decrypt.dart

    Future<String> decrypt(String encryptedMessage) async {
      // 1.
      aesGcmSecretKey = await AesGcmSecretKey.
    (derivedBits);

     // 2.
     List<int> message = Uint8List.fromList(encryptedMessage.codeUnits);

    // 3. 
     Uint8List decryptdBytes = await    aesGcmSecretKey.decryptBytes(message, iv);

    // 4.
      String decryptdString = String.fromCharCodes(decryptdBytes);
      print('decryptdString $decryptdString');
      return decryptdString;
    }