def _make_key(category, key):
    """Generate a binary key for the given category and key.

    Args:
        category (str): The category of the item
        key (str): The unique identifier for the item

    Returns:
        The key to use for storing a the value.
    """
    return (b"TuneRegistry:" + category.encode("ascii") + b"/" +
            key.encode("ascii"))