server.ex

  defp gen_jwk_set do
    {:ok, pem} = genrsa(2048)

    pem
    |> JOSE.JWK.from_pem()
    |> JOSE.JWK.to_public()
    |> JOSE.JWK.to_record()
    |> List.wrap()
    |> :jose_jwk_set.to_map(%{})
    |> Poison.encode()
  end

  defp genrsa(bits \\ 2048) when is_integer(bits) and bits > 0 do
    with {data, 0}   <- System.cmd("openssl", ["genrsa", to_string(bits)], [stderr_to_stdout: true]),
         {start, _}  <- :binary.match(data, "-----BEGIN RSA PRIVATE KEY-----") do
      {:ok, :binary.part(data, start, String.length(data) - start)}
    else error ->
      {:error, error}
    end
  end