encrypting dompdf while opening - php

Question

i am encrypting my pdf but when i open it and enters the password it always says wrong error this is my code:
global $_dompdf_show_warnings;
global $_dompdf_debug;
global $_DOMPDF_DEBUG_TYPES;
$outfile = $id . '_qr_codes.pdf';
$save_file = TRUE; // Save the file or not
$buff = $this->qr_generate_pdf($id);
//echo $buff; die;
$dompdf = new DOMPDF();
$dompdf->load_html($buff);
if (isset($base_path)) {
$dompdf->set_base_path($base_path);
}
$dompdf->render();
$dompdf->get_canvas()->get_cpdf()->setEncryption('admin','ownerpass');
if ($_dompdf_show_warnings) {
global $_dompdf_warnings;
foreach ($_dompdf_warnings as $msg) {
echo $msg . "\n";
}
echo $dompdf->get_canvas()->get_cpdf()->messages;
flush();
}
if ($save_file) {
// if ( !is_writable($outfile) )
// throw new DOMPDF_Exception("'$outfile' is not writable.");
if (strtolower(DOMPDF_PDF_BACKEND) == "gd") {
$outfile = str_replace(".pdf", ".png", $outfile);
}
list($proto, $host, $path, $file) = explode_url($outfile);
if ($proto != "") // i.e. not file://
$outfile = $file; // just save it locally, FIXME? could save it like wget: ./host/basepath/file
//$outfile = dompdf_realpath($outfile);
// if ( strpos($outfile, DOMPDF_CHROOT) !== 0 )
// throw new DOMPDF_Exception("Permission denied.");
file_put_contents($outfile, $dompdf->output(array("compress" => 0)));
}
if (!headers_sent())
$dompdf->stream($outfile);
}
when i enter the password 'ownerpass' in the downloaded pdf it throughs an error-> The Password is incorrect. Please make sure that capslock is not on by mistake,and try again.
Thanks in advance