title: error publishing to a topic using curl from a IoT device (google pub/sub)
body: <p>I have a group of embedded devices running openwrt linux, this devices send data (json) every 3min to an URL using POST.
  As the number of devices is increasing, I'd like to use google pub/sub service to handle that data before doing streaming analysis of the data</p>
  
  <p>But everytime I'm trying to publish something to the topic I get this error:</p>
  
  <pre><code>POST https://pubsub.googleapis.com/v1/projects/myproject/topics/mytopic:publish
  
  { "error": { "code": 401, "message": "The request does not have valid authentication credentials.", "status": "UNAUTHENTICATED" } }
  </code></pre>
  
  <p>I have created a service account and tried to append the id-key at the end of the url, but still same result. 
  What should I do?
  thanks</p>