static public function encrypt($plaintextFileName, $ciphertextFileName, $key_name = null) {
        $kms = self::getService();
        if (is_null($key_name)) {
            $key_name = self::getDefaultKeyName();
        }

        // Use the KMS API to encrypt the text.
        $encoded = base64_encode(file_get_contents($plaintextFileName));
        $request = new \Google_Service_CloudKMS_EncryptRequest();
        $request->setPlaintext($encoded);
        $response = $kms->projects_locations_keyRings_cryptoKeys->encrypt(
            $key_name,
            $request
        );

        // Write the encrypted text to a file.
        file_put_contents($ciphertextFileName, base64_decode($response['ciphertext']));
        Util::cmdline("\tSaved encrypted text to $ciphertextFileName with key $key_name");
        return true;
    }