code: protected function generateHashes($sum)
  	{
  		// Round the time to 30 minutes
  		$time = (int) round(time() / 60 / 30);
  
  		return array_map(
  			function ($hashTime) use ($sum)
  			{
  				return hash_hmac('sha256', $sum . "\0" . $hashTime, System::getContainer()->getParameter('kernel.secret'));
  			},
  			array($time, $time - 1)
  		);
  	}
docs: Generate hashes for the current time and the specified sum
  
  @param integer $sum
  
  @return array