Vigenère Cipher – in Cryptography

What You’ll Learn 📚

• Understanding the Vigenère Cipher and its history
• Key terminology and definitions
• Step-by-step examples from simple to complex
• Common questions and troubleshooting tips
• Hands-on exercises to practice your skills

Introduction to Vigenère Cipher

The Vigenère Cipher is a method of encrypting alphabetic text by using a simple form of polyalphabetic substitution. It was invented by Blaise de Vigenère in the 16th century. Unlike the Caesar cipher, which shifts letters by a fixed number, the Vigenère Cipher uses a keyword to determine the shift for each letter, making it more secure.

Key Terminology

• Encryption: The process of converting plain text into coded text.
• Decryption: The process of converting coded text back into plain text.
• Plaintext: The original message that needs to be encrypted.
• Ciphertext: The encrypted message.
• Keyword: A word used to determine the shift for each letter in the plaintext.

Simple Example

Let’s start with the simplest example. Imagine you want to encrypt the word ‘HELLO’ using the keyword ‘KEY’.

def vigenere_encrypt(plaintext, keyword):
    alphabet = 'ABCDEFGHIJKLMNOPQRSTUVWXYZ'
    keyword_repeated = ''
    keyword_length = len(keyword)
    for i in range(len(plaintext)):
        keyword_repeated += keyword[i % keyword_length]
    ciphertext = ''
    for p, k in zip(plaintext, keyword_repeated):
        p_index = alphabet.index(p)
        k_index = alphabet.index(k)
        c_index = (p_index + k_index) % len(alphabet)
        ciphertext += alphabet[c_index]
    return ciphertext

plaintext = 'HELLO'
keyword = 'KEY'
ciphertext = vigenere_encrypt(plaintext, keyword)
print('Ciphertext:', ciphertext)

Progressively Complex Examples

Example 1: Longer Text

Let’s encrypt a longer message: ‘ATTACKATDAWN’ with the keyword ‘LEMON’.

plaintext = 'ATTACKATDAWN'
keyword = 'LEMON'
ciphertext = vigenere_encrypt(plaintext, keyword)
print('Ciphertext:', ciphertext)

Example 2: Decryption

Now, let’s decrypt the ciphertext ‘LXFOPVEFRNHR’ back to plaintext using the same keyword ‘LEMON’.

def vigenere_decrypt(ciphertext, keyword):
    alphabet = 'ABCDEFGHIJKLMNOPQRSTUVWXYZ'
    keyword_repeated = ''
    keyword_length = len(keyword)
    for i in range(len(ciphertext)):
        keyword_repeated += keyword[i % keyword_length]
    plaintext = ''
    for c, k in zip(ciphertext, keyword_repeated):
        c_index = alphabet.index(c)
        k_index = alphabet.index(k)
        p_index = (c_index - k_index) % len(alphabet)
        plaintext += alphabet[p_index]
    return plaintext

ciphertext = 'LXFOPVEFRNHR'
keyword = 'LEMON'
plaintext = vigenere_decrypt(ciphertext, keyword)
print('Plaintext:', plaintext)

Example 3: Handling Lowercase and Spaces

What if our message has lowercase letters or spaces? Let’s modify our function to handle these cases.

def vigenere_encrypt_handle_case(plaintext, keyword):
    alphabet = 'ABCDEFGHIJKLMNOPQRSTUVWXYZ'
    plaintext = plaintext.upper().replace(' ', '')
    keyword = keyword.upper()
    keyword_repeated = ''
    keyword_length = len(keyword)
    for i in range(len(plaintext)):
        keyword_repeated += keyword[i % keyword_length]
    ciphertext = ''
    for p, k in zip(plaintext, keyword_repeated):
        p_index = alphabet.index(p)
        k_index = alphabet.index(k)
        c_index = (p_index + k_index) % len(alphabet)
        ciphertext += alphabet[c_index]
    return ciphertext

plaintext = 'Hello World'
keyword = 'Key'
ciphertext = vigenere_encrypt_handle_case(plaintext, keyword)
print('Ciphertext:', ciphertext)

Common Questions and Answers

1. What is the Vigenère Cipher used for?

   It’s used to encrypt text by using a keyword to determine the shift for each letter, making it more secure than simple ciphers like Caesar.

2. Why is the Vigenère Cipher more secure than the Caesar Cipher?

   Because it uses multiple shifts based on a keyword, making it harder to crack with frequency analysis.

3. Can the Vigenère Cipher be cracked?

   Yes, with enough ciphertext and no keyword, it can be cracked using techniques like Kasiski examination.

4. How do I choose a good keyword?

   Choose a keyword that is as long as possible and not easily guessable.

5. What happens if the keyword is shorter than the plaintext?

   The keyword is repeated to match the length of the plaintext.

Troubleshooting Common Issues

Make sure your plaintext and keyword are in uppercase and contain no spaces or special characters unless your function handles them.

If your output doesn’t match the expected ciphertext, double-check your index calculations and ensure you’re using modulo arithmetic correctly.

Try It Yourself! 💪

Now it’s your turn! Try encrypting and decrypting your own messages using the Vigenère Cipher. Experiment with different keywords and plaintexts to see how the encryption changes. Remember, practice makes perfect! 😊

Additional Resources

• Wikipedia: Vigenère Cipher
• Cryptii: Vigenère Cipher Tool