diff --git a/lib/matrix_server.ex b/lib/matrix_server.ex
index e9e3e38..5171ecc 100644
--- a/lib/matrix_server.ex
+++ b/lib/matrix_server.ex
@@ -30,17 +30,6 @@ defmodule MatrixServer do
     |> validate_required(required)
   end
 
-  def generate_error(errcode) do
-    {get_errcode_status(errcode), %{errcode: errcode, error: get_errcode_error(errcode)}}
-  end
-
-  # TODO: make a plug for this?
-  def get_errcode_error("M_BAD_JSON"), do: "Bad request."
-  def get_errcode_error("M_USER_IN_USE"), do: "Username is already taken."
-  def get_errcode_error("M_INVALID_USERNAME"), do: "Invalid username."
-
-  def get_errcode_status(_), do: 400
-
   def get_mxid(localpart) when is_binary(localpart) do
     "@#{localpart}:#{server_name()}"
   end
diff --git a/lib/matrix_server_web/controllers/account_controller.ex b/lib/matrix_server_web/controllers/account_controller.ex
index e5fe220..d50e4a2 100644
--- a/lib/matrix_server_web/controllers/account_controller.ex
+++ b/lib/matrix_server_web/controllers/account_controller.ex
@@ -1,7 +1,8 @@
 defmodule MatrixServerWeb.AccountController do
   use MatrixServerWeb, :controller
 
-  import MatrixServer, only: [get_mxid: 1]
+  import MatrixServer
+  import MatrixServerWeb.Plug.Error
 
   alias MatrixServer.Account
   alias Plug.Conn
@@ -9,21 +10,15 @@ defmodule MatrixServerWeb.AccountController do
   def available(conn, params) do
     localpart = Map.get(params, "username", "")
 
-    {status, data} =
-      case Account.available?(localpart) do
-        :ok ->
-          {200, %{available: true}}
+    case Account.available?(localpart) do
+      :ok ->
+        conn
+        |> put_status(200)
+        |> json(%{available: true})
 
-        {:error, :user_in_use} ->
-          {400, %{errcode: "M_USER_IN_USE", error: "Desired user ID is already taken."}}
-
-        {:error, :invalid_username} ->
-          {400, %{errocode: "M_INVALID_USERNAME", error: "Desired user ID is invalid."}}
-      end
-
-    conn
-    |> put_status(status)
-    |> json(data)
+      {:error, error} ->
+        put_error(conn, error)
+    end
   end
 
   def whoami(%Conn{assigns: %{account: %Account{localpart: localpart}}} = conn, _params) do
diff --git a/lib/matrix_server_web/controllers/auth_controller.ex b/lib/matrix_server_web/controllers/auth_controller.ex
index 4c486a0..bb6aa60 100644
--- a/lib/matrix_server_web/controllers/auth_controller.ex
+++ b/lib/matrix_server_web/controllers/auth_controller.ex
@@ -2,6 +2,7 @@ defmodule MatrixServerWeb.AuthController do
   use MatrixServerWeb, :controller
 
   import MatrixServer
+  import MatrixServerWeb.Plug.Error
 
   alias MatrixServer.{Repo, Account}
   alias Ecto.Changeset
@@ -38,7 +39,7 @@ defmodule MatrixServerWeb.AuthController do
           {200, data}
 
         {:error, error} ->
-          generate_error(error)
+          put_error(conn, error)
       end
 
     conn
@@ -48,11 +49,7 @@ defmodule MatrixServerWeb.AuthController do
 
   def register(conn, %{"auth" => _}) do
     # Other login types are unsupported for now.
-    data = %{errcode: "M_FORBIDDEN", error: "Login type not supported"}
-
-    conn
-    |> put_status(400)
-    |> json(data)
+    put_error(conn, :forbidden)
   end
 
   def register(conn, _params) do
@@ -80,9 +77,9 @@ defmodule MatrixServerWeb.AuthController do
   end
 
   defp get_register_error(%Changeset{errors: [error | _]}), do: get_register_error(error)
-  defp get_register_error({:localpart, {_, [{:constraint, :unique} | _]}}), do: "M_USER_IN_USE"
-  defp get_register_error({:localpart, {_, [{:validation, _} | _]}}), do: "M_INVALID_USERNAME"
-  defp get_register_error(_), do: "M_BAD_JSON"
+  defp get_register_error({:localpart, {_, [{:constraint, :unique} | _]}}), do: :user_in_use
+  defp get_register_error({:localpart, {_, [{:validation, _} | _]}}), do: :invalid_username
+  defp get_register_error(_), do: :bad_json
 
   defp register_schema do
     types = %{
diff --git a/lib/matrix_server_web/authenticate.ex b/lib/matrix_server_web/plug/authenticate.ex
similarity index 64%
rename from lib/matrix_server_web/authenticate.ex
rename to lib/matrix_server_web/plug/authenticate.ex
index d517c62..b30e7f2 100644
--- a/lib/matrix_server_web/authenticate.ex
+++ b/lib/matrix_server_web/plug/authenticate.ex
@@ -1,11 +1,12 @@
-defmodule MatrixServerWeb.Authenticate do
+defmodule MatrixServerWeb.Plug.Authenticate do
+  import MatrixServerWeb.Plug.Error
   import Plug.Conn
   import Phoenix.Controller, only: [json: 2]
 
   alias MatrixServer.Account
   alias Plug.Conn
 
-  def init(options), do: options
+  def init(opts), do: opts
 
   def call(%Conn{params: %{"access_token" => access_token}} = conn, _opts) do
     authenticate(conn, access_token)
@@ -17,12 +18,7 @@ defmodule MatrixServerWeb.Authenticate do
         authenticate(conn, access_token)
 
       _ ->
-        data = %{errcode: "M_MISSING_TOKEN", error: "Access token missing."}
-
-        conn
-        |> put_status(401)
-        |> json(data)
-        |> halt()
+        put_error(conn, :missing_token)
     end
   end
 
@@ -34,12 +30,7 @@ defmodule MatrixServerWeb.Authenticate do
         |> assign(:device, device)
 
       nil ->
-        data = %{errcode: "M_UNKNOWN_TOKEN", error: "Invalid access token."}
-
-        conn
-        |> put_status(401)
-        |> json(data)
-        |> halt()
+        put_error(conn, :unknown_token)
     end
   end
 end
diff --git a/lib/matrix_server_web/plug/error.ex b/lib/matrix_server_web/plug/error.ex
new file mode 100644
index 0000000..74ad8db
--- /dev/null
+++ b/lib/matrix_server_web/plug/error.ex
@@ -0,0 +1,23 @@
+defmodule MatrixServerWeb.Plug.Error do
+  import Plug.Conn
+  import Phoenix.Controller, only: [json: 2]
+
+  @error_code_and_message %{
+    bad_json: {400, "M_BAD_JSON", "Bad request."},
+    user_in_use: {400, "M_USE_IN_USE", "Username is already taken."},
+    invalid_username: {400, "M_INVALID_USERNAME", "Invalid username."},
+    forbidden: {400, "M_FORBIDDEN", "The requested action is forbidden."},
+    unknown_token: {401, "M_UNKNOWN_TOKEN", "Invalid access token."},
+    missing_token: {401, "M_MISSING_TOKEN", "Access token required."}
+  }
+
+  def put_error(conn, error) do
+    {status, errcode, errmsg} = @error_code_and_message[error]
+    data = %{errcode: errcode, error: errmsg}
+
+    conn
+    |> put_status(status)
+    |> json(data)
+    |> halt()
+  end
+end
diff --git a/lib/matrix_server_web/router.ex b/lib/matrix_server_web/router.ex
index 66378bd..4e2e215 100644
--- a/lib/matrix_server_web/router.ex
+++ b/lib/matrix_server_web/router.ex
@@ -1,13 +1,15 @@
 defmodule MatrixServerWeb.Router do
   use MatrixServerWeb, :router
 
+  alias MatrixServerWeb.Plug.Authenticate
+
   pipeline :public do
     plug :accepts, ["json"]
   end
 
   pipeline :authenticated do
     plug :accepts, ["json"]
-    plug MatrixServerWeb.Authenticate
+    plug Authenticate
   end
 
   scope "/_matrix", MatrixServerWeb do