function sign($openid,$attribute,$value)
   {
      $samlObj = new SAML();
      $responseXmlString = $samlObj->createSamlAssertion($openid, 
                                                         $this->notBefore, 
                                                         $this->notOnOrAfter, 
                                                         $this->rsadsa,
                                                         $this->acsURI,
                                                         $attribute,
                                                         sha1($value),
                                                      $this->assertionTemplate);
      $signedAssertion=$samlObj->signAssertion($responseXmlString,
                                               $this->private_key,
                                               $this->public_key_certificate);
      return $signedAssertion;
   }