Flutter App for encryption an audio for secure - flutter

Question

I have a project with encrypt an audio for secure my data audio and when the user purchase they can play the audio, but until now didn't find how to do that. I found another way by converting the audio data into a string and then encrypting it, I found a code but it's incomplete so I'm still confused about using it.
import 'dart:io';
import 'dart:typed_data';
import 'package:file_picker/file_picker.dart';
import 'package:path_provider/path_provider.dart';
class MediaFile {
Future<String> get docPath async {
Directory appDocDir = await getApplicationDocumentsDirectory();
return appDocDir.path;
}
Future<Uint8List> get audioFileData async {
final String path = await docPath;
final String theFilePath = '$path/test.mp3';
final File theFile = new File(theFilePath);
if (await theFile.exists()) {
return await theFile.readAsBytes();
} else {
File file = await FilePicker.getFile();
return await file.readAsBytes();
}
}
Future<String> dataToString() async {
Uint8List data = await audioFileData;
return new String.fromCharCodes(data);
}
Future<File> saveMediaAsString(String fileName, String fileContent) async {
String path = await docPath;
Directory dataDir = new Directory('$path/data');
if (await dataDir.exists()) {
File file = new File('$path/data/$fileName');
return file.writeAsString(fileContent);
}
await dataDir.create();
File file = new File('$path/data/$fileName');
return file.writeAsString(fileContent);
}
Future<String> readFromStringMediaFile(String fileName) async {
String path = await docPath;
File file = new File('$path/data/$fileName');
return file.readAsString();
}
Uint8List stringToData(dataStr) {
List<int> list = dataStr.codeUnits;
return new Uint8List.fromList(list);
}
Future<File> createAudioFile(String fileName, Uint8List bytes) async {
String path = await docPath;
File file = new File('$path/$fileName');
return await file.writeAsBytes(bytes);
}
//here you can see how to use them
Future<File> testFile() async {
String dataStr = await dataToString();
File savedFile = await saveMediaAsString('codedFile', dataStr);
String contentSavedFile = await readFromStringMediaFile('codedFile');
Uint8List bytes = stringToData(contentSavedFile);
return await createAudioFile('test.mp3', bytes);
}
}