Vigenère Cipher in Python

This project is a simple implementation of the Vigenère Cipher in Python. The script can encrypt and decrypt text messages using a custom key.

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Description

The Vigenère cipher is a classic method of polyalphabetic substitution. It uses a keyword to shift each letter of the plaintext by a different amount, based on the corresponding letter in the repeating key. This makes it significantly more secure than a simple Caesar cipher.

This script provides two main functions: encrypt() for encrypting and decrypt() for decrypting messages.

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Features

• Encryption & Decryption: Converts plaintext to ciphertext and vice versa.
• Custom Key: Works with any alphabetic keyword.
• Robust: Preserves spaces and non-alphabetic characters in the final output.
• Case-Insensitive: Treats all letters as lowercase during processing.

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How to Use

To use this script, you can run it directly or import its functions into your own project.

1. Running the Script Directly

1. Clone or download the repository.

2. Open the Python file (e.g., main.py).

3. Modify the variables text and custom_key to suit your needs.

   • To encrypt a message, call the encrypt function:

     # Example for encryption
     plaintext = 'vigenere in python'
     key = 'happycoding'
     
     encrypted_text = encrypt(plaintext, key)
     print(f'Encrypted Text: {encrypted_text}')
     # Expected Output -> Encrypted Text: mrttaqrhknsw ih puggrur

   • To decrypt a message, call the decrypt function:

     # Example for decryption
     ciphertext = 'mrttaqrhknsw ih puggrur'
     key = 'happycoding'
     
     decrypted_text = decrypt(ciphertext, key)
     print(f'Decrypted Text: {decrypted_text}')
     # Expected Output -> Decrypted Text: vigenere in python

4. Run the script from your terminal:

   python main.py

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Code Deep Dive

The core of the script is the vigenere() function, which handles both the encryption and decryption logic.

def vigenere(message, key, direction=1):
    key_index = 0
    alphabet = 'abcdefghijklmnopqrstuvwxyz'
    final_message = ''

    for char in message.lower():
        # Append any non-letter character to the message
        if not char.isalpha():
            final_message += char
        else:
            # Find the right key character to encode/decode
            key_char = key[key_index % len(key)]
            key_index += 1

            # Define the offset and the encrypted/decrypted letter
            offset = alphabet.index(key_char)
            index = alphabet.find(char)
            new_index = (index + offset * direction) % len(alphabet)
            final_message += alphabet[new_index]

    return final_message

def encrypt(message, key):
    # Calls vigenere() with direction=1 for encryption
    return vigenere(message, key)

def decrypt(message, key):
    # Calls vigenere() with direction=-1 for decryption
    return vigenere(message, key, -1)