I'm currently building an Eclipse plugin which creates and stores sensitive data, let's say "highscores".
As users can simply unpack the jar file, they are able to see the source code of the project. Thus they are able to cheat and create a faked highscore file by either changing the data in the file itself or writing to the highscore file (as they know how the data is encrypted from the source code).
Can I encrypt my jar file so that users cannot read the source code, but the plugin is still working properly?
As greg-449 already commented, that when the code is with user, it can be hacked in one way or the another. If Encryption of secure data is not going to help, you can move the secure data on your server.
Impact would be, it is available only when the user is online.
Another solution, a bit complex one:
Don't store High Score at all. Store/ log the user's activities to calculate high score. make the logs encrypted to add more security. Use salt, timestamp and secure key while encryption and a checksum to prevent manual changes to the file.