public function getFormKeyEsiUrl() {
        $urlOptions = array(
            $this->getEsiTtlParam()         => $this->getDefaultEsiTtl(),
            $this->getEsiMethodParam()      => 'esi',
            $this->getEsiScopeParam()       => 'global',
            $this->getEsiCacheTypeParam()   => 'private',
        );
        $esiUrl = Mage::getUrl('turpentine/esi/getFormKey', $urlOptions);
        // setting [web/unsecure/base_url] can be https://... but ESI can never be HTTPS
        $esiUrl = preg_replace('|^https://|i', 'http://', $esiUrl);
        return $esiUrl;
    }