 
   @doc "Create a new set of credentials for an asserted ID, given a desired TTL and shared secret"
   def create_credentials(id: id, secret: secret) do
-    username = Enum.join(["foobar", id], ":")
+    username = Enum.join(["foobar", id], @username_separator)
     [username: username, password: sign(username, secret)]
   end
   def create_credentials(secret: secret) do

defmodule MojoAuth do
  use Timex

  @moduledoc ~S"""
  Create and verify MojoAuth credentials

    password = credentials[:password]
    case sign(credentials[:username], secret) do
      ^password ->
        case String.split(credentials[:username], @username_separator, trim: true) do
          [expiry, id] ->
            {:ok, id}
          [expiry] ->
