def machine_id(machine_id):
    """
    Validates the machine_id
    Must be a 64 byte lowercase hex string.
    A sha256sum, in effect.
    """
    if not isinstance(machine_id, str):
        raise TypeError('machine_id must be a string.')
    if len(machine_id) != 64:
        raise ValueError('machine_id must be exactly 64 bytes/characters.')
    for letter in machine_id:
        if letter not in '0123456789abcdef':
            raise ValueError('machine_id must be only 0-9, a-f (lowercase)')
    return True
