def get_mac_a(output: bytes) -> bytes:
    """Support function to get the 64-bit network authentication code (MAC-A)
    from OUT1, the output of 3GPP f1 function.

    :param output: OUT1

    :returns: OUT1[0] .. OUT1[63]
    """
    edge = 8 # = ceil(63/8)
    return output[:edge]