Android provides different solutions for secure data storage on devices. Based on the data type and application use case, developers can choose the solution that fits best.
For primitive data types such as ints, booleans, longs, floats, and strings, which need to persist across user sessions, it is best to use shared data types. Data in shared preferences is stored as a key-value pair that allows developers to save, retrieve, and persist data.
All application data is stored along with the application in the sandbox. This means that this data can be accessed only by that application or other applications with the same signature that have been granted the right to share data. It is best to store private data files in this memory. These files will be deleted when the application is uninstalled.
For large datasets, developers have an option to use the SQLite database that comes bundled with the Android software stack.
All Android devices allow users to mount external storage devices such as SD cards. Developers can write their application such that large files can be stored on these external devices. Most of these external storage devices have a VFAT filesystem, and Linux access control does not work here. Sensitive data should be encrypted before storing on these external devices.
Starting with Android 2.2 (API 8), APKs can be stored on external devices. Using a randomly generated key, the APK is stored within an encrypted container called the asec file. This key is stored on the device. The external devices on Android are mounted with noexec. All DEX files, private data, and native shared libraries still reside in the internal memory.
Wherever network connection is possible, developers can store data on their own web servers as well. It is advisable to store data that can compromise the user's privacy on your own servers. An example of such an application is banking applications where user account information and transaction details should be stored on a server rather than user's devices.
Chapter 7, Securing Application Data, discusses the data storage options on Android devices in great detail.
Rights protected content such as video, e-books, and music, can be protected on Android using the DRM framework API. Application developers can use this DRM framework API to register the device with a DRM scheme, acquire licenses associated with content, extract constraints, and associate relevant content with its license.