You should sign all of your apps with the same certificate throughout the expected lifespan of your applications. There are several reasons why you should do so:
- App upgrade: When the system is installing an update to an app, it compares the certificate(s) in the new version with those in the existing version. The system allows the update if the certificates match. If you sign the new version with a different certificate, you must assign a different package name to the application—in this case, the user installs the new version as a completely new application.
- App modularity: Android allows apps signed by the same certificate to run in the same process, if the applications so requests, so that the system treats them as a single application. In this way you can deploy your app in modules, and users can update each of the modules independently.
- Code/data sharing through permissions: Android provides signature-based permissions enforcement, so that an app can expose functionality to another app that is signed with a specified certificate. By signing multiple apps with the same certificate and using signature-based permissions checks, your apps can share code and data in a secure manner.
If you plan to support upgrades for an app, ensure that your key has a validity period that exceeds the expected lifespan of that app. A validity period of 25 years or more is recommended. When your key's validity period expires, users will no longer be able to seamlessly upgrade to new versions of your application.
If you plan to publish your apps on Google Play, the key you use to sign these apps must have a validity period ending after 22 October 2033. Google Play enforces this requirement to ensure that users can seamlessly upgrade apps when new versions are available.