import argparse


def caesar(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):
    return ciphertext


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

    key = args.key
    ciphertext = encryption(args.ptext, key)

    print("Ciphertext: " + ciphertext)
    print("Decrypted plaintext: " + decryption(ciphertext, key))


if __name__ == '__main__':
    main()