title: SPAppToken (context token) is null
body: <p>I have a Sharepoint 2013 on premise environment for which I am developing an provider hosted app using Visual Studio. </p>
  
  <p>The Visual Studio project has an app project which contains a custom ribbon button that goes to the web project. The web project is hosted on Windows Azure and here I get the error that the context token is null or an empty string. </p>
  
  <p>Getting the token is done using the following code:</p>
  
  <pre><code>var contextTokenString = TokenHelper.GetContextTokenFromRequest(Request);
  SharePointContextToken contextToken =
       TokenHelper.ReadAndValidateContextToken
                               (contextTokenString, Request.Url.Authority);
  </code></pre>
  
  <p>When I see what is passed to the site hosted on Azure I notice that the <code>SPAppToken</code> is null. When using ULS viewer I get the error "<strong>The Azure Access Control service is unavailable</strong>"</p>
  
  <p>I have already checked if the app is registered (using <code>appregnew.aspx</code>) and that the id is the same for this and in the <code>appManifest.xml</code> and <code>web.config</code> files.</p>
  
  <p>How do I get the context token? Do I need to do something else in Azure in order to make it work?</p>