/**
* Sign a request
*
* @param array $params
* @param null|string $method
* @param null|string $url
* @return string
*/
public function sign(array $params, $method = null, $url = null)
{
$rsa = new RSAEncryption();
$rsa->setHashAlgorithm($this->_hashAlgorithm);
$sign = $rsa->sign($this->_getBaseSignatureString($params, $method, $url), $this->_key, RSAEncryption::BASE64);
return $sign;
}
