title: Firebase custom token authentication (firebase version 3)
body: <p>I have an authentication server (NodeJS) where I authenticate a user and create a custom firebase token</p>
  
  <pre><code>   var token = firebase.auth().createCustomToken(userId); 
  </code></pre>
  
  <p>I used to be able to verify a user token (previous version), but now it is not so simple...</p>
  
  <p>I would like to get the decoded userId from the token </p>
  
  <pre><code>  firebase.auth().verifyIdToken(token).then(function)....
  </code></pre>
  
  <p>does not work for server generated custom tokens.</p>
  
  <p>Does anyone know how this could be done?</p>