This document discusses Android storage and Opaque Binary Blobs (OBBs). It defines StorageManager as the interface for storage-related items like OBBs. OBBs allow large binary assets to be stored outside an APK and mounted on demand. The document outlines how to create an encrypted OBB file using mkobb.sh, change its owner, sign it with obbtool, and include it in an OBB-enabled APK to securely provide additional app resources like large files.

1. Android Storage –
StorageManager & OBB
William.L
wiliwe@gmail.com
Date: 2011-11-08

2. Outline
Introduction
Usage of OBB
Good Point of OBB

3. Introduction

4. What is StorageManager?
Since Android Gingerbread 2.3
The interface to the system’s storage service,
MountService, for Android applications
Handle storage-related items such as Opaque Binary Blobs
(OBB)
Code
AndroidSrc/frameworks/base/core/java/android/os/storage/St
orageManager.java
Usage
By calling getService(Context.STORAGE_SERVICE)
STORAGE_SERVICE represents “storage”
Ex : StorageManager stMgr =
(StorageManager)getService(Context.STORAGE_SERVIC
E)

5. What is OBB? (1/3)
Opaque Binary Blob, since Gingerbread 2.3
A file contains a filesystem that maybe be
encrypted on disk and mounted on-demand from
an application
A good way of providing large amounts of binary
assets without packaging them into APKs as
they may be multiple gigabytes

6. What is OBB? (2/3)
Other names
Disk Image / Virtual Drive / Virtual Disk Image /
Virtual Optical Drive file (虛擬光碟虛擬光碟虛擬光碟虛擬光碟)
Mount on the Linux LOOP device(total 7 devices)
Ex : sudo losetup /dev/loop0 Virtual-Disk-File
sudo mount /dev/loop0 /mnt/Mount-Point

7. What is OBB? (3/3)
……….
File
File
File
Folder
File
File
File
An OBB File

8. Usage (1/6)
The following steps are steps to create an OBB
file
Based on Androind Honeycomb 3.2
environment and its tools
Load needed modules :
sudo modprobe cryptoloop
sudo modprobe twofish
sudo modprobe vfat
Under your home directory, create a folder
named "obbdir“

9. Usage (2/6)
Enter "obbdir" folder and create some files
Here are hello321test.txt, hello.txt, test123.txt for example
Create an OBB file using the command
sudo AndroidSrc/frameworks/base/tools/obbtool/mkobb.sh -d OBB-Directory -k
Password -o ObbFile
Ex :
sudo AndroidSrc/frameworks/base/tools/obbtool/mkobb.sh -d ~/obbdir/ -k 1234 -o
obbtest.obb
“1234” is the password / key used by an OBB application
to open this OBB file

10. Usage (3/6)
The output messages after executing "mkobb.sh“ may look
like:
“3c7150b91013237e” of the last line is the salt key and
need to write down for later OBB file signing process

11. Usage (4/6)
Cause to the identity to execute “mkobb.sh” is root,
the created OBB file's owner is root
So we change the owner of "obbtest.obb" before doing next
step
sudo chown YourName:YourName obbtest.obb
Sign the created OBB file using "obbtool“ tool
AndroidSrc/out/host/linux-x86/bin/obbtool a -n APK-Package-Name -v 1 ObbFile -s
SaltKeyFrom-mkobb.sh
Ex :
AndroidSrc/out/host/linux-x86/bin/obbtool a -n com.example.android.obbapp -v 1
obbtest.obb -s 3c7150b91013237e

12. Usage (5/6)
obbtool Parameters
-n with "package name" of your application (APK)
-v with user defined version number of the OBB file
-s with the salt key generated by "mkobb.sh"
Output messages after executing "obbtool" will look like
If you want to REMOVE signature, do :
AndroidSrc/out/host/linux-x86/bin/obbtool r ObbFile
Ex : AndroidSrc/out/host/linux-x86/bin/obbtool r obbtest.obb

13. Usage (6/6)
Now, you can use the OBB file with corresponding
OBB APKs with the valid key(assigned when using
mkobb.sh) and Package-Name (assigned when
using obbtool)

14. Good Point of OBB
Security
Only the APK with the valid key and matched
package name could open the signed OBB file
containing private or important files

15. An OBB APK Example (1/3)

16. An OBB APK Example (2/3)

17. An OBB APK Example (3/3)
Mounted OBB file
OBB Testing File Source (GitHub)
https://github.com/wiliwe/android-obb-example.git

18. Reference
http://julianshen.blogspot.com/2011/01/android-
gingerbread-storagemanager-and.html
http://stackoverflow.com/questions/4524981/what-is-
obbopaque-binary-blob-in-android-develop-site
http://developer.android.com/reference/android/os/st
orage/StorageManager.html
http://en.wikipedia.org/wiki/Opaque_binary_blob