TL;DR
Repacking an Android application with the original developer’s signature is extremely difficult and generally not possible without access to their private key. While tools exist to repackage APKs, they will require you to sign them with your own key, invalidating any previous signatures and potentially causing issues with app updates or functionality.
Understanding Android App Signatures
Android apps are digitally signed by the developer using a cryptographic key. This signature serves several important purposes:
- Verification: Ensures the app hasn’t been tampered with after it was published.
- Identity: Confirms the app’s origin and that it comes from the legitimate developer.
- Updates: The Play Store uses the signature to determine if an update is from the same developer as the original app.
The signature is embedded within the APK file.
Why Repacking with Original Signature is Hard
The private key used for signing is meant to be kept secret by the developer. Without it, you cannot create a valid signature matching theirs. Attempting to do so would involve:
- Reverse Engineering: Trying to extract the key from the APK (very difficult and often unsuccessful).
- Security Breaches: Obtaining the key through illegal means (highly unethical and illegal).
Steps for Repacking an App (with *your* signature)
If you have a legitimate reason to repackage an app – such as modifying it for personal use or creating a custom build – here’s how you can do it, but remember this will not be signed with the original developer’s key. You will need Android Studio and the APK file.
- Decompile the APK: Use a tool like APKTool to decompile the APK into its source code and resources.
apktool d your_app.apk - Make Your Modifications: Edit the source code, images, or other resources as needed within the decompiled directory.
- Rebuild the APK: Use APKTool to rebuild the modified project into a new APK.
apktool b your_app - Sign the APK: This is where you use *your* key, not the original developer’s. You’ll need a keystore file (.jks).
- Generate a Keystore (if you don’t have one): In Android Studio, go to Build > Generate Signed Bundle / APK… Follow the wizard to create a new keystore.
- Sign using jarsigner: From the command line:
jarsigner -verbose -sigalg SHA256withRSA -digestalg SHA-256 -keystore your_keystore.jks your_app.apk alias_name
- Verify the Signature: Use apksigner to verify that the APK is signed correctly.
apksigner verify your_app.apk
Important Considerations
- Legal Implications: Modifying and redistributing someone else’s app without permission is illegal in most jurisdictions.
- App Updates: An app signed with a different key will be treated as a completely new application by the Play Store, meaning you won’t receive automatic updates from the original developer.
- Functionality Issues: Some apps may have security checks that prevent them from running if they are not signed with the correct key.