def add_pkcs1_padding(message):
    """Add PKCS#1 padding to **message**."""
    padding = b''
    typeinfo = b'\x00\x01'
    separator = b'\x00'
    padding = b'\xff' * (125 - len(message))
    padded_message = typeinfo + padding + separator + message
    assert len(padded_message) == 128
    return padded_message
