import argparse


def caesar(ptextletter, keyletter):
    ptextNum = ord(ptextletter) - ord('A')
    keyletterNum = ord(keyletter) - ord('A')
    return chr((ptextNum + keyletterNum) % 26 + ord('A'))


def caesar_reverse(ptextletter, keyletter):
    ptextNum = ord(ptextletter) - ord('A')
    keyletterNum = ord(keyletter) - ord('A')
    return chr((ptextNum - keyletterNum) % 26 + ord('A'))

def encryption(plaintext, key):
    ciphertext = ''
    for i in range(len(plaintext)):
        ciphertext += caesar(plaintext[i], key[i % len(key)])
    return ciphertext


def decryption(ciphertext, key):
    plaintext = ''
    for i in range(len(ciphertext)):
        plaintext += caesar_reverse(ciphertext[i], key[i % len(key)])
    return plaintext


def main():
    # parse command line argument
    parser = argparse.ArgumentParser()
    parser.add_argument('--text', type=str)
    parser.add_argument('--key', type=str)
    args = parser.parse_args()

    key = args.key
    user_text = args.text

    print("Encrypted ciphertext: " + encryption(user_text, key))
    print("Decrypted plaintext: " + decryption(user_text, key))


if __name__ == '__main__':
    main()