def encrypt(key, plaintext):
    out = ""
    for i in range(0, len(plaintext)):
        out += chr(((ord(key[i % len(key)]) +
                     ord(plaintext[i]) - (2 * 0x61)) % 26)+0x61)
    return out


def decrypt(key, cipertext):
    out = ""
    for i in range(0, len(cipertext)):
        pln = ord(cipertext[i])-(ord(key[i % len(key)]) - 0x61)
        pln = pln + 26 if pln < 0x61 else pln
        out += chr(pln)
    return out


if __name__ == "__main__":
    import argparse
    parser = argparse.ArgumentParser()
    parser.add_argument('FILE')
    command_group = parser.add_mutually_exclusive_group(required=True)
    command_group.add_argument('--encrypt', metavar='KEY')
    command_group.add_argument('--decrypt', metavar='KEY')
    parser.add_argument('--out', metavar='FILE')
    args = parser.parse_args()

    # strip non-alphabetic chars from file and convert to lower case
    t = ''.join([x for x in open(args.FILE, "r").read().lower() if x.isalpha()])

    if(args.encrypt != None):
        out = encrypt(args.encrypt, t)

    if(args.decrypt != None):
        out = decrypt(args.decrypt, t)

    if(args.out != None):
        open(args.out, "w").write(out)
    else:
        print(out)