def bytes_in_block(block_size: int, i: int) -> slice:
    """
    Given the block size and the desired block index,
    return the slice of interesting bytes.

    :param block_size: The block size.
    :param i: The block index.
    :return: slice of bytes pointing to given block index.
    """
    return slice(block_size * i, block_size * (i + 1))