Android™ Application Development - Bahar Ali Khan

Chapter 11: Advanced Android Development
Paranoid Android
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Much of Android’s security is native to the underlying Linux kernel. Resources are sandboxed to their
owner applications, making them inaccessible from other applications. Android provides broadcast
Intents, Services, and Content Providers to let you relax these strict process boundaries, using the permission
mechanism to maintain application-level security.
You’ve already used the permission system to request access to native system services — notably the
location-based services and contacts Content Provider — for your applications.
The following sections provide a more detailed look at the security available. For a comprehensive view,
the Android documentation provides an excellent resource that describes the security features in depth
at code.google.com/android/devel/security.html.
Linux Kernel Security
Each Android package has a unique Linux userID assigned to it during installation. This has the effect
of sandboxing the process and the resources it creates, so that it can’t affect (or be affected by) other
applications.
Because of this kernel-level security, you need to take additional steps to communicate between applications.
Enter Content Providers, broadcast Intents, and AIDL interfaces. Each of these mechanisms
opens a tunnel for information to fl ow between applications. Android permissions act as border
guards at either end to control the traffi c allowed through these tunnels.
Introducing Permissions
Permissions are an application-level security mechanism that lets you restrict access to application components.
Permissions are used to prevent malicious applications from corrupting data, gaining access
to sensitive information, or making excessive (or unauthorized) use of hardware resources or external
communication channels.
As you’ve learned in earlier chapters, many of Android’s native components have permission requirements.
The native permission strings used by native Android Activities and Services can be found as
static constants in the android.Manifest.permission class.
To use permission-protected components, you need to add uses-permission tags to application manifests,
specifying the permission string that each application requires.
When an application package is installed, the permissions requested in its manifest are analyzed and
granted (or denied) by checks with trusted authorities and user feedback.
Unlike many existing mobile platforms, all Android permission checks are done at installation. Once
an application is installed, the user will not be prompted to reevaluate those permissions.
There’s no guarantee that your application will be granted the permissions it requests, so it’s good practice
to write defensive code that ensures it fails gracefully in these circumstances.