title: purpose of timestamp in recaptcha's validate response
body: <p>based on the documentation the validation response looks like this</p>
  
  <pre><code>{
    "success": true|false,
    "challenge_ts": timestamp,  // timestamp of the challenge load (ISO format yyyy-MM-dd'T'HH:mm:ssZZ)
    "hostname": string,         // the hostname of the site where the reCAPTCHA was solved
    "error-codes": [...]        // optional
  }
  </code></pre>
  
  <p>I wonder whats the purpose of the timestamp field in the response. Do I need to validate it? </p>
  
  <p>I assume each validation token can only be used once and they expire after some time. So I shouldn't have to worry about the token reuse problem. I can't think of any exploit one could do even if the token doesn't expire at all. </p>
  
  <p>The same goes with hostname. Since the site-key and site-secret must match so I shouldn't have to validate that field either. </p>
  
  <p>Do they exist only for the purpose of logging? </p>