In this post, you'll learn how to implement JWT-based authentication using Phoenix.Token.

In our previous projects, we use guardian library to implement JWT authentication. Guardian is a great library which provides lots of method and tool to work with authentication. But sometime we don't need them all. And recently, I found Phoenix.Token module shipped with phoenix framework that helps me to implement JWT alternative token authentication with few lines of code.

Let's do it.

1. Implement Token module

Here is the document of Phoenix.Token.

We just wrap sign and verify function from Phoenix.Token to create and check for valid token.

defmodule MyApp.Token do
  @signing_salt "octosell_api"
  # token for 2 week
  @token_age_secs 14 * 86_400

  @doc """
  Create token for given data
  """
  @spec sign(map()) :: binary()
  def sign(data) do
    Phoenix.Token.sign(MyAppWeb.Endpoint, @signing_salt, data)
  end


  @doc """
  Verify given token by:
  - Verify token signature
  - Verify expiration time
  """
  @spec verify(String.t()) :: {:ok, any()} | {:error, :unauthenticated}
  def verify(token) do
    case Phoenix.Token.verify(MyAppWeb.Endpoint, @signing_salt, token,
             max_age: @token_age_secs
           ) do
      {:ok, data} -> {:ok, data}
      _error -> {:error, :unauthenticated}
    end
  end
end

Here we wrap in Token module to simplify API. We pass MyAppWeb.Endpoint here to use secret key that config for endpoint. You can pass secret key from config as firt argument.

2. Generate token

defmodule MyAppWeb.SessionController do
  ...
  def new(conn, %{"email" => email, "password" => password}) do
    with {:ok, user} <- Account.authenticate_user(email, password),
         {:ok, token} <- MyApp.Token.sign(%{user_id: user.id}) do
      ...
      # return token to client
    else
      _ ->
        {:error, gettext("email or password is in correct")}
    end
  end
end

Here we create Phoenix token with a map %{user_id: user.id} and return to client.

3. Build Plug to verify token

Client sent token to server via header Authorization. We extract token and call Token.verify to check if token is valid and not expired.

defmodule MyApp.Plug.Authenticate do
  import Plug.Conn
  require Logger

  def init(opts) do
    opts
  end

  def call(conn, _opts) do
    with ["Bearer " <> token] <- get_req_header(conn, "authorization"),
         {:ok, data} <- MyApp.Token.verify(token) do
      conn
      |> assign(:current_user, MyApp.Account.get_user(data.user_id))
    else
      error ->
        conn
        |> put_status(:unauthorized)
        |> Phoenix.Controller.put_view(MyAppWeb.ErrorView)
        |> Phoenix.Controller.render(:"401")
        |> halt()
    end
  end
end

4. Add plug to router

defmodule MyAppWeb.Router do
  use MyAppWeb, :router

  pipeline :api do
    plug :accepts, ["json"]
  end

  pipeline :authenticated do
    plug MyAppWeb.Plug.Authenticate
  end

  scope "/api", MyAppWeb do
    pipe_through :api
    post "/auth/login", SessionController, :new
  end

  scope "/api", MyAppWeb do
    pipe_through [:api, :authenticated]
    delete "/auth/logout", SessionController, :delete
    ...
  end
end

We have done with it. Just a few line of code

Conclusion

Implement token authentication with Phoenix is so easy and you may not need Guardian for your application. In this post I only implement simple version of token authentication. In real application, you should store token signature in database or redis and

  • In Authenticate plug check if the token exists in database.
  • When user logout, clear it from database/redis to make sure that token cannot be used to make request anymore.

Thanks for reading.