Add endpoint to get server publishing keys

lib/matrix_server_web/controllers/key_controller.ex

@@ -0,0 +1,34 @@
+defmodule MatrixServerWeb.KeyController do
+  use MatrixServerWeb, :controller
+
+  import MatrixServerWeb.Plug.Error
+
+  alias MatrixServer.SigningServer
+
+  @key_valid_time_ms 1000 * 60 * 24 * 30
+
+  def get_signing_keys(conn, _params) do
+    keys =
+      SigningServer.get_signing_keys()
+      |> Enum.into(%{}, fn {key_id, key} ->
+        {key_id, %{"key" => key}}
+      end)
+
+    data = %{
+      server_name: MatrixServer.server_name(),
+      verify_keys: keys,
+      old_verify_keys: %{},
+      valid_until_ts: System.os_time(:millisecond) + @key_valid_time_ms
+    }
+
+    case SigningServer.sign_object(data) do
+      {:ok, signed_data} ->
+        conn
+        |> put_status(200)
+        |> json(signed_data)
+
+      {:error, _msg} ->
+        put_error(conn, :unknown, "Error signing object.")
+    end
+  end
+end