How to add App specific permissions to Android DocumentProvider

Question

I want to create a file in AppA and then be able to access it from AppB only. I am able to create the file via the DocumentProvider and then access it via StorageClient see examples here. How do I setup the permission on the file in AppA so that only AppB can access it?

Method for file creation in AppA

        String s = "kv;ab\nkv1;cd";
        try {
            byte[] buffer = s.getBytes();
            String filename = "myfile.txt";
            System.out.println("filename="+filename);
            FileOutputStream fos = getContext().openFileOutput(filename, Context.MODE_PRIVATE);
            fos.write(buffer);
            fos.close();
            System.out.println(s);
        } catch (IOException e) {
            e.printStackTrace();
        }
    }   

Methods for File access in AppB

public void onClick(View view) {
        Intent intent = new Intent(Intent.ACTION_OPEN_DOCUMENT);
        intent.addCategory(Intent.CATEGORY_OPENABLE);
        intent.setType("text/plain");
        Uri pickerInitialUri= Uri.parse("content://com.example.android.storageprovider.documents/document/root%3Amyfile.txt");
        intent.putExtra(DocumentsContract.EXTRA_INITIAL_URI, pickerInitialUri);
        startActivityForResult(intent, READ_REQUEST_CODE);
    }

    @Override
    public void onActivityResult(int requestCode, int resultCode, Intent resultData) {
        super.onActivityResult(requestCode, resultCode, resultData);
        readFileExternalStorage();
    }

    public String readFileExternalStorage() {
        String s = "";
        Uri uri1 = Uri.parse("content://com.example.android.storageprovider.documents/document/root%3Amyfile.txt");
        try {
            InputStream ins = this.getBaseContext().getContentResolver().openInputStream(uri1);
            ByteArrayOutputStream outputStream = new ByteArrayOutputStream();
            int size;
            byte[] buffer = new byte[1024];
            while ((size = ins.read(buffer, 0, 1024)) >= 0) {
                outputStream.write(buffer, 0, size);
            }
            ins.close();
            buffer = outputStream.toByteArray();
            s = new String(buffer);
            System.out.println("output=" + s);
        } catch (FileNotFoundException e) {
            System.out.println("An error occurred.");
            e.printStackTrace();
        } catch (IOException e) {
            System.out.println("An error occurred.");
            e.printStackTrace();
        }
        TextView textview = findViewById(R.id.textView);
        textview.setText(s);
        return "ok\n" + s;
    }

Answer 1

Two probable options-

1. https://developer.android.com/guide/topics/permissions/defining

2. Use a shared encryption key with user based salt to encrypt and decrypt stored files on Android