private function _validateTargeting()
    {
        $exts = $this->_ac->acinfo()->extensions();
        // if target information extension is not present
        if (!$exts->has(Extension::OID_TARGET_INFORMATION)) {
            return;
        }
        $ext = $exts->get(Extension::OID_TARGET_INFORMATION);
        if ($ext instanceof TargetInformationExtension &&
             !$this->_hasMatchingTarget($ext->targets())) {
            throw new ACValidationException(
                "Attribute certificate doesn't have a matching target.");
        }
    }