public function getMetadataUrl($imageIdentifier) {
        $url = sprintf(
            $this->getHostForImageIdentifier($imageIdentifier) . '/users/%s/images/%s/metadata.json',
            $this->getUser(),
            $imageIdentifier
        );

        return Http\MetadataUrl::factory(
            $url,
            $this->getConfig('privateKey'),
            $this->getPublicKey()
        );
    }