Combo.Token (combo v0.8.0)

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Sign/verify or encrypt/decrypt data inside tokens.

The data stored in the token is signed to prevent tampering, and is optionally encrypted. This means that, so long as the key (see below) remains secret, you can be assured that the data stored in the token has not been tampered with by a third party.

However, unless the token is encrypted, it is not safe to use this token to store private information, such as a user's sensitive identification data, as it can be trivially decoded. If the token is encrypted, its contents will be kept secret from the client, but it is still a best practice to encode as little secret information as possible, to minimize the impact of key leakage.

Examples

When generating a unique token for use in an API or Channel it is advised to use a unique identifier for the user, typically the id from a database. For example:

iex> context = Demo.Web.Endpoint
iex> namespace = "user auth"
iex> user_id = 1
iex> token = Combo.Token.sign(context, namespace, user_id)
iex> Combo.Token.verify(context, namespace, token)
{:ok, 1}

In that example we have a user's id, we generate a token and verify it using the secret key base configured in the given endpoint. We guarantee the token will only be valid for one day by setting a max age (recommended).

The first argument to sign/4, verify/4, encrypt/4, and decrypt/4 is called context, and it can be one of:

  • a string - the secret key base itself. A key base with at least 20 randomly generated characters should be used to provide adequate entropy.
  • the module name of an endpoint - the secret key base is extracted from the endpoint.
  • %Plug.Conn{} - the secret key base is extracted from the endpoint stored in the connection.
  • %Combo.Socket{} - the secret key base is extracted from the endpoint stored in the socket.

The second argument is a cryptographic salt which must be the same in both calls to sign/4 and verify/4, or both calls to encrypt/4 and decrypt/4. For instance, it may be called "user auth" and treated as namespace when generating a token that will be used to authenticate users on channels or on your APIs.

The third argument can be any term that you wish to codify into the token. Upon valid verification, this same term will be extracted from the token.

Usage

Once a token is signed, we can send it to the client in multiple ways.

One is via the meta tag:

<meta name="channel_token" content={Combo.Token.sign(@conn, "user auth", @current_user.id)}>

Or an endpoint that returns it:

def create(conn, params) do
  user = User.create(params)
  render(conn, "user.json", %{
    token: Combo.Token.sign(conn, "user auth", user.id),
    user: user
  })
end

Once the token is sent, the client may now send it back to the server as an authentication mechanism. For example, we can use it to authenticate a user on a channel:

defmodule Demo.Web.UserSocket do
  use Combo.Socket

  def connect(%{"token" => token}, socket, _connect_info) do
    case Combo.Token.verify(socket, "user auth", token) do
      {:ok, user_id} ->
        socket = assign(socket, :user, Repo.get!(User, user_id))
        {:ok, socket}
      {:error, _} ->
        :error
    end
  end

  def connect(_params, _socket, _connect_info), do: :error
end

In this example, the client JavaScript code will send the token in the connect command which is then validated by the server.

Combo.Token can also be used for validating APIs, handling password resets, e-mail confirmation and more.

Summary

Types

context()
data()
max_age_opt()
salt()
shared_opt()
signed_at_opt()
token()

Functions

decrypt(context, salt, token, opts \\ [])

Decrypts the original data from the token and verifies its integrity.

encrypt(context, salt, data, opts \\ [])

Encodes, encrypts, and signs data into a token you can send to clients. Its usage is identical to that of sign/4, but the data is extracted using decrypt/4, rather than verify/4.

sign(context, salt, data, opts \\ [])

Encodes and signs data into a token you can send to clients.

verify(context, salt, token, opts \\ [])

Decodes the original data from the token and verifies its integrity.

Types

context()

@type context() :: binary() | module() | Plug.Conn.t() | Combo.Socket.t()

data()

@type data() :: term()

max_age_opt()

@type max_age_opt() :: {:max_age, pos_integer() | :infinity}

salt()

@type salt() :: binary()

shared_opt()

@type shared_opt() ::
  {:key_iterations, pos_integer()}
  | {:key_length, pos_integer()}
  | {:key_digest, :sha256 | :sha384 | :sha512}

signed_at_opt()

@type signed_at_opt() :: {:signed_at, pos_integer()}

token()

@type token() :: binary()

Functions

decrypt(context, salt, token, opts \\ [])

@spec decrypt(context(), salt(), token(), [shared_opt() | max_age_opt()]) ::
  {:ok, data()} | {:error, :expired | :invalid | :missing}

Decrypts the original data from the token and verifies its integrity.

Its usage is identical to verify/4 but for encrypted tokens.

Options

  • :key_iterations - option passed to Plug.Crypto.KeyGenerator when generating the encryption and signing keys. Defaults to 1000.
  • :key_length - option passed to Plug.Crypto.KeyGenerator when generating the encryption and signing keys. Defaults to 32.
  • :key_digest - option passed to Plug.Crypto.KeyGenerator when generating the encryption and signing keys. Defaults to :sha256.
  • :max_age - verifies the token only if it has been generated "max age" ago in seconds. Defaults to the max age signed in the token by encrypt/4.

encrypt(context, salt, data, opts \\ [])

@spec encrypt(context(), salt(), data(), [
  shared_opt() | max_age_opt() | signed_at_opt()
]) :: token()

Encodes, encrypts, and signs data into a token you can send to clients. Its usage is identical to that of sign/4, but the data is extracted using decrypt/4, rather than verify/4.

Options

  • :key_iterations - option passed to Plug.Crypto.KeyGenerator when generating the encryption and signing keys. Defaults to 1000.
  • :key_length - option passed to Plug.Crypto.KeyGenerator when generating the encryption and signing keys. Defaults to 32.
  • :key_digest - option passed to Plug.Crypto.KeyGenerator when generating the encryption and signing keys. Defaults to :sha256
  • :signed_at - set the timestamp of the token in seconds. Defaults to System.os_time(:second).
  • :max_age - the default maximum age of the token in seconds. Defaults to 86400 and it may be overridden on decrypt/4.

sign(context, salt, data, opts \\ [])

@spec sign(context(), salt(), data(), [shared_opt() | max_age_opt() | signed_at_opt()]) ::
  token()

Encodes and signs data into a token you can send to clients.

Options

  • :key_iterations - option passed to Plug.Crypto.KeyGenerator when generating the encryption and signing keys. Defaults to 1000.
  • :key_length - option passed to Plug.Crypto.KeyGenerator when generating the encryption and signing keys. Defaults to 32.
  • :key_digest - option passed to Plug.Crypto.KeyGenerator when generating the encryption and signing keys. Defaults to :sha256
  • :signed_at - the timestamp of the token in seconds. Defaults to System.os_time(:second).
  • :max_age - the default maximum age of the token in seconds. Defaults to 86400 and it may be overridden on verify/4.

verify(context, salt, token, opts \\ [])

@spec verify(context(), salt(), token(), [shared_opt() | max_age_opt()]) ::
  {:ok, data()} | {:error, :expired | :invalid | :missing}

Decodes the original data from the token and verifies its integrity.

Examples

In this scenario we will create a token, sign it, then provide it to a client . The client will then use this token to authenticate requests for resources from the server. See Combo.Token summary for more info about creating tokens.

iex> secret     = "xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx"
iex> namespace  = "user auth"
iex> user_id    = 99
iex> token      = Combo.Token.sign(secret, namespace, user_id)

The mechanism for passing the token to the client is typically through a cookie, a JSON response body, or HTTP header. For now, assume the client has received a token it can use to validate requests for protected resources.

When the server receives a request, it can use verify/4 to determine if it should provide the requested resources to the client:

iex> Combo.Token.verify(secret, namespace, token)
{:ok, 99}

In this example, we know the client sent a valid token because verify/4 returned a tuple of type {:ok, user_id}. The server can now proceed with the request.

However, if the client had sent an expired token, an invalid token, or nil, verify/4 would have returned an error instead:

iex> Combo.Token.verify(secret, namespace, expired)
{:error, :expired}

iex> Combo.Token.verify(secret, namespace, invalid)
{:error, :invalid}

iex> Combo.Token.verify(secret, namespace, nil)
{:error, :missing}

Options

  • :key_iterations - option passed to Plug.Crypto.KeyGenerator when generating the encryption and signing keys. Defaults to 1000.
  • :key_length - option passed to Plug.Crypto.KeyGenerator when generating the encryption and signing keys. Defaults to 32.
  • :key_digest - option passed to Plug.Crypto.KeyGenerator when generating the encryption and signing keys. Defaults to :sha256.
  • :max_age - verifies the token only if it has been generated "max age" ago in seconds. Defaults to the max age signed in the token by sign/4.