在SharedPreferences中加密和解密Android字符串

我需要在SharedPreferences中保存并加载字符串savedText，所以我需要对字符串进行加密和解密。我将字符串保存在saveText（）并加载到loadText（String UNIC）。UNIC是保存我的字符串的ID。我有此代码，它可以工作，但不能加密。

     private void saveText() throws GeneralSecurityException, IOException {

            for (int i = 0; i < myArr.size(); i++) { //here i create my string
                SAVEDITEMS = SAVEDITEMS + myArr.get(i).replace("✔", "") + "&";
            } 

          KeyGenParameterSpec keyGenParameterSpec = MasterKeys.AES256_GCM_SPEC;
            String masterKeyAlias = MasterKeys.getOrCreate(keyGenParameterSpec);


            String fileToWrite = SAVEDITEMS;
           //getFilesDir() is ok? is was variable directory at documentation

            try {
                EncryptedFile encryptedFile = new EncryptedFile.Builder(
                        new File(getApplicationContext().getFilesDir(), fileToWrite),
                        getApplicationContext(),
                        masterKeyAlias,
                        EncryptedFile.FileEncryptionScheme.AES256_GCM_HKDF_4KB
                ).build();


                BufferedWriter writer = new BufferedWriter(new OutputStreamWriter(
                        encryptedFile.openFileOutput()));
                writer.write("MY SUPER-SECRET INFORMATION");
            } catch (GeneralSecurityException gse) {
                // Error occurred getting or creating keyset.
            } catch (IOException ex) {
                // Error occurred opening file for writing.
            }

            sPref = getPreferences(MODE_PRIVATE);
            SharedPreferences.Editor ed = sPref.edit();
            ed.putString(UNIC, SAVEDITEMS);
            ed.commit();

        }

        private void loadText(String UNIC) throws GeneralSecurityException, IOException {

            KeyGenParameterSpec keyGenParameterSpec = MasterKeys.AES256_GCM_SPEC;
            String masterKeyAlias = MasterKeys.getOrCreate(keyGenParameterSpec);

            String fileToRead = SAVEDITEMS;

            EncryptedFile encryptedFile = new EncryptedFile.Builder(
                    new File(getApplicationContext().getFilesDir(), fileToRead),
                    getApplicationContext(),
                    masterKeyAlias,
                    EncryptedFile.FileEncryptionScheme.AES256_GCM_HKDF_4KB
            ).build();

            StringBuffer stringBuffer = new StringBuffer();
            try (BufferedReader reader =
                         new BufferedReader(new FileReader(String.valueOf(encryptedFile)))) {

                String line = reader.readLine();
                while (line != null) {
                    stringBuffer.append(line).append('\n');
                    line = reader.readLine();
                }
            } catch (IOException e) {
                // Error occurred opening raw file for reading.
            } finally {
                String contents = stringBuffer.toString();
            }


            sPref = getPreferences(MODE_PRIVATE);
            String savedText = sPref.getString(UNIC, SAVEDITEMS);

            //here i toast my string
                        Toast toast = Toast.makeText(getApplicationContext(),
                                savedText, Toast.LENGTH_SHORT);
                        toast.show();

            }


        }

请帮帮我，是getFilesDir（）还是其他地方的问题？谢谢你的帮助。对我来说真的很重要。