vigenere_cipher.py

"""
Vigenère cipher uses a keyword to encrypt a message by replacing each letter of the message with a 
letter that is shifted a certain number of places down the alphabet based on the corresponding letter 
of the keyword. The same keyword is used for each letter of the message but shifted to a different 
position for each letter of the message. This makes the Vigenère cipher much more secure than simple 
substitution ciphers, such as the Caesar cipher.

Input:
 Message: Takes input the message you want to encrypt or decrypt.
 key: Takes an alphabetic key as input
 option: asks if you want to encrypt or decrypt the message

Output:
 Cipher text: if you choose to encrypt then the output is a cipher text.
 plain text: if you gave the cipher text as input and the correct key then it gives back the correct plaintext/message
 wrong message: if you mistake the key or plain text then it gives some gibberish.

Algorithm:

1 The plaintext message and the keyword are both converted to uppercase.

2 The alphabet is defined, usually as the 26 letters, all uppercase in English alphabet.

3 For each letter in the plaintext message, the corresponding letter in the keyword is found.   
 
4 The index of the plaintext letter in the alphabet is added to the index of the keyword letter 
 in the alphabet (modulo 26) to get the index of the encrypted letter in the alphabet.
 
5 The encrypted letter is the letter at the calculated index in the alphabet.
 
6 The above process is repeated for each of the letters in the plaintext.
"""

"""Function to encrypt and decrypt"""
def vigenere_cipher(plaintext, key, encrypt=True):
    key = key.upper()
    alphabet = 'ABCDEFGHIJKLMNOPQRSTUVWXYZ'
    result = ''

    for i, char in enumerate(plaintext.upper()):
        if char in alphabet:
            char_index = alphabet.index(char)
            key_index = alphabet.index(key[i % len(key)])
            if encrypt:
                result += alphabet[(char_index + key_index) % 26]
            else:
                result += alphabet[(char_index - key_index + 26) % 26]
        else:
            result += char

    return result

"""Taking input from user"""
plaintext = input("Enter the message to be encrypted/decrypted: ")
key = input("Enter the key: ")
mode = input("Enter 'e' to encrypt, 'd' to decrypt: ")

"""Encryption or decryption phase"""
if mode == 'e':
    ciphertext = vigenere_cipher(plaintext, key)
    print(f"Encrypted message: {ciphertext}")
elif mode == 'd':
    ciphertext = vigenere_cipher(plaintext, key, encrypt=False)
    print(f"Decrypted message: {ciphertext}")
else:
    print("Invalid mode. Enter 'e' to encrypt, 'd' to decrypt.")