A signed APK is essential for Android app development and distribution because:
- Security and Authenticity: Verifies developer identity and ensures the app hasn't been tampered with.
- Distribution Requirements: Required by the Google Play Store and for consistent app updates.
- Permissions and User Trust: Manages app permissions and builds user trust.
- Compatibility and Performance: Ensures device compatibility and optimized performance.
- Enterprise and Custom Distribution: Crucial for enterprise deployment and custom ROM compatibility.
Steps to check if the APK file is signed or not:
Set up the Android SDK: Ensure you have the Android SDK installed. The apksigner tool is located in the build-tools directory.
Open a Command Line: Open a terminal or command prompt.
Run the Command: Use the following command, replacing with the path to your APK file: keytool -printcert -jarfile 'path of the APK file'
Check the Output:
If the APK is signed, you will see a message like "Verified".
If the APK is not signed, you will see an error message indicating that the APK is not signed.

An unsigned APK cannot be published, installed, trusted, updated, or fully functional, limiting its distribution and usability.