title: Google Cloud Storage - Bucket permission within application
body: <p>I am developing a web application using GAE Java and using Google Cloud Storage to store images. The requirements are:</p>
  
  <ol>
  <li>Facebook authentication. When a user is authenticated I want his profile picture to be uploaded to GCS. </li>
  <li>The bucket where I store such images should obviously not allow full user access, but just read-access</li>
  </ol>
  
  <p>I managed to get FB profile picture using Spring Social and upload to GCS but only if I set all users to owners or writers for that bucket. 
  How can I only give write access within the application to upload images, but keep bucket ACL to READ_ONLY for all users?  </p>