title: Youtube PubSubHubBub Integration
body: <p>Been stuck on this integration problem for hours now. I am trying to subscribe to a youtube channel updates using 
  <a href="https://pubsubhubbub.appspot.com/subscribe" rel="noreferrer">https://pubsubhubbub.appspot.com/subscribe</a>.</p>
  
  <p><a href="https://i.stack.imgur.com/6PjTI.png" rel="noreferrer"><img src="https://i.stack.imgur.com/6PjTI.png" alt="Subscription Request"></a></p>
  
  <p>And my server is setup to respond with:</p>
  
  <p><a href="https://i.stack.imgur.com/Xhq5d.png" rel="noreferrer"><img src="https://i.stack.imgur.com/Xhq5d.png" alt="Server Response"></a></p>
  
  <p>But when I press <strong>DO IT</strong>, I always get back the error saying <em>Challenge mismatch.</em></p>
  
  <p>I am unsure which parameter I am missing to set.</p>