/**
* @return string
*/
public function generateSignature()
{
$dateKey = hash_hmac('sha256', $this->credential->getDate(), 'AWS4' . $this->secret, true);
$dateRegionKey = hash_hmac('sha256', $this->credential->getRegion(), $dateKey, true);
$dateRegionServiceKey = hash_hmac('sha256', $this->credential->getService(), $dateRegionKey, true);
$signingKey = hash_hmac('sha256', $this->credential->getRequestType(), $dateRegionServiceKey, true);
$signature = hash_hmac('sha256', $this->getBase64EncodedPolicy(), $signingKey);
return $signature;
}