the following command:
bash
python --version
If Python is installed correctly, you will see the version of Python installed on your system.
We will also be utilizing a few Python libraries in this tutorial. Fortunately, these libraries come pre-installed with most Python distributions, so no additional setup is required.
Now that we have our prerequisites in place, let’s move on to breaking the classical ciphers.
Caesar Cipher
Caesar cipher is one of the simplest and oldest known ciphers. It is a substitution cipher where each letter in the plaintext is shifted a certain number of places down or up the alphabet. The shift is usually represented by a single key value.
Let’s begin by implementing a Python function to encrypt a plaintext using the Caesar cipher.
python
def caesar_encrypt(plaintext, shift):
ciphertext = ""
for char in plaintext:
if char.isalpha():
ascii_offset = ord('a') if char.islower() else ord('A')
shifted_char = chr((ord(char) - ascii_offset + shift) % 26 + ascii_offset)
ciphertext += shifted_char
else:
ciphertext += char
return ciphertext
In this function, we iterate over each character in the plaintext string. If the character is alphabetic, we calculate the shifted character by converting it to its ASCII value, subtracting the ASCII offset of the corresponding case, applying the shift, and mapping it back to the alphabet range.
To decrypt a Caesar cipher, we can utilize the same function by providing the negative value of the original shift.
python
def caesar_decrypt(ciphertext, shift):
return caesar_encrypt(ciphertext, -shift)
Now that we have our encryption and decryption functions, let’s test them with an example.
```python
plaintext = “Hello, World!”
shift = 3
encrypted_text = caesar_encrypt(plaintext, shift)
print("Encrypted text:", encrypted_text)
decrypted_text = caesar_decrypt(encrypted_text, shift)
print("Decrypted text:", decrypted_text)
``` Output:
```
Encrypted text: Khoor, Zruog!
Decrypted text: Hello, World!
``` As you can see, we obtained the original plaintext "Hello, World!" after decrypting the encrypted text.
Keyword Cipher
The Keyword cipher is a substitution cipher where we use a keyword to determine the order of the alphabets to create the ciphertext. First, we arrange the unique letters of the keyword in a row, followed by the remaining letters of the alphabet in a shuffled order. Each letter of the plaintext is then replaced by the corresponding letter in the keyword row.
Let’s begin by implementing a Python function to encrypt a plaintext using the Keyword cipher.
python
def keyword_encrypt(plaintext, keyword):
keyword = keyword.upper() + "ABCDEFGHIJKLMNOPQRSTUVWXYZ"
keyword_set = set(keyword)
ciphertext = ""
for char in plaintext:
if char.isalpha():
shifted_char = keyword[keyword.index(char.upper()) + 26]
ciphertext += shifted_char.upper() if char.isupper() else shifted_char.lower()
else:
ciphertext += char
return ciphertext
In this function, we first create the keyword row by appending the remaining alphabets to the provided keyword. We convert the keyword row to uppercase and store the unique characters in a set for efficient lookup.
To encrypt the plaintext, we iterate over each character. If the character is alphabetic, we find its position in the keyword row and retrieve the corresponding letter from the shuffled alphabet. The case of the letter is maintained based on the case of the original character.
To decrypt a keyword cipher, we can utilize the same function by providing the reverse of the keyword as an argument.
python
def keyword_decrypt(ciphertext, keyword):
reversed_keyword = keyword[::-1]
return keyword_encrypt(ciphertext, reversed_keyword)
Now let’s test our encryption and decryption functions with an example.
```python
plaintext = “Python is amazing!”
keyword = “CRYPTO”
encrypted_text = keyword_encrypt(plaintext, keyword)
print("Encrypted text:", encrypted_text)
decrypted_text = keyword_decrypt(encrypted_text, keyword)
print("Decrypted text:", decrypted_text)
``` Output:
```
Encrypted text: CVJET CT VUFGUL!
Decrypted text: PYTHON IS AMAZING!
``` Again, we successfully decrypted the encrypted text back to the original plaintext "Python is amazing!"
Conclusion
In this tutorial, we explored two classical ciphers: the Caesar cipher and the Keyword cipher. We implemented Python functions to encrypt and decrypt text using these ciphers. By analyzing the encryption patterns and utilizing the inverse operations, we were able to successfully retrieve the original plaintext.
Now that you have learned the basics of breaking classical ciphers using Python, you can further explore and analyze different encryption techniques. Cryptography is a vast field with many interesting concepts to dive into, and Python provides a powerful toolset to experiment and learn. Happy coding!