Application signing
One of the differentiating factors of Android is the way Android applications are signed. All applications in Android are self-signed. There is no requirement to sign the applications using a certificate authority. This is different from traditional application signing where a signature identifies the author and bases trust upon the signature.
The signature of the application associates the app with the author. If a user installs multiple applications written by the same author and these applications want to share each other's data, they need to be associated with the same signature and should have a SHARED_ID flag set in the manifest file.
The application signature is also used during the application upgrade. An application upgrade requires that both applications have the same signature and that there is no permission escalation. This is another mechanism in Android that ensures the security of applications.
As an application developer, it is important to keep the private key used to sign the application secure. As an application author, your reputation depends on it.
