wtf is sign?

Sign.jar is a small tool that automatically applies a digital signature to an Android app file (an APK). Every Android app needs to be signed before it can be installed on a phone, and this tool handles that step using Android's built-in test certificate. The signing keys are already bundled inside the tool, so you don't need to track down separate key files to make it work. Using it is straightforward. You run a single command pointing to your app file, and the tool creates a new, signed copy of that app right next to the original. If you prefer to just replace the original file with the signed version, there's an option for that too. You can then verify the signature worked by running a standard Java verification command on the output file. This tool is useful for developers, testers, or anyone working with Android apps who needs a quick way to sign an APK for testing purposes. For example, if you've built an app or modified an existing one and want to install it on a device to see how it behaves, you'd use this to apply the required signature. Since it uses the standard Android test key, it's specifically suited for development and testing scenarios rather than publishing apps to the Google Play Store, which requires a proper developer key. The project is based on Android's own signing code from version 4.1.1 of the platform. It's packaged as a self-contained Java program, meaning everything you need is in one file with no extra dependencies to install. The main change from the original Android source is the removal of a metadata tag that identified the signing tool.