def add_round_key(block, key):
    """Performs a bitwise XOR between a state block and the key.

    Parameters
    ----------
    block : np.ndarray
        4x4 column major block matrix.
    key : np.ndarray
        4x4  column major block matrix.

    Returns
    -------
    np.ndarray
        4x4 result block matrix

    """
    return block ^ key.T