Calculate the ICAO address from an Mode-S message
    with DF4, DF5, DF20, DF21

    Args:
        msg (String): 28 bytes hexadecimal message string

    Returns:
        String: ICAO address in 6 bytes hexadecimal string

def icao(msg):
    """Calculate the ICAO address from an Mode-S message
    with DF4, DF5, DF20, DF21

    Args:
        msg (String): 28 bytes hexadecimal message string

    Returns:
        String: ICAO address in 6 bytes hexadecimal string
    """

    DF = df(msg)

    if DF in (11, 17, 18):
        addr = msg[2:8]
    elif DF in (0, 4, 5, 16, 20, 21):
        c0 = bin2int(crc(msg, encode=True))
        c1 = hex2int(msg[-6:])
        addr = '%06X' % (c0 ^ c1)
    else:
        addr = None

    return addr