static public function decrypt($ciphertextFileName, Config $config = null) {
        // Instantiate the client, authenticate, and add scopes.
        $kms = self::getService();
        $name = self::getDefaultKeyName($config);
        // Use the KMS API to decrypt the text.
        $ciphertext = base64_encode(file_get_contents($ciphertextFileName));
        $request = new \Google_Service_CloudKMS_DecryptRequest();
        $request->setCiphertext($ciphertext);
        $response = $kms->projects_locations_keyRings_cryptoKeys->decrypt(
            $name,
            $request
        );
        return base64_decode($response['plaintext']);
    }