Autokey_cipher.py

# Python program to implement Autokey Cipher


# Dictionary to lookup the index of alphabets
dict1 = {'A': 0, 'B': 1, 'C': 2, 'D': 3, 'E': 4,
         'F': 5, 'G': 6, 'H': 7, 'I': 8, 'J': 9,
         'K': 10, 'L': 11, 'M': 12, 'N': 13, 'O': 14,
         'P': 15, 'Q': 16, 'R': 17, 'S': 18, 'T': 19,
         'U': 20, 'V': 21, 'W': 22, 'X': 23, 'Y': 24, 'Z': 25}

dict2 = {0: 'A', 1: 'B', 2: 'C', 3: 'D', 4: 'E',
         5: 'F', 6: 'G', 7: 'H', 8: 'I', 9: 'J',
         10: 'K', 11: 'L', 12: 'M', 13: 'N', 14: 'O',
         15: 'P', 16: 'Q', 17: 'R', 18: 'S', 19: 'T',
         20: 'U', 21: 'V', 22: 'W', 23: 'X', 24: 'Y', 25: 'Z'}


# This function generates the key
def generate_key(message, key):
    i = 0
    while True:
        if len(key) == len(message):
            break
        if message[i] == ' ':
            i += 1
        else:
            key += message[i]
            i += 1
    return key


# This function returns the encrypted text
# generated with the help of the key
def cipherText(message, key_new):
    cipher_text = ''
    i = 0
    for letter in message:
        if letter == ' ':
            cipher_text += ' '
        else:
            x = (dict1[letter]+dict1[key_new[i]]) % 26
            i += 1
            cipher_text += dict2[x]
    return cipher_text


# This function decrypts the encrypted text
# and returns the original text
def originalText(cipher_text, key_new):
    or_txt = ''
    i = 0
    for letter in cipher_text:
        if letter == ' ':
            or_txt += ' '
        else:
            x = (dict1[letter]-dict1[key_new[i]]+26) % 26
            i += 1
            or_txt += dict2[x]
    return or_txt


def main():
    message = 'THE GERMAN ATTACK'
    key = 'SECRET'
    key_new = generate_key(message, key)
    cipher_text = cipherText(message, key_new)
    original_text = originalText(cipher_text, key_new)
    print("Encrypted Text =", cipher_text)
    print("Original Text =", original_text)


# Executes the main function
if __name__ == '__main__':
    main()