vigenere.py

alphabet = 'abcdefghijklmnopqrstuvwxyz'

letter_to_index =dict(zip(alphabet, range(len(alphabet))))
index_to_letter= dict(zip(range(len(alphabet)), alphabet))

def encrypt(message, key):
    encrypted = ''
#split the message to the length of key
    split_message = [message[i:i + len(key)] for i in range(0, len(message), len(key))]# (start, end , stop)
#convert the message into index and add the key(mod 26)

    for each_split in split_message:
      i = 0
      for letter in each_split:
        number = (letter_to_index[letter] + letter_to_index[key[i]]) % len(alphabet)
        encrypted += index_to_letter[number]
        i += 1
    return encrypted

def decrypt(cipher, key):
    decrypted= ''
    #split the cipher to the length of key
    split_cipher = [cipher[i:i + len(key)] for i in range(0, len(cipher), len(key))] # (start, end , step)

    #convert cipher to index and subtract thr key (mod 26)
    for each_split in split_cipher:
        i = 0
        for letter in each_split:
            number = (letter_to_index[letter] - letter_to_index[key[i]]) % len(alphabet)
            decrypted += index_to_letter[number]
            i += 1

    return decrypted


def main():
    key = 'secret'
    message = 'havingapetcanmakeyoumorehappy'
    encrypted_message = encrypt(message, key)
    decrypted_message = decrypt(encrypted_message, key)

    print('orignal message: ' + message)
    print('Encrypted message: ' + encrypted_message)
    print('Decrypted message: ' + decrypted_message)


main()