Then you don't have to do any work yourself.

    def SecureRandom.gen_random(n)
      ret = Random.raw_seed(n)
      unless ret
        raise NotImplementedError, "No random device"
      end
      unless ret.length == n
        raise NotImplementedError, "Unexpected partial read from random device: only #{ret.length} for #{n} bytes"
      end
      ret
    end