01-building-a-cipher.py

"""
This script demonstrates how to build a Vigenère cipher in Python.
The Vigenère cipher is a method of encrypting alphabetic text by using a simple form of polyalphabetic substitution.
"""

text = 'mrttaqrhknsw ih puggrur'
custom_key = 'happycoding'

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):
    return vigenere(message, key)
    
def decrypt(message, key):
    return vigenere(message, key, -1)

decryption = decrypt(text, custom_key)


print(f"""
Encryped text: {text}
Key: {custom_key}
Decrypted text: {decryption}
""")