def METHOD_NAME(x, length):
    """Zero pad byte string as defined in NIST SP 800-185"""
    to_pad = concat_buffers(_left_encode(length), x)
    # Note: this implementation works with byte aligned strings,
    # hence no additional bit padding is needed at this point.
    npad = (length - len(to_pad) % length) % length
    return to_pad + b'\x00' * npad