public function parse($credential = array())
    {
        if (array_key_exists(self::PUBKEY_FILE, $credential)) {
            $type = self::PUBKEY_FILE;
            $data = $credential[self::PUBKEY_FILE];
        } elseif (array_key_exists(self::PASSWORD, $credential)) {
            $type = self::PASSWORD;
            $data = $credential[self::PASSWORD];
        } else {
            $type = self::NONE;
            $data = isset($credential[self::NONE]) ? $credential[self::NONE] : array();
        }

        return array($type, $data);
    }